NPN/PNP Transistor Tester

Fri Jul 4 20:17:50 PDT 2025

This video shows a transistor tester I put together based on the circuit available here, and also here. The version I made retained the switch to the base connection for the transistor under test from the first link and also included the diodes from the second link. I also changed some of the resistor values based on what I had available.

What appeals to me about this circuit is its simplicity and its ability to discriminate between NPN and PNP transistors. The circuit uses a multivibrator to create an alternating voltage source. This source is fed to two LEDs connected in opposite directions - so that each is lit only when the voltage is in the appropriate direction for that LED. As the capacitor values in the multivibrator are quite low, the AC generated is of high enough frequency that you cannot see the alternating nature of the LED lighting - but it is present nonetheless. Only one LED is lit at a time, the flash alternately at a high rate.

The transistor under test is connected across the pair of LEDs, and it is biased by the 1k resistor so that it will be turned on for only one of one phase of the AC cycle. NPN transistor transistors are only turned on when when the right hand side of the LED pair is positive and PNP transistors only when the right hand side of the LED pair is negative. When the transistor is turned on, it stops the LED with the same orientation as the transistor under test from lighting. Hence, a good transistor will result in one LED turning off, and depending on which transistor is turned off, the nature of that transistor (NPN or PNP) can be discerned.

The 4 diodes in the collector connection for the transistor under test mean that there will always be two diodes, dropping 1.2 volts, before the transistor under test is reached. This means that the transistor itself must drop less than 0.6 volts to turn off its LED indicator diode. If the 4 diodes were not present, a transistor which was not functioning as a transistor but just as a diode would appear to be a functioning transistor in the tester.

Here is how the astable multivibrator works:

When power is applied, capacitors C1 and C2 begin to charge through R2 and R3 respectively (the transistors are both off initially)

One capacitor, say C1, charging through R2, will reach 0.6 volts first and this will cause Q2 to switch on.

This causes C2 to continue to charge through R3 and after some time, the base of Q1 will have sufficient voltage on it to switch on.

When Q1 switches on, the positively charged side of C1 is taken to ground, meaning that the base voltage of Q2 is taken to a negative value switching Q2 off suddenly.

C1 continues to charge through R2 and soon it voltage reaches 0.6 volts, turning Q2 on again, and the cycle repeats.

And here is the circuit diagram.

Transistor Tester Circuit Diagram (click for a larger image - click the image again to return)

Posted by ZFS | Permanent link | File under: electronics