Electrical Tips and Tricks
Testing Diodes and Transistors BACKGROUND
One of the nice things about solid-state devices is that, under normal conditions, they rarely go bad. However, "rarely" is not the same as "never". And if conditions are not "normal", if an excessive voltage gets to a semiconductor, it can be damaged. In this article we will discuss how to test for a damaged transistor or diode.
Device testing can be done at two levels: functional and parametric. A functional test determines whether or not the device works well enough for the intended use. A parametric test measures all device parameters to see if they meet the specified values. In the production of semiconductor devices, it is often the case that functional testing is done on all units while parametric testing is done on a small percentage of the units as test samples.
For the most part, the performance of semiconductor devices does not deteriorate gradually over a period of time. Typically, transistors and diodes work well up to the point where they stop working completely, so all we will need to do is make a few simple functional tests.
TESTING SILICON DIODES (NOT LED OR ZENER)
If you use a digital multi-meter (DMM), then there should be a special setting on the Ohms range for testing diodes. Often the setting is marked with a diode symbol:
Measure the diode resistance both ways. One way the meter should indicate an open circuit. The other way you should get a reading (often a reading around 600). That indicates the diode is good. If you measure an open circuit both ways, the diode is open. If you measure low resistance both ways, the diode is shorted.
TESTING DIODES IN CIRCUIT
KNOW POLARITY OF YOUR METER
Another way is to take a diode you know is good and find which way you need to put the leads to get an Ohms reading. At that point, the positive lead is on the anode and negative lead is on the cathode (cathode is the banded end.)
One reason to know the polarity of your meter is so you can determine which end of a diode is the cathode if the band has been removed. Also, as we will see below, you can use your Ohm-meter to tell an NPN transistor from a PNP if you know which meter lead is positive.
The power supply voltage should be set to a value slightly higher than the Zener value. For example, for a 12 volt diode, the supply voltage should be about 15 volts. The value of the resistor R should limit the current to about a milliAmp. For example, using 15 volts with a 12 volt Zener, use a 3.3K resistor. The exact value is not critical.
Once the circuit is built, just read the Zener voltage off the meter (if you read 0.6 volts, reverse the diode). NOTE: Any diode will become a Zener diode if you apply enough voltage to it.
If you can't use your multi-meter, then build the circuit shown in Figure 4 and see if the LED light up.
If the LED doesn't light, reverse polarity on the diode. If it still doesn't light, it's bad. (See Figure 4).
TESTING ZENERS AND LEDS IN CIRCUIT
For a Zener, just measure the voltage across it. Using a VOM or a battery-operated DMM, put the black lead on the anode and the red lead on the cathode. You should read the Zener voltage. If you read zero volts, the Zener is shorted or the resistor feeding the Zener is open or not getting voltage. if you read a value higher than the Zener voltage, the Zener is open.
For an LED that is supposed to be lit but isn't, use a VOM or battery-operated DMM to measure the voltage across it. If you measure more than 3 volts or so, the LED is open.
You can think of a transistor as two back-to-back diodes in one package as shown in Figure 5.
Note that transistors come in two basic types: NPN and PNP. The letters C, B, E stand for COLLECTOR, BASE, EMITTER which are the names of the three leads which come out of a transistor.
Transistors come in many different case styles, three of which are shown in Figure 6. It is important to know where C, B, E are for any given case.
Also, check the resistance from C to E using a higher Ohms scale (say, the 2 Meg scale). Be sure your fingers don't touch the metal test points or you will just measure your skin resistance.
If the transistor is good, you should get an open-circuit reading from collector to emitter. NOTE: the above assumes silicon. With germanium transistors you may measure a high resistance from C to E.
USING METER TO SEPARATE NPN FROM PNP
Assuming you know where C, B, and E are on the transistor, do the following. Connect the positive lead of your Ohm-meter to the base. Touch the other lead of your meter to the collector. If you get a reading, the transistor is NPN. To verify, move the lead from the collector to the emitter and you should still get a reading.
If your meter reads open-circuit, then connect the negative lead to the base and touch the positive lead to the collector. If you get a reading, then the transistor is PNP. Verify by measuring from base to emitter.
THINGS TO WATCH FOR
Some transistors have resistors from base to emitter built into them. They will read that resistance when measuring Ohms B to E.
Some transistors are Darlingtons. They have a higher reading base to emitter which may appear as an open on a VOM.
CHECKING TRANSISTORS IN CIRCUIT