This note describes procedures for testing of bipolar (NPN or PNP) transistors for catastrophic failures like shorts and opens. In most cases, this will identify bad Silicon transistors. Gain, frequency response, etc. are not addressed here. While the tests can be applied to Germanium devices, these are more likely to change characteristics, it would seem, without totally failing.
Note: Analog and Digital meters behave quite differently when testing nonlinear devices like transistors or diodes. It is recommended that you read through this document in its entirety. Most digital meters show infinite resistance for all 6 combinations of junction measurements since their effective resistance test voltage is less than a junction diode drop (if you accidentally get your skin involved it will show something between 200K and 2M Ohms). The best way to test transistors with a DMM is to make use of the "diode test" function which will be described after the analog test. For both methods, if you read a short circuit (0 Ohms or voltage drop of 0) or the transistor fails any of the readings, it is bad and must be replaced. This discussion is for OUT OF CIRCUIT transistors *ONLY*.
One exception to this ocurrs with some power transistors which have built in diodes (damper diodes reversed connected across C-E) and resistors (B-E, around 50 ohms) which will confuse these readings. If you are testing a transistor of this type - horizontal output transistors are the most common example - you will need to compare with a known good transistor or check the specifications to be sure. There are some other cases as well. So, if you get readings that do not make sense, try to confirm with a known good transistors of the same type or with a spec sheet.
Before testing an unknown device, it is best to confirm and label lead polarity (of voltage provided in resistance or diode test mode) of your meter whether it be an analog VOM or digital DMM using a known good diode (e.g., 1N4007 rectifier or 1N4148 signal diode) as discussed below. This will also show you what to expect for a reading of a forward biased junction. If you expect any Germanium devices, you should do this with a Ge diode as well (e.g., 1N34).
The assumption made here is that a transistor can be tested for shorts, opens, or leakage, as though it is just a pair of connected diodes. Obviously, simple diodes can be tested as well using the this technique. However, LEDs (forward drop too high more most meters) and Zeners (reverse breakdown - zener voltage - too large for most meters) cannot be fully tested in this manner.
For NPN transistors, lead "A" is black and lead "B" is red; for PNP transistors, lead "A" is red and lead "B" is black (NOTE: this is the standard polarity for resistance but many multi-meters have the colors reversed since this makes the internal circuitry easier to design; if the readings don't jive this way, switch the leads and try it again). Start with lead "A" of your multi-meter on the base and lead "B" on the emitter. You should get a reasonable low resistance reading. Depending on scale, this could be anywhere from 100 ohms to several K. The actual value is not critical as long as it is similar to the reading you got with your 'known good diode test', above. All Silicon devices will produce somewhat similar readings and all Germanium devices will result in similar but lower resistance readings.
Now move lead "B" to the collector. You should get nearly the same reading. Now try the other 4 combinations and you should get a reading of infinite Ohms (open circuit). If any of these resistances is wrong, replace the transistor. Only 2 of the 6 possible combinations should show a low resistance; none of the resistances should be near 0 Ohms (shorted). As noted above, some types of devices include built in diodes or resistors which can confuse these measurements.
Set your meter to the diode test. Connect the red meter lead to the base of the transistor. Connect the black meter lead to the emitter. A good NPN transistor will read a JUNCTION DROP voltage of between .45v and .9v. A good PNP transistor will read OPEN. Leave the red meter lead on the base and move the black lead to the collector. The reading should be the same as the previous test. Reverse the meter leads in your hands and repeat the test. This time, connect the black meter lead to the base of the transistor. Connect the red meter lead to the emitter. A good PNP transistor will read a JUNCTION DROP voltage of between .45v and .9v. A good NPN transistor will read OPEN. Leave the black meter lead on the base and move the red lead to the collector. The reading should be the same as the previous test. Place one meter lead on the collector, the other on the emitter. The meter should read OPEN. Reverse your meter leads. The meter should read OPEN. This is the same for both NPN and PNP transistors. As noted, some transistors will have built in diodes or resistors which can confuse these readings.
