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.

Re: Indentifying transistor pinouts with a DMM
 

Before I got a component analyser I just used to use trial and error. There are only 4 common arrangements, CBE, EBC, BCE and ECB, each as PNP and NPN. If the transistor doesn't give a sensible hfe reading in one of these positions then it's a CEB/BEC HF transistor like an MPSH10, not a bipolar transistor at all, or faulty. Possible CEB types received further attention.

The Chinese analysers do make life a lot simpler though, despite sometimes getting it wrong.

Re: Indentifying transistor pinouts with a DMM
 

We used to test with an Avo8 (on ohms x100 i think) by finding the base lead then connecting the black (+ve on an Avo) and the red lead (-ve on an Avo) to the other two leads. A wet finger between the open base and the black lead would cause a 'reduction' in resistance if the black lead was connected to the c of an NPN. If little change was seen then the black lead was assumed to be connected to the e, and the connections could be reversed to confirm. I have never tried it with a DMM.
BC

Re: Indentifying transistor pinouts with a DMM
 

You can do this with a DMM too, though most have built in transistor sockets and an hfe test mode, so it's not really necessary.

Beware of faulty or intermittent contacts in the socket if using a cheap DMM.

Re: Indentifying transistor pinouts with a DMM
 

I have seen a trick used to quickly identify the C from the E with a meter, Though the meter must be a type with an Rx10k range and a 22.5V battery, like the Hioki A-10. It is based on the fact that the typical zener voltage of the B-E is around 7v for a silicon signal transistor, and usually much higher for the B-C. So the B-E conducts with both meter polarities on the B-E connections but only one way on the B-C connection.

However, I've never thought this was a good idea because each time the B-E is zenered, it slightly degrades the transistor's hfe, without altering the transistor's other parameters (the mechanism of this effect has been the subject of some good research papers by Motorola). I have experimented with this effect as a tool to hfe match transistor pairs, its very real.

I have seen it claimed (by Audiophiles) that due to coupling capacitors to bipolar OP amps, when the power is switched on and off, that the input transistors b-e 's get zenered, slowly degrading the OP amp performance. I haven't bought into this notion completely, but I guess its possible.

Re: Indentifying transistor pinouts with a DMM
 

I have also experimented with beta degradation unintentionally and it is indeed very real. I was attempting to use a 2n3904 as a varactor and zenered it. Next thing I know, it make its way back into the parts box and it wouldn't oscillate in a VXO. Ended up replacing the transistor and hey presto it worked. I did a spot check with a cheap DT830 DMM and the beta was around 40 compared to the average 190-250. Took me a long time to put two and two together, then I tried it again and it was reproducible.

Re: Indentifying transistor pinouts with a DMM
 

I think what this means, for testing signal transistors, is to avoid zenering the B-E junctions ever. Also, for vintage germanium signal/RF transistors be wary of how much base current is passed by the test system/meter as they are more fragile and have lower max current ratings than typical silicon type. There was once a recommendation not to test these on a standard meter on Rx1, but it might pay to look up the specs of the actual device.Though with power capable germanium devices, AC128/188, AD149 etc etc, I don't think any multi-meter I know of could harm them.

Re: Indentifying transistor pinouts with a DMM
 

I'd always thought low ohms ranges were more of a problem than high resistance ranges.

An AVO 8 on the low range is effectively a 1.5V battery with a 20ohm series resistor, giving a short circuit current of 75mA. Some far-eastern meters have a low range with 3V and 20ohm drive, a current of 150mA, and can blow the filament of Dx96 valves!

An AVO 8 on its high resistance range uses a 15V battery, but with a 200kohm resistance, I'd always thought the current (75uA) too small to harm most devices. Does anyone have any information on the current levels which cause degradation when zennering a B-E junction?

Stuart