Testing NPN Germanium Transistors

[JJPJ83]

Hello. I need help evaluating an NPN Germanium Transistor (AC127).

I've read the Geofex article. And I don't get it. There's so much to read through and I ain't smart.

In the Geofex article, it says to reverse the polarity and DMM leads. When I do this, I get no reading. Do I also need to flip the transistor? Or only flip the transistor? Does E or C get + or - ? What about the 2M2 base resistor, to + or - ? I've experimented with multiple variations of this configuration, and have finally got a reading that is acceptable, but my layout is not what the "internet" has been telling me and so I don't trust the readings. I understand "room temperature" and "handling" factors. Ignore that. I just need the polarity, NPN transistor orientation, and resistor connections. Then the algebra for what to do with the readings. I'm looking for stereo instructions.

Thank you.

[mth5044]

'just reverse the battery polarity and the meter leads in the tester diagram'

So the negative terminal of the battery will go to the emitter of your AC127, and the positive will now connect to the two resistors. For the meter leads, switch them to the opposite sides of the resistor. You do not need to flip the transistor during readings, but you do need to make sure you have the pinouts correct. 

[PeterPan]

Hello. I need help evaluating an NPN Germanium Transistor (AC127).

I've read the Geofex article. And I don't get it. There's so much to read through and I ain't smart.

In the Geofex article, it says to reverse the polarity and DMM leads. When I do this, I get no reading. Do I also need to flip the transistor? Or only flip the transistor? Does E or C get + or - ? What about the 2M2 base resistor, to + or - ? I've experimented with multiple variations of this configuration, and have finally got a reading that is acceptable, but my layout is not what the "internet" has been telling me and so I don't trust the readings. I understand "room temperature" and "handling" factors. Ignore that. I just need the polarity, NPN transistor orientation, and resistor connections. Then the algebra for what to do with the readings. I'm looking for stereo instructions.

Thank you.

I don't know what article you're referring to (link?) but i can help with some general info.

So for a transistor, you should be able to pass positive current from the bass (B) to either the collector (C) or emitter (E). If you have a DMM that has a diode setting, you can see this by connecting the positive (usually RED) lead to B and the negative (BLACK) to C or E. For a germanium transistor you'll read between 2 and (at most) 4 volts, for silicon it would be between 5 and 7 volts. If you reverse the leads, you should not get a reading. the exact opposite of all this would happen with an NPN transistor. And it doesn't hurt to do a leakage test with the DMM on a resistance setting , measuring from C to E and E to C, with the base open. It should be very high, and a germanium may have a little more measurable leakage than a silicon.

Now if your DMM does not have a diode setting, then you'll need to supply your own battery, and a resistor. You'll want to have at least 3V , so a pair of AA cells should do fine. A resistor value of about 4.7K should work OK for most transistors. So in this case, for your NPN, the battery (3V) can go to the resistor, from there to the transistor B terminal, and then the battery - will go to either the C or E, depending on which side you're testing. You would then use your DMM on a DC voltage scale between the B terminal and battery -.  Again the meter should read from about 2 to (at most) 4V. Reverse the battery, and you should see the full battery voltage (3V in this case) instead.

[JJPJ83]

Got it. I have the AC127 NPN Transistor in place. Emitter (red dot) to 9v-. And B / C to 9v+ via respective resistors. I have re-checked the resistors and they measure 2M2 and 2.74.

DC: 9.38v.
1st reading: 0.057v.
2nd reading: 0.114v.

Leakage = 0.057v / 2.472 = 0.023ma Leakage.

Gain = (0.114 - 0.057) x 100 = 5.7hfe.

Is this math correct?

[JJPJ83]

*Edit. Resistors + 2M2 and 2.472k.

[JJPJ83]

Update: AC127 Red dot is Collector. Not Emitter.

I'm a dummy.

Thanks for your help.