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confused about PNP Transistors

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Screech

New Member
I realise that in a npn transistor, the collector and emitter contacts are normally open until current enters the base and then the collector/ emitter closes..bla bla bla.


but, for the PNP transistor, is the emitter/collector contacts closed untill the base is has current flowing though it,,,,,bla,bla bla?????????


I have two PNP transistors at home, and one hase the collector/emitter contacts normally open.
My other PNP(darlington) has the two contacts normally closed.

I realise that npn and pnp transistors are wired up opposite.



Thanks for your replies.
 

Travo

New Member
Let me see if I can keep this thread alive,

NPN transistors require a .6-.7 Volt on the base more than the emitter to turn the transistor on and allow emitter to collector to conduct.

PNP transistors require a negative .6-.7 Volt on the base less than the emitter voltage to allow collector to emmitter to conduct.

This is negative to positive current theory. If you want positive to negative theory then collector to emmitter NPN and emmitter to collector PNP.

If this is wrong let me know, I've been out of my repair school for over 7 years now.
Glad to find a board not dominated by engineers who think they are God's gift to electricity.

8)
 

Klaus

New Member
The way I was taught that (many years ago) is to look at the P standing for positive and the N standing for negative.
Now, with PNP, since the middle letter is always the base, it has to be negative with respect to the positive emitter next to it, to conduct.
For NPN it is exactly the opposite, the base has to be P = positive compared to the emitter to conduct.

Screech compared this with a switch (n/o and n/c contacts), that is misleading as a switch is bi directional whereas a semiconductor junction passes current only in one direction.

One has to make sure when testing a transistor with an analog meter in the Ohm scale the meter leads are of the correct polarity it is usually the opposite than that indicated on the meter), amongst other precautions such as the maximum current and voltage of the particular range chosen.
With a digital meter one needs to select the semiconductor testing range as the Ohm range may have insufficient voltage to forward bias a junction. Digital meters give a direct indication of the voltage across the junction when it is turned on.

With regard to your darlington, if you measure a dead short (in both directions) across the E and C terminals it is kaputt.
Klaus ( who is not an engineer

:wink: )
 

Screech

New Member
Thank you very much guys.
I've been searching the net for weeks ,trying to find the answer.

OPPOSITE TRANSISTOR?
Is there a transitor that is normally open (flowing full current) until a small current flows through the base that will then reduce current at collector/emittor?
 

Roff

Well-Known Member
Screech said:
<snip>OPPOSITE TRANSISTOR?
Is there a transitor that is normally open (flowing full current) until a small current flows through the base that will then reduce current at collector/emittor?<snip>
No, but you can prebias a transistor and then turn it off by "stealing" the prebias. See the examples below. You can do the same with a PNP by changing the polarities of the two voltages.
 

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Screech

New Member
Thanks for answering my Question, Ron.

:idea: I think I'll call my opposite transitor the SCREECH TRANSITOR after I invent it.
 
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