Date: Tue, 31 Jan 1995 21:10:17 -0500 (EST)
From: Machine Media <media@pcnet.com>
Subject: Re: _VERY_ Basic question about 2N2907 and 2N2222 Transistors...
Cc: analogue@magnus.acs.ohio-state.edu

> Yesterday, I purchased some 2N2222 ( NPN ) and 2N2907 ( PNP ) transistors...

uh-huh . . .

> Unfortunately, the things are round in shape, and I have no idea which
> leads from them are the emitter, base, and collector....
> 
> the BOTTOM view of the 2N2222 is like this :

(use your imagination :)

> In general, HOW DO I TELL the emitter, base, and collector apart on these
> Motorola transistors?

OK, unless someone at Motorola is a serious crackhead, lets assume that 
the base is in the middle. I have no idea what a 2N2222 is, but  
assuming they're your garden variety bipolar junction transistors, the 
collector/emitter location can be determined in two of several ways:

1) This is the easiest, stick the leads into the Hfe meter or transistor 
checker found on many DVM's, it should flash "cbe PNP" (or whatever) and 
then display the beta value.

2) If all you have is a regular Ohmmeter ( <-- spelled right, but still 
looks funny :) you can preform the following battery of tests:

A. Assuming it's PNP:
   With the positive (red :) lead of the meter on the collector and the 
negative lead on the emitter the resistance should read quite high. With 
the positive lead on the emitter and the negative lead on the collector 
the resestance should read low.
   With the positive lead on the base, resistance measurements from the 
base to the collector, or from the base to the emitter should read high. 
With the negative lead on the base, both restances should be low.

B. Assuming it's NPN
   Edit the above paragraph, replacing the word "positive" with the word 
"negative." Replace the word "negative" with the "word" positive. Repeat 
test A.

C. In either case . . .
   If both the foward and reverse readings are low, or nearly the same,
it's fried. If they are both very high, it's open. Start with the mid ranges 
on your ohmmeter as both the RX1 (too much current) and the megazorn ohm 
range (too much voltage) can fry smaller transistors. Typically, your 
garden variety transistors have a ratio between the foward and reverse 
resistances of about 100 or so.

I hope this has been helpful :)  

PEACE OUT :)
MARK

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