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Topic: PNP transistors... how to use them? (Read 4 times) previous topic - next topic

May 17, 2011, 12:48 pm Last Edit: May 17, 2011, 01:11 pm by mcookieman Reason: 1
Hello all,
I understand how a NPN transistor works and what it does, but I can't figure out how to use a PNP transistor and what it does. At first, sorta due to only having really basic electronics at school, I thought it did the opposite of a NPN and when the base was low the output would be high and vice versa. But after some research this doesn't seem to be the case. I tried using the Java circuit sim/visualiser/thingymajigy to help explain it but I'm still confused. Could someone explain what it does please?

Thanks in advance.

edit: I want to make a really really simple intervalometer for my camera, but do not have any NPN transistors handy.

Grumpy_Mike

You just use a PNP like an NPN transistor but the power rails are swapped over.
It is an upside down transistor.
Quote
I thought it did the opposite of a NPN and when the base was low the output would be high and vice versa

Yes it does but remember that the emitter instead of going to ground has to go to plus. And the collector instead of going through the load to plus goes through the load to ground.

MarkT

If you use the convention that positive voltages are always above ground and negative below ground, then the conversion from NPN to PNP is exactly like a mirror flipping the circuit upside-down - positive supply becomes negative.

In a circuit where 'ground' is the negative rail then the PNP treats the positive supply in the same way an NPN treats ground.  For instance a PNP can switch a load on the 'high side' whereas an NPN switches the ground (low side) of the load.

Hope that less confusing rather than more confusing!!  Basically in a PNP all voltages and currents swap sign from the NPN case.

There is a similar relation between n-channel MOSFETs and p-channel MOSFETs BTW
[ I won't respond to messages, use the forum please ]

eddiea6987

Hey guys , excuse me for kicking up settled dust, i understand how the PNP switches the high side which means to go about actually switching we apply negative to the base...but how does one go about that? let me explain
I have an arduino , and  i would like to pulse width modulate to vary the speed of a small dc motor.
and yes  i do have NPN transistors and i have done it like this just fine  , but if in reality a PNP can replace an NPN then how do i pulse width modulate with a PNP
using the arduino?
If my understanding of PWM is simply ranging from 0-5 volts,lets say on pin 9 , that would be feeding the base of an PNP positive charge , which is not what i need.
so how do i go about this.
I could print the Arduino logo on a box of cereal and sell it as "Arduin-O's"

Grumpy_Mike

No it is only a negitave voltage with respect to the emitter. As the emitter is sitting at +5V then applying a 0V signal is like applying. -5V signal.

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