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CIRCUIT PROTECTIONguide

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ISO 9001:2000 Registeredwww.ttiinc.com

TVS Diodes

ResettableFuses

TelecomThyristors

Standard Fuses

PTC Thermistors

NTC Thermistors

Polymer Suppression

Devices

Metal OxideVaristors

MultilayerVaristors

GDTs

CIRCUIT PROTECTION COMPONENTS AVAILABLE FROM TTI

AVX Bourns KOA Littelfuse Murata Panasonic Raychem

Resettable Fuses ◆ ◆ ◆

Standard Fuses ◆ ◆ ◆ ◆ ◆

Varistors ◆ ◆ ◆ ◆ ◆ ◆

Thermistors ◆ ◆ ◆ ◆

Thyristors ◆ ◆ ◆

GDTs ◆ ◆

TVS Diodes ◆

Polymer Suppresion Devices ◆

About TTI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

What is Circuit Protection? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Circuit Protection Line Card . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

AVX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Bourns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

KOA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Littelfuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Murata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Panasonic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Raychem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Vishay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

While every effort has been made to ensure that the information contained within is accurate and up to date, TTI, Inc. makes no warranty, representation or undertaking whether expressed or implied, nor does it assume any legal liability, whether direct or indirect, or responsibility for the accuracy, completeness, omissions, or usefulness of any information.

Copyright © 2003 TTI, Inc.

Table of Contents

2 C i r c u i t P r o t e c t i o n 3D i a l 1 - 8 0 0 - C A L L - T T I f o r S o l u t i o n s

About TTI, Inc.

Why Acquire Circuit Protection From TTI?TTI is the world’s largest distributor of circuit protection components. TTI maintains an extensive inventory and offers the best circuit protection choices in the industry, meeting almost any circuit protection requirements of customers. The company’s basic strategy is to focus on a specific range of products and strive to be the industry’s leading circuit protection distributor. No other company offers the unique combination of benefits that TTI provides.

Product Knowledgeable ExpertsThe team of specialists at TTI has more experience in circuit protection product areas than any other distributor. All TTI employees participate in company-wide training programs, which have helped create the most product-knowledgeable team in the industry. Additionally, customers dealing with the complexities of magnetic components can rely upon the TTI sales team as a valuable resource of information and guidance.

The 80/20 RuleThe parts sold by TTI comprise 80-85 percent of the components used in electronic equipment. But, because the components are so inexpensive, they represent only about 5-10 percent of the total cost. When customers rely on TTI to supply passive, interconnect, and electromechanical components, they address 80 percent of their potential problems—freeing them to focus on semiconductors, subassemblies, and PCBs which offer a better return on their efforts. Broader And Deeper InventoriesTTI maintains extensive inventories, stocking more than 200,000 parts, with more than 80 percent available for immediate delivery. That’s more passive and interconnect parts than from any other source. TTI’s product department ensures that the parts stocked are the right parts, needed by customers today.

Close Supplier RelationshipsTTI partners with only the leading components manufacturers in the world, structuring a solid supply base to establish a partnership with each manufacturer. As a group, they allow TTI to provide the best approved vendor list (AVL) coverage imaginable. And the suppliers chosen by TTI are the companies that lead the industry into new component technologies. This means that TTI Is able to enlist the help of a manufacturer to solve any potential design problems that may arise for customers.

Quality And ReliabilityTTI’s 30 year commitment to quality and service excellence means reliability for the customer. Whenever possible, TTI takes advantage of technology to shorten process times and prevent opportunities for errors. TTI implemented Total Quality Management in 1989 to further solidify a commitment to quality and has been registered to the most current ISO 9000 standard since 1993. Today, TTI has one global documentation system, allowing every branch and warehouse operated by TTI throughout the world to utilize the same processes. Year after year, customers rate our quality and management practices to be among the very best in the industry. Contact your local sales branch to learn more about TTI quality systems and results.

Services And Special ProgramsTTI is experienced, committed and well-prepared to meet your circuit protection and magnetic components needs. From component labeling to in-plant stores, TTI helps customers find, buy, receive, and use components in the most cost-efficient way. For more information on parts modification, electronic information exchange, and supply chain management programs contact a TTI branch location today. ❖


What Is Circuit Protection?

Circuit protection is an insurance policy. Most customers buy circuit protection never expecting to use it, but to protect circuits from unexpected damaging surges of current or voltage. Circuit protection is available in two types: over-current and over-voltage.

• What is Over-Current Protection?An over-current condition is caused by excessive current (more than the maximum amount of current a product is designed to withstand under normal conditions) passing through a circuit. Over-current conditions have the potential to cause severe damage to a circuit, possibly leading to fi re.

Over-current protection is placing a component, usually a standard fuse or a PTC (resettable fuse or PTC thermistor), in series with the device being protected, to limit the current to an acceptable level during over-current conditions, thus preventing catastrophic events. When no over-current condition exists, these products lie dormant within the circuit with a minimal amount of resistance to the circuit.

What Technologies Are Available to Provide Over-Current Protection? TTI carries three technologies classifi ed as over-current devices to provide protection to customers: fuses, resettable fuses, and PTC thermistors.

What Do They Do?Standard Fuses: Fuses are an intentionally weak link in a circuit. A standard, one-time fuse with the correct rating will “blow” before the circuit reaches a damaging level or fi re hazard. The event of the fuse blowing breaks the circuit and prevents current from fl owing past that point. A standard fuse will blow only once and then must be replaced.

Resettable fuses: A resettable fuse does not blow like a standard fuse. It actually creates a very high resistance that prevents most of the current from fl owing past it into the device on the circuit being protected. The resettable fuse then “resets,” as the name implies, once the over-current condition subsides.

PTC Thermistor: Similar in function to a resettable fuse, a PTC creates a very high resistance to the current at a particular temperature caused by additional current. This temperature (called “switching temperature”) shelters the device from overheat or over-current. Once the overheat or over-current event is removed, the thermistor will cool down and reset, just like a resettable fuse.

Why Are They Important?There are three main reasons why over-current protection is needed in a circuit: safety, reliability and compliance.

Safety: Using circuit protection will help guarantee safety as it protects from catastrophic events such as large surges of current that could cause fi res.

Reliability: Using over-current protection provides the reliability that components will be protected in the event that an over-current condition occurs. It also provides assurances that when the condition is cleared, the product will continue to function.

Compliance: Many agencies now require circuit protection in electronic devices.

Where Is Over-Current Protection Needed?Everywhere there is current running through an application. Over-current protection is a necessity. ☛

: Similar in function to a resettable fuse, a PTC creates a very high resistance


• What Is Over-Voltage Protection?Like over-current protection, over-voltage protection is the safeguarding of products from over-voltage conditions. An over-voltage condition occurs when an excessive amount of voltage is applied to the circuit. Over-voltage conditions are often called “transients,” and are defi ned as a short duration spike in voltage resulting from a sudden release of previously stored energy. Over-voltage conditions can be short or prolonged. Most applications will require both over-voltage and over-current protection for the circuit.

What Causes An Over-voltage Condition?An over-voltage condition is caused by a variety of reasons, including but not limited to, transformers converting energy from one voltage to another, lightning, ESD, motor start-ups, and automotive load dumps

Why Is Over-Voltage Protection Important?There are many reasons that people use overvoltage protection. They include, but are not limited to, the following:

◆ 75% of fi eld equipment failure is caused by Electrical Over-Stress ◆ With increasing speeds, semiconductor devices are becoming increasingly intolerant to voltage transients◆ A lightning strike several miles away can induce transients into your customer’s equipment ◆ Failures in IC’s can start and are common at 2kV (not noticeable to human touch) whereas most ESD events are at 8kV and higher◆ Anything else causing an overvoltage condition that would endanger parts on the board

What Over-Voltage Protection Devices Are Offered By TTI?TTI offers several options to limit excessive voltage experienced in today’s circuits. These are metal-oxide varistors (MOV), multi-layer varistors (MLV), thyristors, TVS diodes, gas discharge tubes (GDT) and polymer suppression devices.

How Do Over-Voltage Protection Devices Work?These devices can either clamp or crowbar the undesired voltage to protect the circuit.

Clamping devices are devices placed parallel to components being protected. When an over-voltage condition arises, the device goes from an extremely high impedance to one of almost zero, permitting the transient to follow the path of least resistance through the clamping device. The device then clamps at a voltage above the standard operating voltage, but below the level that would damage the device being protected. This additional voltage is dissipated by absorbing part and shunting the rest to ground. TTI offers clamping devices that include MOVs, MLVs, TVS Diodes, and polymer suppression devices.

Crowbar device are placed parallel to components being protected, and change from very high impedance to very low impedance during an over-voltage event. Energy is dissipated differently. During an over-voltage condition, the device operates like a switch, turning on to a low voltage state that is well below the normal operating voltage of the circuit. All energy is then shunted straight to ground and dissipated. TTI offers two crowbar devices, GDTs and Thyristors.

What Are MOVs?MOVs are metal-oxide varistors, and when exposed to high voltage transients, their impedance, resistance within the circuit, alters from very high to a highly conductive level (very low impedance to the circuit). The conductive state of the varistor is then much lower than any other path along the circuit and the transient energy diverts into the varistor where it is clamped and dissipated. ☛


What Are MLVs?An MLV is a multi-layer varistor, or a compact, surface mountable chip that is voltage dependent (like an MOV) and bidirectional. MLVs offer protection in forward and reverse directions, and like MOVs, stay in a very high impedance state when no over-voltage condition exists. When a condition arises, the MLV changes to a very high conductive state, allowing the voltage to pass into the MLV to be dissipated.

MLVs are constructed in various chip sizes and are capable of handling signifi cant surge energy for their size, and are used in lower voltage applications than MOVs.

What Are TVS Diodes?TVS diodes are silicon-based devices that allow current and voltage to fl ow in only one direction (unidirectional) when it reaches a specifi ed breakdown voltage. Bidirectional diodes protect lines from either direction, effectively clamping the voltage to a safe amount for the circuit.

What Are Polymer Suppression Devices?Polymer suppression devices are typically designed for ESD protection. The manufacturing process creates a device with extremely low capacitance , very low leakage current and extremely fast turn on time. Due to these characteristics, this type of device is suited to very fast applications. These type of ultra-fast applications must have low capacitance to keep from the accidental attenuation of high-speed data signals due to capacitive loading.

What Are Thyristors?Thyristors are silicon-based devices that protect a circuit from over-voltage conditions by switching to a low “on” state voltage and shunting additional voltage to ground, or exhibiting a crowbar effect. A thyristor acts like two devices: one that provides continuous voltage protection but also has a high current capability. One characteristic that differentiates thyristors from other devices is that once the thyristor has turned “on,” the current level must fall below a certain level for the device to turn off and reset. In many cases, the level is so low the circuit needs to shut down for the device to reset.

What Are GDTs? Gas discharge tubes are also known as spark gap tubes or gas tube surge arresters. These are crowbar devices that act as switches when in an “on” state, shunting over-voltage to ground, and leaving only minimal voltage on the circuit. GDTs are composed of ceramic tubes or envelopes with two or three electrodes lying opposite each other, separated by inert gas. When the voltage is below its turn on level, the gas acts as an insulator and creates a very high impedance to the circuit as if it were open. When the voltage reaches a certain “DC sparkover” voltage, the inert gases ionizes and creates an environment of very low resistance to the circuit. The ionized gas allows the circuit to arc within the tube and between the electrodes, completing the circuit and shunting the over-voltage condition to ground.

Why Would Each Of The Over-Voltage Devices Be Used?Polymer suppression are typically used in high-speed applications such as high speed data, signal or control lines for ESD protection. Its fast turn-on time and very low capacitance makes it suitable for these applications.

Common applications include:◆ airbags, ABS, telematics automotive systems such as GPS and security◆ high-speed customer premise equipment (including DSL, gateway, set top box, T1/E1/ISDN and more)◆ high-speed for computers (USB 2.0, IEEE 1394, RF antenna circuits, etc.) ☛


MLVs are designed to suppress a variety of transient events. These transient events are typically limited to ESD, EMI, inductive switching, etc up to, but not including, lightning surges. MLV’s are typically applied to protect integrated circuits and other components at the circuit board level. Their wide operating voltage and energy range make them suitable for numerous applications on power supply, control and low to medium speed signal lines. Their larger capacitance ratings give them fi ltering abilities for applications that have EMI problems as well. In many cases these can be used to replace larger SMD TVS diodes. In many cases a varistor’s and TVS diode’s abilities to protect overlap wherein you then choose the part via what is most important to you: price, size, etc.

Common applications include:◆ computers and peripherals ◆ handheld portables ◆ alarm systems◆ customer premise equipment ◆ multimedia auto systems ◆ auto engine control modules

Thyristors are designed to suppress lightning and other transients induced in telecommunication systems. Thyristors help provide secondary protection for telecommunication equipment such as telephones, modems, line cards and other devices subject to damage from over-voltage transients.

Common applications include:◆ secondary protection for telecommunication equipment such as telephones, modems, line cards◆ alarm systems ◆ CATV lines ◆ remote sensors◆ power lines ◆ cellular base stations

MOVs are intended for a comprehensive range of applications that typically are higher voltage and slower in nature such as lightning opposed to a faster event such as ESD.

Common applications include: ◆ AC line protection (in power supplies and consumer electronics) ◆ motor drives◆ high power input customer premise equipment ◆ repeaters◆ high power input automotive systems and electronics ◆ TVSS

devices

TVS Diodes overlap with the over-voltage applications of MOVs. Differences are primarily in package styles and pricing.

GDTs are specifi cally designed to protect primary telecom systems from lightning surges.

Common applications include:◆ interface equipment (PBX systems, Internet gateways, DSLAM)◆ conversion equipment (cellular, microwave and satellite base stations)

CIRCUIT PROTECTION COMPONENTS AVAILABLE FROM TTI

Manufacturer Resettable Fuse Standard Fuse Varistors Thermistors Thyristors GDT TVS Diodes

AVX ◆ ◆ ◆

Bourns ◆ ◆ ◆ ◆ ◆

KOA ◆ ◆ ◆

Littelfuse ◆ ◆ ◆ ◆ ◆ ◆

Murata ◆ ◆

Panasonic ◆ ◆

Raychem ◆ ◆ ◆

Vishay ◆ ◆


Circuit Protection Solutions From AVX CorporationThe evolution of circuit protection is driven by four major factors and AVX circuit protection devices address these needs today:

Electro-Magnetic Compatibility: As circuits become more complex, the need to minimize noise created by the circuits increases. These circuits must also be protected from environmental noise.

Signal Integrity: Circuit designers demand that the signals of the future carry much more information than the simple on-off signals currently in use, making the signals more susceptible to disruption.

IC Trends: Features of integrated circuits will continue to become smaller in the future. Although these features will make the IC more powerful, their size will allow the IC to be more easily disrupted

Miniaturization and Integration: As the demand for smaller and more functional electronic devices increases, demand for circuit protection devices will also grow. Miniaturization and integration of these devices will play a key role.

Multilayer VaristorsAVX offers Multilayer Varistors (MLVs) in standard EIA surface mount case sizes from 0402 to 2220, as well as a conformally coated axial leaded confi guration for use in through-hole applications. The monolithic multilayer construction of the TransGuard ® line provides protection from voltage transients caused by ESD, lightning, NEMP, inductive switching, and more.

For applications requiring the protection of multiple lines, AVX offers the MultiGuard series. These Transient Voltage Suppressor (TVS) Arrays address three important trends in today’s electronic circuits: mandatory ESD protection, PCB downsizing, and reduced component placement costs. MultiGuards are available in package size ranging from a two-element 0405 case size to a four-element 0612 case size.

AVX has developed several series of MLVs designed for specifi c circuit protection applications. These include StaticGuards for applications where ESD is the only concern; AntennaGuards; and the USB series for applications involving very high data transfer rates; and the UltraGuard, for applications requiring extremely low leakage current.

MOVs And SLVsFor applications that require very high energy handling capability, AVX offers the Metal Oxide Varistor (MOV) series. These devices are leaded disc varistors, operating at working voltage up to 500VDC and with an energy rating as high as 230J. These properties make MOVs suitable for applications such as telecom, automotive and consumer electronics, and industrial use.

Many manufacturers are moving away from leaded devices. AVX offers these customers the Single Layer Varistor (SLV) series. These devices have similar ratings as the MOVs, so are used in the same applications. The SLV series comes in 3220 and 4032 case sizes, and are suitable for surface-mount assembly techniques.

TransFeedsAVX has combined the geometry of a feed-through capacitor with the characteristics of its MLVs to produce a unique device with the fastest response time of any MLV product on the market. The TransFeed product provides EMI attenuation over a broad range of frequencies, and protects the circuit from over-voltage conditions. The TransFeed is available in an 0805 case size, and—like all AVX MLV products—has electroplated terminations to assure ease of assembly. ❖

Single Layer Varistor (SLV) series. These devices have similar ratings as the MOVs, so


Circuit Protection Solutions From BournsBourns, Inc. offers a wide range of technologies and products to meetcustomers’ requirements for over-voltage and over-current protectors. Bourns has a global reputation for quality products, and its circuit protection devices have demonstrated consistent reliability in fi eld applications.

Gas Discharge Tubes (GDTs)Gas Discharge Tubes (GDTs) are used to protect personnel and sensitive equipment from hazardous transient voltages. Bourns GDTs can withstand multiple applications of high surge current energy in excess of 25 KA and are used in primary and secondary applications.

Bourns offers 8mm and 5mm Mini Series GDTs in two- and three-electrode versions. Both feature long service life and low capacitance and insertion loss. An optional, proprietary switch-grade fail safe is offered on the three-electrode version to provide superior protection from thermal runaway. Bourns GDTs meet or exceed industry specifi cations that govern the use of gas discharge tubes and are UL registered.

Thyristor Surge ProtectorsThyristor Surge Protectors provide protection for wired telecommunications equipment against over-voltages such as lightning, AC power contact and induction. TISP products are designed to protect sensitive line card components such as subscriber line interface circuits (SLICs), line card access switches (LCAS) and modem chipsets from disturbances on the telephone network. A common protection scheme protects the 400 V rated DC blocking capacitor on ADSL modems using a 350 V TISP4350T3BJ and a Bourns Telefuse B1250T. Device confi gurations include two-wire to fi ve-wire protection solutions, with programmable and fi xed voltage options available. Using TISP products, equipment can be designed to meet international standards including Telcordia GR-974, GR-1089, UL 60950, ITU-T K.20 and TIA-968-A. Bourns TISP products are UL registered.

Resettable FusesMultifuse polymeric positive temperature coeffi cient (PPTC) resettable fuses can be used in a wide range of electronic products, including battery, automotive, industrial and telecommunications applications, anywhere there is a potential for damaging over-currents. Multifuse devices exhibit a low resistance under normal working conditions and switch to high resistance state when a circuit fault occurs, resetting after the fault is cleared and power is removed. Multifuse PPTC resettable fuses are packaged in radial leaded PTH, surface mount, axial battery strap and uncoated disk confi gurations with a wide range of voltage and current ratings.

Surge Line Protection ModulesSurge line protection modules (LPMs) provide secondary protection required to protect telecommunication line interface devices. Surge LPMs are manufactured using precision thick-fi lm technology. Products are UL registered

and withstand lightning and AC power cross conditions, meeting Telcordia GR-1089 and ITU-T K.20 standards. In addition to various standard versions, Bourns offers extensive custom

options, including packaging options, additional resistors, fuses, PPTCs, and diodes.

ChipGuard®ChipGuard® is a multilayer varistor (MLV) family of electrostatic sensitive device (ESD) protection devices. ChipGuard products’ metal oxide technology provides high impulse current capability of 20A at 8/20 µs in the ultra-small 0402 package. The ChipGuard®

family is available in a variety of voltage options from 5.5 V to 18 VDC, and 0402 and 0603 type packaging to meet industry standards. ❖

K.20 standards. In addition to various standard versions, Bourns offers extensive custom


Circuit Protection Solutions From KOA Speer Electronics, Inc.KOA Speer’s circuit protection components include thermal sensors, chip fuses and varistors. The company’s thermistors are available in a range of sizes, including 0402, 0603, 0805 and 1206. Fuse components are offered in a number of styles and sizes including fusing fl at chip resistor, chip fuses and thin fi lm chip fuses. Metal oxide varistors are available in surface-mount and leaded styles, offering higher surge current ratings in a number of sizes.

Thermal SensorsKOA’s linear and non-linear thermistors feature anti-leaching nickel barrier terminations (90/10 solder plated, standard) and both negative and positive tempco styles. The LT73 is a thin fi lm resistor with thermo-perceptivity and the LA73 is a thick fi lm sensor, featuring 25 specifi c temperature characteristics. NT73 is available in 12 standard resistance values, and the PT72 is lightweight with a reduced size and mounting area. Resistance range is 1MΩ ~ 22MΩ, depending on the component, with tolerances of +1 percent, +2 percent, +5 percent, and an operating temperature range of -40°C to +125°C. KOA Speer’s thermal sensors are used for circuit protection, temperature compensation, and temperature control in the automotive, computer, appliance and industrial markets.

FusesKOA’s fuses include the RF73 series, featuring anti-leaching nickel barrier terminations. Under normal conditions, these perform like RK73 with a resistance range of 0.2Ω ~ 510Ω and a maximum open circuit voltage of 24V~100V. Features of the CCP fuse include immediate cut-off of excessive heat and no heat generation, with a current rating of .400mA~10A and internal R. max of 23mΩ~1000mΩ. The TF fuse is a small, lightweight, low-compensation component with current ratings of 0.25A~3.15A, and a rated voltage of 32V. CCF fuses have a ceramic case to provide superior mechanical strength, with a current rating of 400mA~10A, and are suitable for primary and secondary circuits. Applications include power supplies for LCD inverters, personal and notebook computers, CD-ROMs and camcorders.

VaristorsA new line of surface mount and disc type metal oxide varistors (MOVs) are now

available from KOA Speer. The NV73 surface mount MOV protects electronics components against static electricity, switching and incoming surges. NV73

series voltage ranges from 4.2V~56V with a maximum peak current of 30A, 10A~50A and 30A~70A. The NVD series varistors are non-polarized and offer superior transient voltage suppression characteristics and the ability to withstand higher surge currents. Maximum allowable voltage ratings are from 14-1000VAC and 18-1465VDC. Capacitance ratings are from 70-10,000pF, and all ratings depend upon the type of varistors selected.

KOA’s varistors are designed for use in a number of electronic applications including offi ce, audio and visual equipment, computer components, primary

and secondary batteries and telecommunications equipment. ❖


Littelfuse, Innovative Circuit Protection SolutionsLittelfuse innovative circuit protection solutions are based on a thorough understanding of our customer’s current and future circuit protection needs and an uncompromising approach to quality. Our broad line of circuit protection solutions include fuses, resettable PTCs, ESD suppressors, thyristors, TVS diodes, GDTs and metal oxide varistors. Whether you are designing a TVSS module for AC Line protection in a factory, a motherboard for a PC or a Body Control Module for an automobile, Littelfuse has the circuit protection solution to meet your most demanding application requirements. Our technical support staff will work with your design team to develop a complete application specifi c solution using our broad range of technologies to ensure that you get the right solution to fi ll your circuit protection need. Look to Littelfuse for all of your circuit protection needs.

Metal Oxide VaristorsWith peak current ratings ranging from 40A to 70,000A and peak energy ratings ranging from .1J to 10,000J, Littelfuse has the varistor regardless of the application. Available in radial leaded, axial leaded, surface mount, packaged and bare-disk options, Littelfuse Metal Oxide Varistors (MOVs) are specifi cally designed to suppress transient voltages such as lightning and other high level transients found in industrial, AC line application or lower level transients found in automotive DC line applications. The TMOV and iTMOV varistors include an integrated thermally activated element designed to open in the event of overheating due to sustained abnormal overvoltage, limited current, conditions outlined in UL1449.

Multilayer VaristorsLittelfuse multilayer varistors are designed to protect a broad range of low voltage (0-120VDC) applications in the computer, handheld device, industrial and automotive markets. The MLE and MHS series are designed to suppress ESD. The MHS is a low capacitance variation specifi cally designed to suppress ESD in high-speed (RS232 and USB 1.1) data lines. The ML series has a high energy capability and low clamping voltage, making it ideal for suppression of overvoltage transients such as inductive switching, EFT and repetitive ESD. With the exception of the MLN series, all of the multilayer varistor families are available in packages as small as 0402 size with a wide range of end-termination options including Nickel Barrier for improved solderability.

Polymer ESD SuppressorsPulseGuard® suppressors use polymer composite materials to suppress fast-rising ESD transients as specifi ed in IEC 6100-4-2. Their ultra-low capacitance makes them ideal for protection of high-speed data and signal lines that link an IC or ASIC to outside electronic equipment. For example USB 2.0, IEEE 1394, DVI and Infi niBandSM systems cannot tolerate capacitive loading due to signal integrity issues. The devices are available in single-line (0402 & 0603) and multiline SOT23 (2-line) and 0805 (4-line) packages for optimal design fl exibility. All PulseGuard PGB series devices are bi-directional with a response time less than 1 ns, an inherent capacitance of .05 pF and a fl at

frequency response from 1Hz to 1GHz. The devices have a clamping voltage of 150V @ 8kV and an operating voltage ranging from 0 to 24VDC.

Silicon Protection—TVS ArraysLittelfuse offers a full range of silicon based circuit protection devices designed for use in a wide variety of computer, handheld, telecom, automotive and industrial applications. The SP5xxx and SP72x series of silicon protection arrays are specifi cally designed to protect Analog and Digital signal lines from ESD and other overvoltage transients. Designers should choose the SP series when their design requires the

lowest possible clamp voltage and multiple lines of protection in the smallest possible package. The SPUSB1 series integrated protection device consists of Transient ☛

an operating voltage ranging from 0 to 24V


Voltage Suppression (TVS) avalanche diodes, impedance matching/termination resistors and fi lter capacitors that provide ESD protection, EMI Filtering and termination as specifi ed in USB1.1. The device is ideal for highly space sensitive applications such as PDAs and digital still cameras.

Silicon Protection—TVS DiodesIn addition to our array products, Littelfuse also offers a full range of discrete TVS diodes designed to suppress overvoltage transients such as EFT, inductive load switching and lightning in a wide variety of applications in the Computer, Industrial, Telecom and Automotive markets. These devices offer Peak Pulse Power (10/1000µs) ratings from 500W to 15,000W and Reverse Standoff Voltages from 5V to 376V. These devices are available in standard surface mount (DO-214AA and Pill) and axial leaded (DO-15 & DO-13) packages. With peak pulse current (8/20µs) ratings of 6,000A and 10,000A, the AK6 and AK10 series are designed specifi cally for use in high-power AC Line protection applications.

Silicon Protection—SiBOD ThyristorsLittelfuse now offers a complete line of thyristor products specifi cally designed to suppress overvoltage transients, such as those resulting from lightning, in a wide range of telecom and datacom applications. Single element devices are available in DO-214AA (SMB) surface mount packages and axial leaded through-hole packages while two and three chip devices for ground referenced protection are available in low height modifi ed TO-220 packages. In addition the T10C series primary protection range is available in a three pin ‘gas tube’ package that is compatible with Krone strip insertion.

Gas Plasma DischargeAvailable in small foot print leaded and surface mount confi gurations, Littelfuse Gas Plasma OVP devices (GDTs) have a fast response time to transient overvoltage events. This fast response time translates into a much lower breakdown voltage and a reduced risk of equipment damage. These new devices have the ability to handle very high current surges - up to 10,000A - while effectively suppressing overvoltage transients. Their low capacitance (typically 1-2pF), high insulation resistance (greater than 1GΩ) and low leakage ensure that it has virtually no effect on the protected system during normal (non-surge) operating conditions. These electrical characteristics make them ideal use in broadband cable, MDF (Main Distribution Frame), Central Offi ce and Access applications.

FusesLittelfuse is the world leader in the design and manufacturing of fuses for the Automotive, Industrial, Handheld, Computer and Telecom markets. Whether your looking for surface mount or axial; glass or ceramic; thin-fi lm or Nano2® style; fasting-acting or Slo-Blo® fuses; Littelfuse has the part to meet your need. Operating characteristics include current ranges from .010A - 35A, maximum voltage ranges from 24V - 600V, and interrupting ratings from 35A - 200,000A. Look to Littelfuse as your lead-free fuse source. Our new line of lead-free thin-fi lm fuses, are perfect for your environmentally friendly design. Contact Littelfuse for future availability of other lead-free fuse products.

Resettable PTCsLittelfuse offers a full range of surface mount and radial leaded PTC resettable devices. Designed for “plug and play” applications, the 1206L and 1812L series surface mount PTCs provide protection for electronic circuits from overcurrent conditions in a wide range of “plug and play” applications. Littelfuse surface mount PTCs comply with electronic industry standards for lead reduction. The 30R and 60R series radial PTCs are rated at 30VDC and 60VDC

respectively, and are designed for use in higher voltage applications that require minimal maintenance and are subject to repetitive overcurrent conditions. Applications include industrial controls and home security systems. All Littelfuse PTC device series are recognized under the Components Program of Underwriters Laboratory, the Component Acceptance Program of CSA, and TUV. ❖

Available in small foot print leaded and surface mount confi gurations, Littelfuse Gas Plasma


Circuit Protection Solutions From Murata ElectronicsDemand for thin, compact and lightweight designs extends well beyond portable equipment to encompass nearly all categories of electrical and electronic devices. Simultaneously, users demand both enhanced power and speed from today’s equipment. In an effort to meet these demands, developers have made substantial progress in the technologies for miniaturizing parts, using laminated substrates, and dense mounting.

Circuit boards carrying dense arrangements of mounted parts cannot radiate heat as easily today as in the past. It is important to apply rigorous controls when designing the thermal characteristics of these circuit boards. Positive-temperature coeffi cient (PTC) and negative-temperature coeffi cient (NTC) thermistors can serve as temperature sensors in these applications.

PTC ThermistorPosistor is Murata Manufacturing Co., Ltd.’s trademark for a positive temperature coeffi cient (PTC) thermistor using ceramics of sintered barium titanate (BaTiO3) mixed with a trace of rare-earth metal. At specifi ed temperatures, the resistance of PTC thermistors rises sharply. Engineers can use this characteristic to create an overheating-protection circuit. Such circuits can, for instance, raise the rotating speed of a cooling fan or limit the equipment output when the temperature of the circuit board reaches a predetermined level.

Murata Manufacturing’s Posistors can also detect overheating in the power transistors of step-down regulators. If the power transistor radiates unexpected heat arising from any cause, including an increase in output current or heat radiation failure, the resistance value of the thermally-coupled Posistor rises sharply before the transistor reaches itsallowable temperature. The rise of resistance value activates the fi rst transistor (Tr1) to stop the operation of the power transistor, and thus avoids serious failure, fuming and fi ring.

PTC Benefi ts And VarietiesPosistors offer several advantages. Unlike thermal fuses, Positors have no

parts that expand, and do not have to be replaced. They also do not generate noise, or suffer contact failures. Unlike bimetal thermostats,

Posistors do not have any contacts. They also exhibit large gain and can control power devices directly, using transistors including fi eld effect transistors (FETs).

Murata Manufacturing offers three kinds of Posistors to commercial customers. The PTFL04 series consists of models with lead wires; and the PTFM04 series utilizes screwed electrodes. The PRF18 series consists of chip Posistors with dimensions of

1.6mm x 0.8mm.

The temperature-sensing accuracy of the PTFL04 series reaches ±5°C for every 10 degrees between 55° and 105°C. The accuracy of the chip

Posistor is ±5°C for every 10 degrees between 85° and 145°C. ☛

parts that expand, and do not have to be replaced. They also do not


Murata Manufacturing is working to improve the temperature-sensing accuracy to ±3°C. The company also is looking for ways to eliminate lead and other environmentally unsound materials from the product. Product variety will be expanded in the near future.

NTC ThermistorsTo make thermistors with a negative-temperature coeffi cient (NTC), engineers create a ceramic, heat sensitive resistor consisting of a sintered body of transition metal oxide. The temperature characteristic of NTC thermistors is the opposite of that for PTC thermistors. The resistance value of NTC thermistors decreases as the temperature rises giving its temperature coeffi cient a negative value. NTC thermistors are often used as temperature sensors in many kinds of devices because they offer a sharp change in the resistance value, relative to the temperature change, usually about 3.0 to 5.3 percent per degree of centigrade temperature.

NTC thermistors have a relatively wide range of selectable resistance values, allowing engineers to modify the value, as required. These thermistors have excellent environmental resistance, even though they are quite small, and remain physically stable for long periods. NTC thermistors are easily mass-produced, making them a cost-effective solution.

Engineers apply NTC thermistors as temperature sensors in different ways, depending on the target temperature range and the necessary detection accuracy. For these purposes, thermistors feature various connection options. One option enables the use of the thermistor’s resistance-temperature characteristics without alteration.

Another approach adds a resistor in parallel, to ensure linearity of the thermistor’s resistance-temperature characteristics. Other methods give the resistant temperature characteristic linearity across a wide temperature range, even though the temperature coeffi cient is small.

ConclusionToday’s designers of electronic equipment face multiplying technical challenges. Among them is the requirement to enhance product safety and product liability. Engineers must determine the most effective ways to reduce product dimensions and lower the cost. Murata Manufacturing focuses on refi ning product design and materials to meet these demands, especially in Positors serving as temperature sensors. By accomplishing this goal, the company also plans to expand the potential uses of thermistors. ❖

Engineers apply NTC thermistors as temperature sensors in different ways, depending


Circuit Protection Solutions From PanasonicPanasonic provides a full range of solutions that protect against the many hazards facing electronic circuitry. These include Metal Oxide Varistors, Thermistors, fast-acting Microchip Fuses, and Chip Fuse Resistors.

“ZNR” Transient/Surge AbsorbersPanasonic ZNRs are zinc-oxide nonlinear resistors whose resistance changes as a function of the applied voltage. The basic characteristics of ZNRs are expressed by varistor voltage and clamping voltage (or voltage ratio). The ZNR utilizes a ceramic element composed of zinc oxide and several kinds of metal oxide additives that have been sintered at relatively high temperatures. The ZNR has a bilateral and symmetrical V-I characteristic similar to back-to-back zener diodes and unparalleled large peak current capability. It can be used for absorption of transient voltage, suppression of pulse noise, and as an arrester element to provide circuit voltage stabilization.

The surface-mount ERZ-CF/SF/VF series has a wide range of energy absorption and is packaged in a molded case. It has varistor voltages from 22 to 470 VDC and is capable of 0.4 to 14 Joules and 50 to 600A.

The surface-mount EZJ-S Series has large capacitance values (4,700, 8,200, or 20,000pF) making it ideal for protecting DC voltage lines. It has varistor voltages of 12, 30 or 50 VDC and is available in 0805 case size. It is capable of handling Electro Static Discharges of 30KV.

The surface-mount ERJ-Z has small capacitance values (3 to 330 pF) which makes it ideal for use in protecting high-speed signal lines. It has varistor voltages of 12, 27, 33 or 170 VDC and is available in 0402, 0603, and 0805 case sizes. It is capable of handling peak currents as large as 20A.

The radial-leaded ERZ-VxxD Series has voltages from 18 to 1800VDC. It is available with diameters from 5 to 20mm and has a wide range of energy absorption, 0.6 to 1,020J. It is capable of handling large peak currents of 250 to 7,500A.

Thermistors (NTC)—Compensation For Temperature ChangesNTC Termistors are Negative Temperature Coeffi cient Resistors whose resistance changes linearly as ambient temperature changes. Thermistors are comprised of 2 or 4 kinds of metal oxides of iron, nickel, cobalt, manganese, and copper, and are shaped and sintered at high temperatures (1200 to 1500ºC). Panasonic manufactures both positive and negative temperature characteristic Thermistors, although the more widely used variant is the NTC Thermistor. The NTC Thermistors are available in both surface-mount and leaded packages.

The surface-mount ERT-J Series NTC Thermistor is available in 0603, 0402, 0201 case sizes and has maximum power dissipations of 100, 66, 33 mW. It is available in resistance values (at 25°C) of 22Ω - 150kΩ in the 0603 case size, 2k - 470kΩ in the 0402 case size, and 2k and 3kΩ in the 0201 case size.

Microchip FuseThe surface-mount ERB-F Series Microchip Fuse is available in 0603 case size and, in over-current situations, has very fast, sharp fusing characteristics, with current ratings from 0.5A to 5.0A.

Chip Fuse ResistorThe surface-mount ERQF06 Series Chip Fuse is available in 0805 case size with resistance values of, 0.47Ω to 1.0Ω. Under normal conditions it acts like a chip resistor, but in over-current situations, it opens within 60 seconds at 30 times the rated power for 0.47 to 1.0Ω and at 24 times the rated power for 1.2 to 10Ω. ❖


Raychem Circuit Protection—The Leader In Resettable Circuit ProtectionResettable circuit protection is now an important aspect of product reliability and safety, helping to prevent costly damage to printed circuit boards, and guarding against damage caused by unsafe battery charging conditions. So much more than a simple fuse alternative, resettable circuit protection protects motors from failure and increases the value of products and systems. Raychem Circuit Protection drives the development and application of this technology with a broad range of product solutions which are continually refi ned to keep pace with market demand, and provide the enabling material science technology for new product innovation.

PolySwitch® resettable devices are the world’s leading polymeric positive temperature coeffi cient (PPTC) devices for circuit protection. PolySwitch devices are commonly called resettable devices to distinguish them from traditional fuses that work only once and then must be replaced. While the generic term “resettable fuses” is sometimes used for these devices, they are not technically fuses. They are highly non-linear thermistors that allow a circuit to return to normal operation after a fault has been cleared and power to the circuit has been removed. Resettable fuses reduce the need for product service and repair by protecting against damage from harmful over-current surges and over-temperature faults in electronic equipment, thus resulting in improved customer satisfaction and reduced warranty costs.

Resettable FusesThe LVR series of PPTC devices are rated at 240 VAC, permitting maximum voltages of up to 265VAC, and are available in hold currents from 50 to 400 mA. The thermally active devices can help protect against both over-current and over-temperature faults on the primary side of power supplies and transformers. Developed to help prevent damage to control boards and components, LVR devices limit current in the event of a load-side short-circuit or overdraw, or improper incoming voltage. Unlike a single-shot current fuse, the resettable LVR device can help protect against conditions where faults may cause a rise in temperature with only a slight increase in current draw. When installed on the primary side of the circuit in proximity to potential heat-generating components such as magnetics, FETs, or power resistors, the LVR device can help provide both over-current and over-temperature protection with a single installed component.

Raychem Circuit Protection’s PolySwitch® TSM600-250 over-current protection devices are designed for DSL and other telecommunications network equipment applications. The small-footprint, low-resistance, surface-mount device provides tandem resettable over-current circuit protection for use in high-density multi-port line cards and other telecommunications network equipment. The TSM600-250 device incorporates two matched, low-resistance PPTC components into a single device, yielding balanced protection on tip-and-ring lines with minimal attenuation of data transmission. The TSM600-250 device occupies half of the board space of two previous generation 600V PPTC devices and eliminates the risk of unmatched tip-and-ring line protection. In addition to interrupting AC power faults, the device is robust to Telcordia GR-1089 lightning and has a rated hold current of 250mA.

With its 1206 (3216mm) form factor, the nanoSMDM is one of the smallest lead free surface-mount PPTC resettable devices on the market. Comprised of four devices

in two basic package confi gurations, the nanoSMD series offers a 40 percent smaller footprint than the company’s microSMDM series fuses, already one of the industry’s smallest.

Designed with the latest generation of portable electronics in mind, nanoSMDM resettable devices allow designers of cell phones and other devices to protect

circuits up to 6V from over-current surges of up to 40A. NanoSMDM devices are suitable for use in notebook PCs, monitors, motherboards and USB hubs and

peripherals. ❖


Circuit Protection Solutions From Vishay ElectronicsAs demand for thin, compact and lightweight designs extends well beyond portable equipment encompass nearly all categories of electrical and electronic devices, users simultaneously demand enhanced power and speed from products. In an effort to meet these requirements, developers are making consistent progress in the technologies for shrinking parts, using laminated substrates, and dense mounting.

Circuit boards carrying dense arrangements of mounted parts cannot disseminate heat as easily as less complex boards, making it critical to maintain rigorous control when designing thermal characteristics of these boards. Positive-temperature coeffi cient (PTC) and negative-temperature coeffi cient (NTC) thermistors can be used as temperature sensors for this purpose. Voltage-dependent resistors or varistors can also be used with thermistors to provide protection against high voltage transients.

PTC ThermistorCeramic positive temperature coeffi cient (PTC) thermistors provide reliable and repeatable circuit protection. Unlike thermal fuses which provide one time protection and need to be replaced, ceramic PTCs reset with very low drift. NTCs are widely used for sensing temperature but PTCs can also be used for thermal protection. The three main application areas for PTCs are temperature sensing and protection, overload protection, and inrush current generation. The temperature coeffi cient of resistance of a PTC thermistor is positive only between certain temperatures. Thus, these devices exhibit a sharp rise in resistance within this specifi ed temperature range. Designers can use this characteristic to create a protection circuit in applications where the temperature (hence the resistance) is primarily determined by the current fl owing through the thermistor.

Vishay offers a wide variety of popular confi gurations of PTC thermistors, including surface mount, leaded, leadless discs, molded types, and chips. Among the surface mount types are a lead frame PTC with nominal R25 values from one ohm to 500 ohms and a response time of less than one second. Leaded parts have R25 values from 0.3 ohms through 3000 ohms. For temperature protection and sensing applications there are 15 well-defi ned protection temperature curves with nominal reference temperatures from 70 degrees C to 170 degrees C. The designer can select from numerous mechanical and electrical confi gurations to select the best PTC for the application.

NTC ThermistorsNTC thermistors are composed of sintered metal oxides such as manganese,

nickel, cobalt, iron, copper and aluminum. The exact shape of the fi nished disc or chip is dictated by the specifi c application, with the most frequent

confi gurations being leaded or surface mount discs and chips which are available in both leaded and surface mount versions. Unlike PTC thermistors, the resistance value of NTC thermistors decreases as the temperature rises, resulting in a negative temperature coeffi cient. NTC thermistors are commonly used as temperature sensors in many kinds of devices because they feature a sharp change in the resistance value relative to the temperature change. Usually, this

change is about -3.10 to –6.22 percent per degree of temperature. ☛

select from numerous mechanical and electrical confi gurations to select the best PTC for the application.


NTC thermistors are available in a wide range of resistance values, including surface mount (EIA sizes 0402-1206) in values of 200 ohms to 400k ohms, with seven standard curves. Leaded parts range from 27 ohms to 40meg ohms with ten standard curves, with many different leads and diameters. With a variety of choices, engineers can modify the value as required. These thermistors have high sensitivity, making them cost-effective temperature measurement devices that remain physically stable for long periods.

NTC AssembliesThermistor assembly probes are also available in a wide variety of confi gurations. Choices of assembly mounting, housing, and other options are available depending on the application. Primary factors that can determine thermistor assembly confi guration are operation environment, method of mounting and required thermal time response. For specialized applications Vishay offers a custom design service to allow the customer to advance from conception to design, manufacturing and installation.

VaristorsVoltage Dependent Resistors, or varistors, provide reliable and economical protection against high voltage transients and surges produced by lightning strikes and switching or electrical noise on AC or DC power lines. Surface mount varistors protect ICs and transistors from surge voltage and other transients at the board level. Vishay’s zinc-oxide

varistor technology can absorb much higher transient energies than transient suppressor diodes and can suppress positive and negative transients. When a transient occurs, the varistor resistance

changes from a very high stand-by value to a very low conducting value. The transient is thus absorbed and clamped to a safe level, protecting sensitive circuit components. Leaded confi gurations are available in a wide range of voltages from 14 V to 680 Vrms with peak surge ratings up to 10 kA, single 8 x 20 pulse. Radial lead package sizes from 7 to 20mm are

offered in a variety of lead forms and packaging. These components are recognized under the components program of UL, VDE, and CSA. 0805, 0603, and 0402 size surface mount devices are

available from 4.0 volts to 25 volts with inherent bi-directional clamping voltage up to 65 volts, and with peak surge ratings of 30A and 80A, single 8 x 20 pulse.

ConclusionVishay considers that there are eight steps to selecting the proper thermistor for an application: 1. Dissipation Constant (D.C.)—the amount of power required to self-heat the thermistor. 2. Thermal Time Constant (T.C.)—the time in seconds required for a thermistor to change through 63.2% of its difference between its initial and fi nal body temperature. 3. Selection of resistance value—typically NTC thermistors are specifi ed and/or referenced to +25°C. 4. Resistance versus temperature curve selection—Vishay currently offers 11 different curves. 5. Tolerance—there are tolerances at 25c which are called point match, or there is a tolerance called curve tracking. 6. Tolerance availability versus R-T curve. 7. Tolerance availability versus confi guration. 8. Measurement accuracy. ❖


Glossary

AC Discharge Current: The maximum amount of continuous current the TVS device—usually a GDT/arrester—can handle without damage. This test is normally designed to replicate a condition where AC main lines come into contact with telephone line and is a much longer and continuous event as opposed to a lightning strike.

AC Voltage Rating (symbol VMAC or RMS): (alternating current applications only). The maximum continuous AC (rms) voltage at which the CP device, usually a TVS device, is designed to operate without switching on. It should be 10 to 20 percent above the customer’s normal operating voltage.

Agency Approvals: There are many different agencies that set approval standards for components. Some standard agencies include: UL, CSA, ANCE, VDE (IEC), METI, MITI, Semko (IEC), BSI (IEC) or military.

Breakdown Voltage (symbol VBR): The voltage at which a TVS device begins the clamping process. It is lower than the clamping voltage once the TVS device has reached its full capabilities and has “turned on.” Both a minimum and a maximum breakdown voltage are measured.

Capacitance (symbol C): The capacitance of the device normally measured in picofarads.

Chipguard: A multi-layer varistor product manufactured by Bourns.

DC Sparkover Voltage: The voltage at which a TVS device, usually a GDT/arrester, breaks down when subjected to a slow rising voltage, normally at a rate of 100V/second. The DC sparkover value may be specified as an upper and lower limit, or a nominal voltage with a tolerance, normally ± 20 percent, unless otherwise stated. This is typically from a continuous over-voltage condition, which typically has a slower rise time than a transient surge.

DC Voltage Rating (symbol VMDC): In direct current applications, the maximum continuous DC voltage at which the CP device, usually a TVS device, is designed to operate without switching on. It should be 10 to 20 percent above the normal operating voltage.

Hold Current (symbol IH): The largest steady state current that, under specified ambient conditions, can be passed through a resettable fuse device without causing the device to trip. For thyristor devices, the current at which the device resets to a high-impedance state once the surge current dissipates.

Holdover Voltage: The voltage value at which a CP device (usually a GDT/arrester) remains in low impedance arc mode following a transient surge. It is important that the holdover voltage of a GDT/arrester is greater than that of the system voltage.

Insulation Resistance: The amount of resistance to the circuit when a CP device, usually a GDT, is in an off state. This is normally in the gigaohm range.

Interrupt Rating: Also known as breaking capacity, short circuit rating, maximum current rating, or maximum overload current. This is the amount of maximum approved current that a fuse can safely interrupt at the rated voltage. During a short circuit or fault condition, a circuit protection device may encounter an instantaneous overload current many times greater than its normal operating current. According to guidelines set by electronics safety governing agencies, a CP device must be able to withstand this excessive current without an explosion or rupture of the device’s body. It is many times the normal operating current in order to protect the circuit from unexpected excessive surges of current. ☛


Impulse Discharge Current: The maximum value of current that the TVS device, usually a GDT/arrester, can withstand while remaining within the specified limits without damage to the GDT. This current may be specified as 5KA or 0KA, depending on the type of device. The current used to determine the impulse discharge current rating has a waveform of 8/20µs, which is the associated rating for a lightning strike.

Impulse Sparkover Voltage: The voltage at which the TVS device, usually a GDT/arrester, breaks down when subjected to a much faster rate than the DC sparkover. It will be typically two or more times the DC sparkover voltage. The rate of rise for the impulse sparkover is 1KV/µs.

Leakage Current: The measure of current that is absorbed by the device when it is switched off. Typical leakage current passing through this type of device is less than 1nA.

Maximum Breakdown Current (symbol IBO): The maximum amount of current that the device will handle before switching to its latched state.

Maximum Breakdown Voltage Temperature Coefficient (%VBR/ °C): Also known as Max Voltage/Temperature Variation of VBR. Like most other devices, TVS devices will typically derate as the temperature increases over the standard 25° Celsius. This is the amount – measured in millivolts—that the breakover voltage will change with this increase in temperature.

Maximum Breakover Voltage (symbol VBO): The maximum amount of voltage that the device will allow before switching to its on latched mode.

Maximum Clamping Voltage (symbol VC): The peak voltage that will appear across the TVS device when subjected to the peak pulse current. This should always be lower than the maximum voltage rating of a protected device.

Multifuse: A resettable fuse product manufactured by Bourns.

Multiguard: A TVS array product manufactured by AVX.

Negative Temperature Coefficient (acronym NTC): A NTC device has a decrease in resistance with an increase in temperature.

Nominal Melting Rating (symbol I2T): The measurement of energy required to melt s fuse’s element (wire) and open the circuit to protect your load.

Nominal Voltage Rating (symbol VNOM): The voltage at which a CP device, usually a TVS device, turns on from a highly non-conductive device to a highly conductive device and begins to absorb transient energy.

On State Voltage (symbol VT): An insignificant amount of voltage. The nominal amount of voltage the TVS device allows through when switched to its “on” state.

Peak Pulse Current (symbol IPP): The maximum amount of current a TVS device can withstand without damage to the device that would limit or eliminate the device’s capability to protect a circuit.

Polyswitch: A resettable fuse product manufactured by Raychem. ☛

20 C i r c u i t P r o t e c t i o n

Positive Temperature Coefficient (acronym PTC): The increase in resistance with an increase in temperature usually caused by an increase in current experienced by a PTC device.

Pulseguard: A polymer suppression device product manufactured by Littelfuse.

Resistance: Also known as nominal cold resistable, this is the amount of resistance a component contributes to a circuit when there is no over-current condition, measured at room temperature (20-25° Celsius). This is significant because the more resistance there is in a component’s “off” state, the greater the power loss due to reduced voltage.

Resistance minimum (symbol Rmin): The minimum resistance in a resettable fuse generated by a component before being tripped the first time.

Resistance maximum (symbol Rmax): The maximum resistance in a resettable fuse generated by a component before being tripped the first time.

Resistance1 maximum (symbol R1max): The maximum resistance in a resettable fuse generated by a component after it is tripped the first time.

Slo-blo fuse: This type of fuse incorporates a thermal delay design to enable it to survive normal start-up pulses characteristic with some applications. It allows the initial pulse to pass through, depending on the duration it lasts. Most applications are designed to withstand these pulses, a slo-blo fuse provides protection against prolonged overloads.

Surge Current Rating (symbol ITM): Also known as peak current rating, it is the maximum amount of transient current a CP device—usually a TVS device—can handle one time only. This is measured in amps and based upon the same waveform as a lightning strike.

Surge Energy Rating (symbol WTM): This is the maximum amount of transient energy, measured in Joules (J), that a TVS device can dissipate without causing device failure.

Switching Temperature (symbol TSW): The temperature at which a component switches from a low resistance to a very high resistance. This is caused by the increased current flow and will coincide with the trip current value.

TISP: A thyristor product manufactured by Bourns.

Transguard: A multi-layer varistor product manufactured by AVX.

Transfeed: A combination feed-through capacitor and MLV product manufactured by AVX.

Transient Voltage Suppression Device (acronym TVS): A device to protect a circuit from short duration excursions or surges of electrical energy.

Trip Current (symbol IT): The trip current is the minimum amount of current at which a component will switch from low resistance to high resistance at room temperature (20-25° Celsius), normally twice the hold current.

Voltage Rating: The maximum recommended voltage for a component.

Voltage Standoff (symbol VDRM): A value greater than the normal operating voltage of the circuit, so as to not clip the normal voltage needed to drive the circuit. ❖

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