simple transistor question???

rr

is it a bad idea to put a transitor on the positive side of a circuit

12volts+

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Transistor

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led--led--led--led

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Ground

it seems in most designs the transistor on on the ground side...

Chris Savage

Hello,

·· It depends on which type of transistor is used and how much current it can handle.· In your example you would need to be using a PNP transistor, and it would require a LOW from a Stamp I/O PIN to turn it on.· Of course, you should have a 1K resistor between the base and the Stamp I/O PIN.

·· If you used an NPN transistor, the LEDs would be on the positive side of the transistor, since the emitter would be tied to ground.· In this case, a HIGH on the Stamp PIN would turn it on through the 1K resistor.· Again, the transistor needs to be able to handle the voltage and current.


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Chris Savage
Parallax Tech Support
csavage@parallax.com

pjv

Chris,

I know nothing about Stamps (never seen one) but in the case of the PNP transistor example, the Stamp would need to pull the base down as you said. In order to be able to turn the LEDs off, the Stamp would need to be able to switch its output up to 12 Volts supply rail, or alternately, have an open collector style output.

Something to be aware of; perhaps the Stamp meets those requirements.

Peter (pjv)

Chris Savage

Peter,

·· You are correct...Some minor modifications would need to be made to handle that, which is outside the scope of this discussion.· Suffice it to say he should use the NPN transistor.




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Chris Savage
Parallax Tech Support
csavage@parallax.com


Post Edited (Chris Savage (Parallax)) : 4/29/2005 1:59:36 AM GMT

KenM

rr,

Using an NPN is far more common than using a PNP.

Is there a reason you must source the LED's?

It is possible to use a PNP, but it is not so easy using 4 LEDs. If you use 5 LEDs the attached circuit will work (I tested it).

But there is no advantage to the posted circuit. Personally, I would go with the NPN as Chris suggested.