Download Lab Experiment III

L. Fennigkoh, Ph.D.
Milwaukee School of Engineering
Electrical Engineering & Computer Science Department
EE-2705 – Linear Circuits I: DC
Experiment 3
LED i-v Characteristics & Specification Sheets
Name:
Lab Partner:
Date:
Objectives:
 To experimentally determine the current-voltage (i-v) relationship of a typical light-emitting
diode (LED).
 To understand the importance of proper and safe LED biasing and the interpretation of
specification sheets.
Needed Equipment and Parts:

Before coming to lab, go to Tech Support and check out the following (one set per two
person team): breadboard, parts box, hook-up wire, either a red or yellow LED.
Determining LED i-v- characteristics:
1. Locate, measure, and record the
resistance of either a 220 or 330,
and 100 ohm resistors then
construct the circuit shown. Be
sure to pay particular attention to
the polarity of the LED. Use the
+25 vdc supply.
2. With the supply set to zero volts,
and the multimeter connected
across the 100Ω resistor and set to
read DC volts, gradually increase the supply until the multimeter reads 0.200 or 200 mV.
Total circuit current can now simply be derived from Ohm’s law, i.e., I=200 mV / 100 Ω = 2
mA. (Note that this is an indirect but very useful way of measuring current in a circuit
without having to insert an ammeter in series).
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3. Move the multimeter leads to measure the corresponding
voltage drop across the LED. Record this voltage in the
table shown.
4. Continue to increase LED current in 2 mA increments and
recording the associated LED voltages. After 20 mA,
increase in 5 mA increments.
Analysis:
1. Use an appropriate software package, e.g., Matlab, Minitab, to
plot a fitted line to these i vs v data. Include such a plot in your
notebook. How would you best describe this relationship?
2. Compare the LED forward voltage at a current of 20 mA to the
typical LED specifications shown below. Is your particular LED
reasonably within these specifications?
LED current
(mA)
0
2
4
6
8
10
12
14
16
18
20
25
30
35
40
LED voltage
(vdc)
0
3. If you now needed to use this LED in an automotive application where the source voltage was
13.6 vdc, and LED current limited to 18 +/- 3 mA, calculate the needed value of a series
dropping resistor and the power that such a resistor would dissipate. Based upon your cal
culations, what standard value resistor and power rating would you specify and why.
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