Transistors and Multimeters

The first thing you need to find out is the polarity of your multimeter leads. Do not assume that the negative lead is really negative for there is no standard for ohmmeter leads. The following will tell you what you need to know about bipolar transistors if all you have is multimeter:

1. Find the two leads which give a high resistance reading for either polarity. These will be the collector and emitter leads. The remaining lead is the base.

2. Put the positive lead on the base and the negative lead on either of the remaining two leads. If you get a low resistance reading you have an NPN transistor. You may need to change ohmmeter ranges.

3. Repeat step 2 except with the negative lead on the base. A low resistance reading tells you that it's a PNP transistor.

4. Now you know if its PNP or NPN and which lead is the base. What's left is determining emitter versus collector and relative gain.

5. Moisten your finger or grab a resistor (value is not all that important, 1K to 10K) and bridge the base to positive ohmeter connection. Note the drop in resistance. Swap the leads between emitter and collector and repeat this bridging. The connection which gives the lowest resistance reading tells you that the positive lead is on the collector and the relative drop gives you an indication of gain for NPN.

6. Repeat step 5. The connections which give the lowest reading tells you that the positive lead is on the emitter and the relative drop gives you an indication of gain for PNP.

Deviations from the above indicate a faulty transistor.

Best wishes.

That would work quite well for single transistor configurations, 1.5 volt battery in series with a 1K resistor. However, for darlington transistors (two transistors connected in compound), a 1.5 volt battery and a 1K resistor won't do the trick reliably because you're looking at essentially two junctions in series or about a drop of 1.4 volts, perhaps a little less at low current. In this case, two 1.5 volt cells and about 2.7K - 3.3K in series will work for all transistors.

Another, slightly more complex, arrangement would be to use an LED in series with a resistor and 9 volt battery to give a visual indication of conduction. 680 ohms would work. This would eliminate the need for a DVM and would work for single and darlington transistors.