TECH 150
LAB #2
ELECTRONIC DEVICES AND CIRCUITS
The Semiconductor Diode
Name _________________
Introduction
You will examine basic diode properties in this lab and learn how to test them using a DVM.
Although several semiconductor materials might be used, silicon is by far the most common and
will be used here. The simplest semiconductor device and the first one you study is the diode
(an example of a p-n junction).
Equipment Required
Parts kit
Procedure:
Diode Identification
1. Most diodes use an identification number starting with the prefix “1N”. Locate a 1N4004 or a
1N4005 from your lab kit. It is a small resistor sized component…black…with a white stripe
at the negative (cathode…N material) end. The other lead is the positive (anode…
p type material) lead.
1N4004
ANODE
CATHODE
ANODE (P)
CATHODE (N)
Diode “ON” Characteristics
2. The most basic property of the diode is that it will conduct only when the anode is more
positive than the cathode (by about 0.7 V).
3. Set up the circuit of figure 1.
FIGURE 1:
The AVERAGE forward voltage drop across a
conducting (forward biased) diode is usually
taken as 0.7V.
Measure and record, to 3 figures (after the decimal point) on the DVM, the voltage across
the diode.
VD =_______
4. Measure the resistor
(remember Ohm’s law?)
VR = ________
voltage
I
D
=I
R
and
from
= VR/RLimit
Determine the resistance of the diode as:
=
it
calculate
the
diode
current
.
________
RD =
VD
 =
ID
________
Repeat for R = 1000Ω, 4.7 kΩ and R = 10 kΩ. Enter all the results in the table.
RLimit
100 Ω
1 kΩ
4.7 kΩ
10 kΩ
5.
VR
VD
ID
RD
Take the average of the four diode voltage drops and round off the value to one decimal
place
VD = _______
6. Plot a hand-drawn graph of ID (mA) versus VD (volts) and comment on its linearity.
ID
VD
Diode Switch/Conduction
7. Set up the next circuit shown. You are using an LED (light
emitting diode) which is still just a diode, but with a
different voltage drop and it glows (conducts) when
forward biased.
The LED should now be illuminated.
Measure the voltage drop across the LED.
VLED = _______
Measure the voltage drop on the 330Ω resistor and determine the circuit current.
V
330
= ___________
I
circuit
=___________
The diode is said to be FORWARD BIASED and circuit current is flowing.
Circle the Correct word
For a diode to conduct the Anode voltage must be more Positive or Negative than the
cathode.
8. Reverse the 1N 4004 diode. Did the LED turn off?
Yes/No.
If so, this indicates that the 1N 4004 diode is now OFF and is acting like an open switch.
The diode is said to be REVERSE BIASED.
Measure the voltage drop on the 330Ω resistor again and determine the circuit current.
V
330
= ___________
I
circuit
=_____________
Diode Testing
9. Testing the diode with an ohmmeter will determine if the diode is Good or whether it is open
or shorted internally. Normally a diode will fail open circuit.
Set the DVM to OHMS and on the range marked with a diode symbol.
An ohmmeter measures resistance by applying an internally generated voltage
of about 0.25V to the component and it determines the unknown resistance from the resulting
current flow. 0.25V is not enough to turn on the diode, so for diode testing a special diode
marked range has an output which is high enough to turn on the diode and measure its
resistance. The test gives a reading which is of no particular significance other than indicating
whether the diode is conducting or not. When the meter forward biases the diode there should
be a reading, and when reverse biased it should indicate an open circuit. Most DMMs show the
“On” voltage across the diode if it is good, if it is bad it will show an open or a “1”.
Leave the meter leads open and note how your meter indicates an open circuit. Different
meters have different ways of showing an open (or off scale value).
Question: What does the meter display for an Open Circuit? ____________________
Connect the red meter lead (+) to the anode and the black lead (- or gnd or com) to the
cathode of the diode. Record your reading as “OPEN” or “CONDUCTING”
READING _________________
READING _________________
QUESTIONS:
1. The + lead of an ohmmeter is connected to the cathode and the negative to the anode of a
diode. The meter reads 0Ω. Is the diode good or bad?
2. Does the diode have a constant resistance? YES
NO
3. Diode resistance decreases as the diode current increases.
True
False
4. The voltage drop across a diode ( increases/decreases/does not change) as diode current
increases?
5. Is the “On” voltage for the LED (Greater or Less) than the “ON” voltage for the 1N4004
diode?
6. For the LED the (Long/Short) lead is the cathode. (Circle correct word)
Signature __________________