Go Down

Topic: PNP Transistor not 'shutting off' when increasing voltage. (Read 1 time) previous topic - next topic

Docedison

A PNP transistor is effectively a diode from base to emitter ( base cathode ) and pulling too much current from the base WILL destroy the transistor, Short the base to ground and the transistor WILL explode or shut down the power supply, in either case the transistor is toast. ALL PNP transistors need some method of current limiting (A resistor) in the base circuit. If it was an NPN directly pulling the base down, likely the NPN is toast as well.

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

Grumpy_Mike

Then you are missunderstand what I was saying. Can you draw what you are doing.

Inevitableavoidance


A PNP transistor is effectively a diode from base to emitter ( base cathode ) and pulling too much current from the base WILL destroy the transistor, Short the base to ground and the transistor WILL explode or shut down the power supply, in either case the transistor is toast. ALL PNP transistors need some method of current limiting (A resistor) in the base circuit. If it was an NPN directly pulling the base down, likely the NPN is toast as well.

Bob


To be honest, i don't quite see how this is relevant..?

Also, when not connecting anything to the transistor except for 12v on the emitter and the collector to the 'anode' of the led strips, i measure about 11v from the collector to ground..

Grumpy_Mike

Quote
Also, when not connecting anything to the transistor except for 12v on the emitter

Transistors are current devices. Do not leave the base floating, the results you see will not make sense.

Inevitableavoidance

#14
Oct 15, 2012, 12:52 am Last Edit: Oct 15, 2012, 12:54 am by Inevitableavoidance Reason: 1

Then you are missunderstand what I was saying. Can you draw what you are doing.



My initial setup was:

Leds
|
|
Collector

Base ------2k5-----ULN2803 output

Emitter
|
|
18v


What i understood you meant was:

Leds
|
|
Collector

Base --- 2k5 --- ULN2803 output
|                      |
4k7                    |
|                      |
Emitter ----------4k7
|
|
18v

(edited, misdrew something)

Go Up