electronic circuits use lots of resistors

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Electronic circuits use lots of resistors. This isn't always obvious when you look atmodern circuits which appear to consists of a group of ‘silicon chips’ or Integrated Circuits wired together. In fact, IC's (even digital ones) don't just contain transistors,they also contain a lot of hidden resistors. Although resistors come in various formswe can divide them up into just two basic types.

• ‘Fixed’ resistors• Variable resistors (or ‘potentiometers’)

A fixed resistor is a component with two wires which obeys Ohm's Law — i.e. it's a bit of material which behaves as we described in the last section. Electronic engineersand manufacturers have adopted some standards for resistors. These are intended tokeep the cost down and make it easier for you to buy them from whichever supplier you like without having to redesign the equipment you want to put them in.

The first standard concerns the resistor values you can buy. If you look through acatalogue of electronic components you'll see that the resistance values offered nearlyalways follow the same series:-

1·0 , 1·2 , 1·5 , 1·8 , 2·2 , 2·7 , 3·3 , 3·9 , 4·7 , 5·6 ,

6·8 , 8·2 , 10 , 12 , 15 , 18 , ... etc.

This is called the ‘E12’ series. The values are chosen to give twelve values in eachdecade (tenfold change in value). Most of the time, engineers who want a particular resistance value just pick the ‘nearest’ one from this list. You can buy other values.You can even have them specially made to whatever value you like. But the E12series are much cheaper because so many of them are made. If you take the logs of theabove series you'll find that they're roughly equally spaced. This ensure that thefractional error of choosing the closest value to the one you really wanted stays aboutthe same for any size of resistor.



curved resistor. When we twist the rod we can move this wiper along the resistor,shifting it away from one end and towards the other. As a result, the resistance

between this tag and the ones at the ends goes up and down as we rotate the rod. In thediagram shown below, the horseshoe shape of resistive material is coloured orange,and the wiper is coloured blue.

As with fixed resistors, variable ones tend to come in standard end-to-end values inthe same E12 series. In this case the most ‘popular’ ones have end-to-end resistances

which are simply 1, 10, 100, 1000 (=1k), 10k, etc, Ohms. (25k and 50k pots alsoturn up a lot in Hi-Fi's as volume/balance potentiometers even tho' they aren't E12values.)

Oh, if you are wondering where “pots and pans” came from in the title of this page.Pots is an abbreviation of potentiometer. ‘pan’ is a term used to mean setting the



stereo location of a sound by adjusting the relative volume levels it has from thespeakers in a stereo system! Thus pots can be used to pan the sound of something.

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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.

Resistor Types

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PRECISION WIREWOUND The Precision Wirewound is ahighly accurate resistor with a

very low TCR and can be accurate within .005%. A temperaturecoefficient of resistance (TCR) of as little a 3 part per million per degree Celsius (3ppm/oC) can be achieved. However thesecomponents are too expensive for general use and are normally used in highly accurate DCapplications. The frequency response of this type is not good. When used in an rf application allPrecision Wirewound Resistors will have a low Q resonant frequency. The power handlingcapability is very small. These are generally used in highly accurate DC measuring equipment, andreference resistors for voltage regulators and decoding networks.

The accuracy is maintained at 25oC(degrees Celsius) and will change with temperature. Themaximum value available is dependent upon physical size and is much lower than most other typesof resistor. Their power rating is approximately 1/10 of a similar physical size in a carbon

composition. They are rated for operation at +85oC or +125oC with maximum operatingtemperature not to exceed +145oC. This means that full rated power can be applied at +85 ( 125)oC with no degradation in performance. It may be operated above +125 (85) oC if the load isreduced. The derating is linear, rated load at +125(85) oC and no load at +145oC. Life is generallyrated for 10,000 hours at rated temperature and rated load. The allowable change in resistanceunder these conditions is 0.10%. Extended life can be achieved if operated at lower temperaturesand reduced power levels. End of life requirements are generally defined by the manufacturer or insome case by user specification. Some degradation in performance can be expected. In some cases,

particularly if the tolerance is very low and the TC is low, the rated power is reduced to improveresistor stability through life. Precision Resistors regardless of type, are designed for maximumaccuracy and not to carry power. The materials used in these resistors are highly stable heat treatedmaterials that do change under extended heat and mechanical stress. The manufacturing processesare designed to remove any stresses induced during manufacture. There is little detectable noise inthis type of resistor. The stability and reliability of these resistors is very good and their accuracycan be enhanced by matching the absolute value and the temperature coefficient over their operating range to achieve very accurate voltage division.

NIST STANDARDThe NIST (National Institute of Standards and Technology) Standard can be as accurate as .

001% with roughly the same TCR as Precision Wirewound Resistors and are very stable. Theseare used as a standard in verifying the accuracy of resistive measuring devices. They are normallythe Primary Standards of a company's test lab.

They are returned to the NIST for measurement and their accuracy is tracked through out thestandards life to determine the Standard's stability. Most companies will have two sets of standardsso that they can continue to measure while one set of standards are being measured by the NIST .They will alternate returning these NIST Standards to the NIST , one set one year and the other setthe next year. For extremely accurate measurements, the Standard with the longest history and the

best stability will be used. If erratic readings are received from the NIST over a period of years,the Standard is retired. Also, if the reading has significantly changed since the last NIST reading,the standard is suspect and all measurements made using that standard must be checked. Normally,a standard will take about 3 years to stabilize and becomes more stable with time unless it has hadexcessive power applied or has been dropped. These standards are generally stored in an oil bath at+25oC. During measurement, a thermometer is placed in a cavity in the top of the Standard, calledthe oil well, and the temperature is recorded for each measurement so that the exact value can bedetermined. That is the value at +25oC plus or minus the change in value caused by thetemperature coefficient. Each standard will have a temperature correction chart for exact values.Being stored in the oil bath prevents the Standard from being stressed by changes in room



temperature. These arehighly precision devicesand are expensive to buyand expensive tomaintain, but they are the primary resistor reference for any test lab.

These resistors are furnished in a totallyenclosed metal case and for values above 1ohm, this enclosure is filled with mineral oil(other type of oil may contain additives thatcan cause corrosion in later life). The values

below 1 ohm may be built in an enclosurethat is perforated and these must be

submersed in oil. If power is applied without it being submersed, the Standard will be ruined.All NIST Type Standards are equipped with provisions for two, three, or four terminal

measurements. The applied power is calculated and the temperature of the Standard is monitoredduring test. The lowest power level consistent with sufficient resolution to get the desiredmeasurement is used (in the area of 0.01 watts) and any appreciable rise in temperature will dictatethat the measurement should be suspended and the test set-up reviewed for ways to reduce the

power level. These Standards are rated for operation at room temperature only but their other

characteristics are the same as Precision Wirewound Resistors.

POWER WIREWOUND RESISTORSPower Wirewound Resistors are used when it is necessary to handle a lot of power. They will

handle more power per unit volume than any other resistor. Some of these resistors are free woundsimilar to heater elements. These require some form of cooling in order to handle any appreciableamount of power. Some are cooled by fans and others are immersed in various types of liquidranging from mineral oil to high density silicone liquids. Most are wound on some type of windingform. These winding forms vary. Some examples are ceramic tubes, ceramic rods, heavilyanodized aluminium, fibreglass mandrels, etc.

To achieve the maximum power rating in the smallest package size, the core on which thewindings are made must have a material with high heat conductivity. It may be Steatite, Alumina,Beryllium Oxide, or in some cases hard anodized Aluminium. Theoretically, the anodizedAluminium core has a better heat conductivity than any other insulated material, with BerylliumOxide being very close. There are specific problems with the anodized aluminium cores such asnicks in the coating, abrasion during capping and controlling the anodized thickness. There arevarious shapes, oval, flat, cylindrical, and most shapes are designed to optimize heat dissipation.The more heat that can be radiated from the resistor, the more power that can be safely applied.

There is a group of these called "Chassis Mounted Resistors". These are generally cylindrical power resistors wound on a ceramic core moulded and pressed into an aluminium heat sink andusually with heat radiating fins. These are designed to be mounted to metal plates or a chassis tofurther conduct heat. This result in a rating approximately 5 times or more its normal rating.

These resistors come in a variety of accuracy's and TCRs. They can be custom made as a cross breed between a Precision Resistor and a Power Resistor; capable of handling more power than thestandard Precision Wirewound but not as accurate. Practically speaking, tolerances of 1% andtemperature coefficients of 20 ppm can be achieved on all except the parts that are coated withVitreous Enamel and low values. The curing process for Vitreous (a type of glass) requiresextremely high heat and shrinks applying pressure to the winding. This particular group normallywill run tolerances of 10% with a TCR of 100ppm/oC. Power Resistors come in a variety of ratings. Most are rated at +25oC and derated linearly to either +275oC or +350oC. Again if theambient temperature of operation is +275oC, no power can be applied and at +125ooC 1/2 rated

power can be applied.These power rating are based on mounting the resistor in free air with the leads terminated at the

recommended point. On axial lead components, this is 3/8 of an inch from the body. If they can bemounted closer, the resistor will run cooler or you can apply slightly more power and if mountedfurther out, you must reduce the power. CAUTION, if mounted directly over and in contact with a

printed circuit board, the heat from the resistor can char the board if full power is applied. I don't



know of any PC Boards that are rated at+275oC.Other means of increasing the amount of power you can apply(a) bond the resistor to the chassis or other metal parts(b) mount vertically to get the chimney effect (this is very helpful when using those woundceramic tubes)(c) terminate as close to the body as practical(d) submerse in oil (CAUTION some types of resistor coating, particularly silicone based coatingswill disintegrate when immersed in oil and heated). This will increase the rating as much as 5times. or reduce the temperature rise of the resistor due to self heating.

The small power resistor can serve a two fold purpose, that is to fulfil it's purpose as a resistor and act as a heater in an enclosure. Some users have used them in crystal ovens to maintain thecrystal at the desired temperature. It makes a reasonably cheap off the shelf heater that comes in avariety of wattage's , sizes and values.

One unique type of power resistor is the "Bathtub Boat Type". This consists of resistance wirewound on a fibreglass cord.. This is a continuously wound strip, cut into strips of the appropriatelength with leads crimped. These resistive elements are placed in a ceramic shell (boat) and anhighly filled cement is used to fasten these in the boat. The filler often used in the cement is aceramic material with high heat conductivity. These are very inexpensive, no effort is made toachieve tight tolerances, low TCRs, and the range of values is extremely limited. They are oftenfound as surge resistors in TVs and other electronic /electrical equipment. Their main selling pointis low cost. They are often sold with an enamel coating for a low power precision wirewoundresistor that is even lower in cost.

One more item to consider, Power Wirewounds are made using alloys with melt temperatures

ranging from +1200o C to +1500o C and may be operated cherry red without failure for short periods of time, however the resistance value and TCR will change significantly and the insulatingmaterial will severely degrade. The bathtub boat type can not be subjected to this type of overload,the fibreglass winding form will disintegrate.

FUSE RESISTORS Fuse Resistors serve a dual purpose, a resistor and a fuse. They are designed so that they will

open with a large surge current. The fusing current is calculated based on the amount of energyrequired to melt the resistive material (the melt temperature plus the amount of energy required tovaporize the resistive material).

These resistors will normally run hotter than a normal precision or power resistor so that amomentary surge will bring the resistive element up to fusing temperature. Some designs create ahot spot inside the resistor to assist in this fusing. Calculations are made and samples are producedto verify the calculations. The major unknown is the heat transfer of the materials, which can be

quite significant for pulse of long duration, and is very difficult to calculate. Mounting of thesedevices is critical because it will effect the fusing current. These are quite often made to mount infuse clips for more accurate fusing characteristics.

CARBON COMPOSITIONCarbon composition resistors were once the most common resistor on the market. They still

have a very large market and prices are highly competitive. They are made from carbon rods cut inthe appropriate length then moulded with leads attached. The mix of the carbon can be varied tochange the resistivity for the desired values.

High values are much more readily available. Very low values are more difficult to achieve. A



5% tolerance is available. This is usually done bymeasuring and selecting values. Normal toleranceswithout measurement and selection is in the areaof 20%.

The temperature coefficient of resistance is inthe range of 1000 ppm/oC and is negative, that is

when the temperature goes up the resistance goesdown and when the temperature goes down, theresistance goes up. This is due to the carbon

particles being relaxed (with increase intemperature) and being compressed (with the reduction in temperature).

These resistors also has a voltage coefficient. That is the resistance will change with appliedvoltage, the greater the voltage, the greater the change. In addition to a power rating, they alsohave a voltage rating. (The wirewound voltage rating is determined by the value and the wattagerating). The voltage rating of Carbon Composition Resistors is determined by physical size as wellas the value and wattage rating.

One more item to consider is that due to their construction, they generate noise and this noiselevel varies with value and physical size. The power capability in relation to physical size isgreater than Precision Wirewounds but less than Power Wirewounds.

CARBON FILM RESISTORS Carbon Film Resistors have many of the same characteristics as carbon composition resistors.

The material is similar therefore they have noise, a voltage coefficient, the TCR can be muchlower because the formula can be varied to achieve this, the tolerance is much tighter due to thedifference in manufacturing processes.

The Carbon Film Resistor is made by coating ceramic rods with a mixture of carbon materials.This material is applied to these rods in a variety of means, the one most familiar to me aredipping, rolling, printing , or spraying the rods in the appropriate solution. The thickness of thecoating can be determined by the viscosity of the solution. This as well as the material compositionwill determine the ohms / square. Some of you may not be familiar with this term. It simply meansthat if a material has a resistivity of 100 ohms / square, one square inch with have the sameresistance as 1 square mm, or 1 square foot or 1 square yard or 1 square mile all equalling 100ohms but the power handling capability is proportional to the size.

One batch of material can produce resistors in a wide range of values. These rods are cut to thelength required for a specific size of resistor. These rods can then be spiral cut to a wide range of values. The original method of spiralling these was done with grinding wheels on a machinesimilar to a lathe. I am sure that later processes use lasers that are programmed to cut to specificvalues. The maximum ohmic value of this group is the highest in the discrete resistor group.

Tolerance of 1% can be achieved with out measuring and selecting. Tolerance of less than 1%can be achieved by measuring and selecting. You should use caution in getting tight tolerances inthis type because the temperature coefficient, voltage coefficient and stability may mean that it isonly good for that tolerance at the time it was installed. The TCR of carbon film resistors is in theneighbourhood of 100 to 200 ppm and is generally negative. Measuring and selecting can yieldeven tighter TCRs.

The frequency response of this type of resistor is among the best, far better than Wirewounds,and much better that carbon composition. The wirewound resistors are inductive at lower frequencies and values and somewhat capacitive at higher frequencies regardless of value. Alsowirewound resistors will have a resonant frequency. Carbon Composition Resistors will be

predominately capacitive .

METAL FILM RESISTORS Metal Film resistors are the best compromise of all resistors. They are not as accurate and have

a higher temperature coefficient of resistance and are not as stable as Precision Wirewounds. Theyare more accurate, do not have a voltage coefficient, have a lower temperature coefficient thanCarbon Film. TCRs of 50 to 100 ppm can be achieved.

They have a very low noise level when properly manufactured. In fact some of the screening processes measure the noise level to determine if there are problems in a particular batch of



resistors.Metal film resistors are manufactured by an evaporation/deposition process.

That is the base metal is vaporized in a vacuum and deposited on a ceramic rod or wafer. Several attempts have been made to vaporize low TCR materials anddeposit on these substrates, but to my knowledge, these attempts have not beensuccessful. This is partially due to the different boiling points of the various base

metals in these alloys (I use the word alloy not entirely accurately, for these materials are not truealloys but amalgamations --- they do not bond to form a molecule as does a true alloy). The verylow TCR resistive materials are heat treated to achieve the resistivity and low TCR. This is notcompatible with an evaporation process.

The frequency characteristics of this type are excellent and better than Carbon Films. The onearea that carbon films exceed metal films is the maximum values. Carbon films can achieve higher maximum values than any other group.

FOIL RESISTORSFoil resistors are similar in characteristics as metal films. Their main advantages are better

stability than metal films and lower TCRs. They have excellent frequency response, low TCR,good stability, and very accurate. They are manufactured by rolling the same wire materials asused in precision wirewound resistors to make thin strips of foil. This foil is then bonded to aceramic substrate and etched to produce the value required. They can be trimmed further by

abrasive processes, chemical machining or heat treating to achieve the desired tolerance. Their main disadvantage is the maximum value is less than Metal Film Resistors.

The accuracy is about the same as metal film resistors, the TCR and stability approachesPrecision Wirewounds but somewhat less because the rolling process and the packaging process

produce stresses in the foil. The resistive materials used in Precision Wirewound Resistors is verysensitive to stresses which result in instability and higher TCRs. Any stresses on these materialwill result in a change in the resistance value and TCR, the greater the stress, the larger the change.This type can be used as strain gauges, strain being measured as a change in the resistance. Whenused as a strain gauge, the foil is bonded to a flexible substrate that can be mounted on a partwhere the stress is to be measured.

FILAMENT RESISTORS The Filament Resistors are similar to the Bathtub Boat Resistor except they are not packaged in

a ceramic shell (boat). The individual resistive element with the leads already crimped is coatedwith an insulating material, generally a high temperature varnish. These are used in applicationswhere tolerance, TCR, and stability are not important but the cost is the governing consideration.The cost on this type is slightly higher that carbon composition and the electrical characteristicsare better.

POWER FILM RESISTORSPower film resistors are similar in manufacture to their respective metal film or carbon film

resistors. They are manufactured and rated as power resistors, with the power rating being the mostimportant characteristic. Power Film Resistors are available in higher maximum values than thePower Wirewound Resistors and have a very good frequency response. They are generally used inapplications requiring good frequency response and/or higher maximum values. Generally for

power applications, the tolerance is wider, the temperature rating is changed so that under full loadresistor will not exceed the maximum design temperature, and the physical sizes are larger, and insome cases, the core may be made from a higher heat conductive material and other means to helpradiate heat.

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ResistorsResistor ConstructionResistors are components having a stated value of RESISTANCE. Many types of resistors areused having different uses and construction. The most common types have a fixed value of resistance so are often called fixed resistors. They are shown on circuit schematic diagrams(theoretical diagrams that show how the circuit components are connected electrically, rather than what a circuit looks like physically) using one of the following symbols.

Various types of fixed resistors are used in circuits, they are the most numerous of all electroniccomponents and their most common job is to reduce voltages and currents around a circuit sothat "active components", transistors and integrated circuits for example, that carry out tasks suchas producing or amplifying signals within the circuit are supplied with the correct voltages andcurrents to work properly.

Resistors are also used in conjunction with other components such as inductors and capacitors to process signals in many ways.

Because resistors are "passive components" they cannot amplify or increase voltages currents or signals, they can only reduce them. Nevertheless they are a most essential part of any electroniccircuit.

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SMT (Surface Mount Technology)Many modern circuits use SMT resistors. Their manufacture involves depositing a film of resistive material such as tin oxide on a tiny ceramic chip. The edges of the resistor are thenaccurately ground, or cut with a laser to give a precise resistance (which depends on the width of the resistor film), across the ends of the device. Tolerances may be as low as ±0.02%. Contacts ateach end are soldered directly onto the conductive print on the circuit board, usually byautomatic assembly methods. SMT resistors normally have a very low power dissipation. Their main advantage is that very high component density can be achieved.

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Carbon Film ResistorsSimilar construction to Metal film resistors but generally with wider tolerance (typically +/- 5%)Shown here mounted on paper strips for machine insertion into printed circuit boards. Smallresistors are extremely inexpensive components and are also often sold in batches of 10s or 100sin this form for easier handling.







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Carbon Composition Resistor

Carbon composition is the oldest design and usually the cheapest of the resistors. Carbongranules are mixed with a filler material and inserted into a tubular casing. In earlier typesvulcanised rubber was used but in modern designs the carbon is mixed with a ceramic filler. Thevalue of resistance is determined by the amount of carbon added to the filler mixture. Carboncomposition resistors do not have the close tolerances of either carbon or metal film types.Typical tolerances are +/-10% or 20%. One advantage however is that they are better suited toapplications involving large voltage pulses than the more modern types.

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1Watt resistorCarbon composition, carbon and metal film resistors are available in a range of power ratings. Ina resistor, the power that the resistor must dissipate (get rid of as heat) depends on the voltagedifference (V) across the resistor, and the current (I) flowing through it. These are multipliedtogether to obtain the amount of power (P) that must be dissipated using the formula P = IV . For any particular type or value of resistor, the greater the power rating, the larger the physical sizeof the resistor.


Wire-wound resistors

Wirewound resistors are very variable in construction and physical appearance. Their resistiveelements are commonly lengths of wire, usually an alloy such as Nickel/Chromium (Nichrome)or Manganin (Copper/Nickel/Manganese) wrapped around a small ceramic or glass fibre rod and









coated in an insulating flameproof cement film. They are normally available in quite low valuesof resistance (single ohms to a few Kilohms) but can dissipate large amounts of power. In usethey may get very hot.

For this reason high power wirewound resistors may be housed in a finned metal case that can be bolted to a metal chassis to dissipate the heat generated as effectively as possible. With all typesof wirewound resistor, fire protection is important and flame proof cases or coatings are vital.Lead-out wires are normally welded rather than soldered to the resistor.


Metal film resistors.

These resistors are made from small rods of ceramic coated with metal (such as a nickel alloy) or a metal oxide (such as tin oxide). The value of resistance is controlled firstly by the thickness of the coating layer (the thicker the layer, the lower the value of resistance). Also by a fine spiralgroove cut along the rod using a laser or diamond cutter to cut the carbon or metal coatingeffectively into a long (spiral) strip, which forms the resistor. Metal film resistors can beobtained in a wide range of resistance values from a few Ohms to tens of millions of Ohms witha very small TOLERANCE. For example a typical value might be 100KΩ ±1% or less i.e. for astated value of 100KΩ the actual value will be between 99KΩ and 101KΩ. Note that althoughthe body colour (the colour of the laquer coating) on metal film resistors is often grey, this is nota reliable guide. Small carbon, metal and oxide resistors may be made in various body colourssuch as dark red, brown, blue, green, grey, cream or white.


5 Watt Wirewound ResistorA wirewound resistor can have a smaller physical size for a given power rating than carboncomposition or film resistors, compare this 5W resistor with the 1W resistor at 3. Wirewound







resistors however, do not not have the close tolerance of composition or film types. This 4R7resistor has a tolerance of ±10%.


PCB Mounting Wirewound ResistorWirewound resistors usually have a resistance range from around 1Ω to about 50KΩ. Becausethey use a coil of wire as their resistive element they tend to act as inductors to some degree.This limits their use to low frequency circuits up to around a few tens of kiloHertz (kHz). Thisexample, available in power ratings up to 25W, is for mounting on a printed circuit board and to

prevent heat damage to the board, the specially shaped legs ensure an air gap between theresistor and the board. The whole resistor is enclosed in a flameproof ceramic layer.


High Power Metal FilmMetal film resistors are also available in high power types with power ratings less thanwirewound types (typically less than 5W) but having closer tolerances.


Fusible Wirewound ResistorIn this fusible resistor, the current flowing through the resistor first flows through a spring loadedconnection that is positioned close to the body of the resistor. The heat generated by thewirewound resistor under normal conditions would not be sufficient to melt the blob of solder holding a spring wire in place. If too much current flows through the resistor it overheats, thesolder melts and the wire springs up, opening the connection and stopping the current. This thenrequires a service technician to find the cause of the overcurrent before re−soldering the springconnection to restore normal operation. It is important to use the correct type of solder (usuallystated in the service manual for the equipment) when re−soldering, since this will affect thetemperature at which the spring opens.


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