UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Indentifying transistor pinouts with a DMM

TonyDuell · 6th Jun 2017, 11:23 am

If you have one of those cheap component testers you don't need this, but if not, I have found this to be a useful way of working out the pinouts of a transistor using most digital multimeters. You need the diode test feature and the Hfe transistor test feature. I have seen the latter claimed to be useless (and to be fair it is not a good test of a suspect transistor), but here is a use for it.

Start by using the diode test function, both ways round on each pair of leads (6 tests in all). You should find it tests like 2 diodes with one lead either a 'common cathode' or 'common anode'. That lead is the base, in the first case it's PNP, the second is NPN. This is just the common '2 junctions' test for a transistor, of course.

Now use the Hfe range. You know which the base lead is, connect that to the 'b' hole of the test socket. Try the other 2 leads each way round in the 'e' and "c' sockets. One way will give a much higher reading than the other (for the 2N3906 I've just tried it was around 200 one way, 3 the other). The connection that gives the higher reading is the right one, you have the collector in 'c', etc.

6th Jun 2017, 11:23 am	#1
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Indentifying transistor pinouts with a DMM If you have one of those cheap component testers you don't need this, but if not, I have found this to be a useful way of working out the pinouts of a transistor using most digital multimeters. You need the diode test feature and the Hfe transistor test feature. I have seen the latter claimed to be useless (and to be fair it is not a good test of a suspect transistor), but here is a use for it. Start by using the diode test function, both ways round on each pair of leads (6 tests in all). You should find it tests like 2 diodes with one lead either a 'common cathode' or 'common anode'. That lead is the base, in the first case it's PNP, the second is NPN. This is just the common '2 junctions' test for a transistor, of course. Now use the Hfe range. You know which the base lead is, connect that to the 'b' hole of the test socket. Try the other 2 leads each way round in the 'e' and "c' sockets. One way will give a much higher reading than the other (for the 2N3906 I've just tried it was around 200 one way, 3 the other). The connection that gives the higher reading is the right one, you have the collector in 'c', etc.