Experiment for diode and transistor testing using multimeter. Find the diode bad or good. Identify the terminals of a good diode. Find the hfe of a transistor. Identify the PNP or NPN type of transistor. Identify the diode for open or short circuited.

1. Experiment
Objective:
To test a Diode and Transistor using a digital multimeter.
Equipment:
Digital Multimeter, Connecting probe, Diodes etc.
Theory:
A) Diode testing with Digital Meter
Most digital multi-meters are suitable for diode testing, and in many cases will have a special ‘diode
test’ range usually marked with a diode symbol. This range should always be used when testing diodes
or any other semiconductor device. The reason for this is that the meter tests the diode by applying a
voltage across the diode junction. The normal voltages used by the meter on other resistance ranges
may not be high enough to overcome the diode´s forward junction potential and so will not make the
diode conduct, even in the forward direction. This would give an indication that the diode was open
circuit (very high resistance). If the diode range is used, the test voltage applied by the meter will be
high enough in most cases to overcome the forward junction potential and the diode will conduct.
Therefore in the forward direction (meter positive lead to the diode anode, and the negative lead to the
cathode) the diode's resistance can be measured.
The actual value of resistance will depend on the slope of the forward characteristic of the diode at the
voltage applied by the meter, and so will vary from device to device and from meter to meter, so a
precise value cannot be given. When measuring a good silicon diode (not connected to any circuit), a
reading in the forward direction of about 500Ω to 1kΩ could be expected, similar or slightly less with
germanium diodes. With the meter leads reversed, an out of range (infinity) or open circuit reading
(usually indicated by a display something like ‘1.’ on a digital meter, as shown in Fig. 2.8.1) should be
expected.
If the diode is already in a circuit, the resistances measured, always with the circuit switched off, will
be affected by any parallel paths. Therefore readings will be lower than those indicated above. However
very low or zero ohm readings may indicate a short circuit diode (the most common fault with diodes)
making it worthwhile, if no other obvious reason for the very low reading can be seen, to remove at
least one end of the diode from the circuit and re-check the diode's forward and reverse resistance.
Digital multimeters have a special setting for testing a diode, usually labelled with the diode symbol.
Connect the red (+) lead to the anode and the black (-) to the cathode. The diode should conduct and
the meter will display a value (usually the voltage across the diode in mV, 1000mV = 1V).
Reverse the connections. The diode should NOT conduct this way so the meter will display "off the
scale" (usually blank except for a 1 on the left).

2. B) Test a transistor with a digital multimeter
Ask any field or bench technician what their most-used piece of test equipment is and they will
probably say a DMM (Digital MultiMeter). These versitiale devices can be used to test and
diagnose a wide range of circuits and components. In a pinch, a DMM can even substitute for
expensive, specialized test equipment. One particularly usefull skill is knowing how to test a
transistor using a digital multimeter.
Transistor Pinouts
Fortunately, using a DMM to get a basic pass/fail reading from a suspected faulty NPN or PNP bipoloar
transistor is a simple and quick task. Some multimeters have a built-in transistor testing function,
simply insert your transistor into the socket on the multimeter and set the meter to the correct mode.
You will probably get information such as the gain (hFE) that could be checked against the datasheet
as well as a pass/fail reading. If meter does not have a transistor testing function, then transistors can
easily be checked with the “Diode” testing setting. (Some meters have the diode test function coupled
with the continuity test – this is OK).
Remove the transistor from the circuit for accurate test results.

3. Step 1: (Base to Emitter)
 Hook the positive lead from the multimeter to the to the BASE (B) of the transistor.
 Hook the negative meter lead to the EMITTER (E) of the transistor.
 For an good NPN transistor, the meter should show a voltage drop between 0.45V and 0.9V. If
you are testing PNP transistor, you should see “OL” (Over Limit).
Step 2: (Base to Collector)
 Keep the postitive lead on the BASE (B) and place the negative lead to the COLLECTOR (C).
 For an good NPN transistor, the meter should show a voltage drop between 0.45V and 0.9V. If
PNP transistor is testing, OL (Over Limit) value is showing.
 Hook the positive lead from the multimeter to the to the EMITTER (E) of the transistor. Hook the
negative meter lead to the BASE (B) of the transistor.
 For an good NPN transistor, you should see “OL” (Over Limit).If you are testing PNP transistor,
the meter should show a voltage drop between 0.45V and 0.9V.
Step 4: (Collector to Base)
 Hook the positive lead from the multimeter to the to the COLLECTOR (C) of the transistor.
 Hook the negative meter lead to the BASE (B) of the transistor.
 For an good NPN transistor, you should see “OL” (Over Limit).If you are testing PNP
transistor, the meter should show a voltage drop between 0.45V and 0.9V.
Step 5: (Collector to Emitter)
 Hook the postitive meter lead to the COLLECTOR (C) and the negative meter lead to the
EMITTER (E) – A good NPN or PNP transistor will read "OL"/Over Limit on the meter. Swap
the leads (Positive to Emitter and Negative to Collector) – Once again, a good NPN or PNP
transistor should read “OL”.
If bipolar transistor measures contrary to these steps, consider it to be bad.
You may also be able to use the voltage drop to determine which lead is the emitter on an unmarked
transistor, as the emitter-base junction typically has a slightly higher voltage drop than the collector-
base junction.
Remember: This test only verifies that the transistor is not shorted or open, it does not guarantee that
the transistor is operating within its designed parameters. It should only be used to help decide
if you need "replace" or "move on to the next component". This test works on bipolar
transistors only – you need to use a different method for testing FETs.