characteristic curve diodes and leds

8/9/2019 Characteristic Curve Diodes and Leds

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L.105.2 PG. 12. WHAT IS A CHARACTERISTIC CURVE? DIODES AND LED'S, BASIC OPERATION,

TESTING, CURVES

I. Characteristic Curves

1. why study

A. electronic devices we will study can be described by means of specialgraphs called characteristic curves (REMINDER: bringgraph paper to class in future)

B. available in data manuals, catalogs, and on manufacturers' web sites(SHOW example)

C. can tell you what you need to know to use a device

D. how it will perform in a circuit

2. definition—a graph of DC current through vs. voltage across the device

A. current on y (vertical) axis, voltage on x (horizontal) axis

B. I vs. V

C.

DRAW example, Fig. 1

0V = voltage across the device

1 2 3 4 5 6 7 8 9 10

Fig. 1—Example characteristic curve

20

10

0

30

40

50

I (ma) = current through the device

+

DEVICE

V

A

I →→→→


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3. uses

A. tells us how much current will flow through the device when a certainvoltage is placed across the device's terminals

B. or how much voltage it would take to get a certain amount of current toflow

C. SHOW examples based on Fig. 1

4. example—a 1000- Ω resistor

A. make a table of I vs. V—how? Ohm's Law

Voltage across 1000- ΩΩΩΩ resistor Current through resistor0 V 0 mA1 V 1 mA5 V 5 mA

10 V 10 mA

B. make a graph of this data, Fig. 2

C. characteristic curve of a resistor is a straight line

0V (V) = voltage across the device

2 4 6 8 10

Fig. 2—Characteristic curve of a 1000- ΩΩΩΩ resistor

4

2

0

6

8

10

I (mA) = current through the device

+

V

A

I →→→→


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TESTING, CURVES

5. for an unknown device

A. resistor was trivial example, because we already know relationship of I andV—direct proportion

B. not all characteristic curves are straight lines

C. must determine experimentally

D. procedure (DRAW circuit of Fig. 3, piece by piece, and SHOW procedureusing unknown device no. 2A)

i) connect ammeter in series with device—why do this?

ii) connect separate voltmeter (where?) across device

iii) now set up to measure voltage across and current through

iv) next add voltage source, adjustable (why?)

v) problem—what if too much current flows and destroys device?

(a) example—device is a 1.5-V lamp, and we apply 6 V to it?


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(b) happens a lot with semiconductor devices—excess current

may flow

vi) solution—add series resistor to limit current

(a) voltage across device will not equal supply voltage anymore—why?

(b) what size resistor?

• if too large, you won't be able to get large enoughcurrent to complete your graph

• if too small, you may damage device, or find it toodifficult to adjust the current to the exact desiredvalue

vii) prepare a table of I vs. V

(a) indicate your units

(b) fill in one column, ahead of time, with values covering the

range of interest

• which column? if you're using a digital voltmeterand an analog ammeter, fill in the I column withround values (why?)

• how many points should you measure? generalanswer: enough to complete the curve—take morepoints where changes are occurring, fewer pointswhere the curve is straight—you should never haveto guess where to draw the curve

viii) adjust the voltage source until the desired current appears on theammeter

ix) record the corresponding voltage reading alongside it in the table

x) what meter ranges should you be using?


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xi) continue until the table is complete

xii) problem: what happens if you can't reach the highest desiredcurrent?

(a) change to smaller R (why?)

(b) (important concept) will you then have to re-measure theentire curve?

(c) should continually check device and resistor for excessiveheat buildup as you go

(d) if excessive, may have to use water bath or other heat sink

(e) could you use a single multimeter and simply move itbetween series and parallel? why not?

xiii) don't take apart apparatus yet

xiv) draw graph (SHOW how, projecting Fig. 3A below on whiteboard)

(a) I on y axis, V on x axis—draw the axes, label them

(b) start both I and V scales at zero

(c) indicate units of each scale

(d) make uniform scale graduations—others may seem moreconvenient, but would distort the characteristic curve

(e) purpose of graph is to represent the trend, not preciselytabulate each data point—don't worry if points appear ongraph to be equal in value

(f) make each square count as a round value: 1, 2 or 5 only—no 3's, 2.5's, etc.


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TESTING, CURVES

(g) try to make the finished graph fill the entire paper, not

squeezed into one corner—plan ahead

(h) avoid numbering every single square on the paper—majordivisions only, then tic marks for minor divisions

(i) draw a smooth curve through the points, not dot-to-dot linesegments

(j) if more than one curve, label each carefully—e.g., 25degrees, 50 degrees, etc.

(k) finished graph should be clean and easy to read at a glance

6. what characteristic curve can tell you about a device in any circuit

A. the current through it when a given voltage is applied across it

V(V)

I(mA)

0

0

Fig. 3A


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TESTING, CURVES

i) example from data measured

ii) from homework

B. the voltage across it when a given current is flowing through it in anycircuit

i) example from data measured

ii) from homework

C. the rate of change of current with voltage

i) called dI/dV

ii) tells how much the current will change for a given small change involtage

iii) units are mA per V, typically

iv) examples

(a) for the 1000-Ohm resistor

• look at curve

• current always goes up 1 mA for every volt increase

• dI/dV the same anywhere on curve = 1 mA/V

(b) for the unknown device

• how does given increase in voltage affect currentlevel?

• depends where you are on curve

• not much change at first, more as you go up on thecurve


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TESTING, CURVES

(b) trial and error

• guess a voltage for V A

• less than supply voltage (why?)

• this is voltage across device

• compute voltage across R = supply voltage -estimated voltage across device

• compute current—voltage across R / R

• this same current flows through device (why?)

• look on characteristic curve and see if this currentagrees with the voltage assumed to be across thedevice

• if not refine your guess of the voltage across thedevice and repeat above procedure

• keep going until your assumed voltage agrees withcomputed current

(c) load line method

• on the same graph as the characteristic curve, drawa dot on the x axis at the power supply voltage (thisis the so-called open circuit condition—if the devicewere an open circuit, there would be zero currentthrough and full supply voltage across the device)

• draw another dot on the y axis at the short circuitcurrent = supply voltage / resistor value (if thedevice were a short, there would be zero voltsacross it and a current equal to the supply voltageover R)


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TESTING, CURVES

• connect the two dots with a straight line (equation

of this line is I = (V SUPPLY –V) / R, which is theequation of a line)

• where this line crosses the characteristic curve is the"operating point" of the device

• read off the devices I and V from this point

• test your answer in circuit

• will use in the future

II. The Diode

1. why study

A. basis of all solid state (semiconducting) devices, e.g., transistors

B. used in many application circuits

i) power supplies

ii) clippers, clampers, peak detectors, logic, to be studied later

2. definition—a device which conducts current in one direction but not the other

A. SHOW with lamp

B. like a one-way electronic valve

3. construction (DRAW Fig. 5)


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TESTING, CURVES

A. p type material

B. n type material

C. joined to form "junction"

D. silicon or germanium used, silicon now most common

4. symbol and physical package (DRAW Fig. 6)

A. bar on symbol called "cathode"

B. other terminal "anode"

C. bar on physical device marks cathode (same as on symbol)

5. part numbering system


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TESTING, CURVES

A. 1NXXXX commonly used in U.S.

B. ("2NXXXX" is used for transistors)

C. e.g., 1N4001

D. unfortunately, numbers are not in themselves significant—have to use datasheet to find out specifications

E. data sheets found on the Internet (SHOW for 1N4001)

6. operation—which way can electrons flow through the diode? (SHOW with circuitsof Fig. 7)

A. in cathode, out anode

B. opposite the "arrow"


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TESTING, CURVES

C. arrow notation designed for conventional current

7. ideal equivalent circuit (Fig. 8)

A. "forward bias"

i) diode is conducting current

ii) equivalent to a closed switch

iii) anode more positive in voltage than cathode

B. "reverse bias"

i) diode is blocking current

ii) equivalent to an open switch

iii) cathode more positive than anode

8. testing


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TESTING, CURVES

ii) Fig. 10

iii) Fig. 11

iv) Fig. 12


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TESTING, CURVES

10. ideal characteristic curve

A. forward bias

i) first quadrant (positive voltage means forward bias on diode)

ii) like closed switch

(a) never any voltage across (V = 0)

(b) no matter what current flows

iii) vertical line (start to DRAW Fig. 13)


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TESTING, CURVES

(a) how compare with resistor?

(b) more like a high valued or low valued resistor?

B. reverse bias

i) 3rd quadrant

ii) like an open switch

(a) never any current through (I = 0)

(b) no matter what voltage across

iii) horizontal line (add to Fig. 13)

(a) compare with resistor

(b) high or low value?

11. actual characteristic curve

V (V) = voltageacross the diodeFig. 13—Characteristic curve of an ideal diode

0 2 4 6 8 10

4

2

6

8

10

I (mA) = current through the diode

-4 -2-6

FORWARD BIASREVERSE BIAS


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TESTING, CURVES

A. different from ideal, both + and - parts

B. exact shape depends on temperature and type of diode

C. to be measured in lab

12. diode parameters—what you pay for when buying a diode

A. maximum forward current

i) how much current diode can carry before it blows out

ii) e.g., for 1N4000, it's 1 A

iii) show various samples of different ratings, if available

B. reverse voltage rating

i) how much reverse bias voltage you can apply before diode startsconducting (the wrong way)

ii) like one-way valve—will blow when back pressure is excessive

iii) may blow if exceeded—depends on whether current flow is limited

iv) e.g., for 1N4000, it's 50 V

v) test circuit (DRAW circuit of Fig. 14)


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TESTING, CURVES

III. LED

1. applications

A. lamp replacement (SHOW online ads for LED lamps)

i) draw less current for same amount of light as incandescent lamps

ii) last a lot longer—no filament to burn out

B. digital displays (SHOW example of 7-segment display—light up a segmentusing DC supply and 330-Ohm resistor)

C. fiber optic communications

i) LED used as light source

ii) shines down fiber optic cable

iii) invisible infrared light used

iv) picked up at other end by photo detector and decoded

v) SHOW basic operation of fiber optic cable using flashlight, andfiber optic cable, observing other end of cable on CCD videocamera

D. TV remote controls


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TESTING, CURVES

E. security alarms

F. laser printers (use laser LED)

G. CD players

H. bar-code scanners

2. symbol and physical package (DRAW Fig. 15)

A. cathode and anode designated the same as a regular diode

B. cathode of discrete LED slightly flattened

3. operation

A. basically the same as regular diode

i) electrons go in cathode, out anode (SHOW)

ii) WARNING: always use series resistor to limit current

cathode

Fig. 15—Light-emitting diode (LED) symbol and construction


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TESTING, CURVES

B. but gives off light as electrons cross the junction and drop to a lower

energy level

C. reversible effect—shine light on LED and will put out voltage—try it

4. characteristic curve—similar to diode, but slight difference, to be measured in lab(REMINDER—bring graph paper to lab)

5. testing

A. same method as for diode

B. most analog ohmmeters can't test due to higher voltage required

C. some digitals will (try it)

IV. MATERIALS

1. silicon diode

2. lamp

3. DC power supply

4. analog ammeter

5. digital multimeter

6. LED

7. demonstration fiber optic cable

8. 7-segment LED display

9. 350-Ohm resistor

10. sample diodes of different sizes and current ratings

11. flashlight


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TESTING, CURVES

12. unknown two-terminal device #2A

13. 100-Ohm resistor

characteristic curve diodes and leds

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