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Transistor Question


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It is kind of tricky.
The problem is that the test it's self could destroy the device depending
on it's design.  One method is to compare the breakdown voltages of
the E to B diode VS. the C to B diode.  I would have to break out my theory
books but I suspect (based on the physical model most NPN/PNP devices use
that the C to B diode will go into a ziner behaviour at a voltage lower than the
E to B diode.  THe problem is that if you are trying to test a darlington, or a JFET
or any other type device you will end up getting no where at best.
(You might end up in a smoke filled room at worst)

If you can read the part number off the part then looking up the datasheet is the best method.
If you can't find the datasheet, Post the numbers here and someone will gladly post your answer.

When all else fails.  Build a very low current circuit (High Ohm resistors) and Guess.
If the device doesn't work one way, flip it around and try another way.


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I understand his question in another way

may be he ask like this

if he has a transistor on the table and he wand to identify its terminals and type

use your digital multimeter diode function, first assume your trans is npn type, then put the red wire in one leg and the black on another leg .... repeat this untill you have a reading with the red on one lege and the black one time on the 2nd and then on the 3d
then the leg in contact with red is base and the others are C and E the bigger reading is E and the smaller is C

if u can't get any reading then assume it is pnp and repeat the above with the black first to one leg assumed base....

may be u hardly understand me but i want to help

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Hi again Walid,
About how to find the Emitter and Collecter, when I used the diode function on an NPN  transistor, I found out that the test worked, but the collecter and the Emitter only had a difference of about .004 V. Is this suppose to be that small?

The B-C had about .654V
and the B-E had about .657V.


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yes it is very small like that

I'm pleasure, it is the first time i can help a friend in this great community

good luck truf

always I get all my project's parts from my old junk so i check them especially the transistors, you can say that i check more than 5000 transistors in that way and always the difference be that small.

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thats what I said.

No you didn't.

IF you reverse bias test the transistors, the Collector to base will be LOWER
than the Emitter Base.

No it isn't.
The collector-base is almost always reversed-biased in circuits and has a high breakdown voltage.
For a 2N3904 it is 60V. The emitter-base is not usually reverse-biased in circuits and has a max rating of only 6V for a 2N3904.

SHesh, I not wrong ALL the time.

Hee, hee. ;D
No, just most of the time! ;D
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