Transistor Help

[Alex Hunter]

Hello. I am building a circuit where a PNP transistor is used to switch
a 12v relay. I was using a general purpose transistor but the 12v was
enough to pass through the transistor even when the base was not
grounded.

My question is what kind of transistor should I use that will work with
12v? I know very little about different kinds of transistors, so any
help would be appreciated.

Thanks,
Alex Hunter

 

[Thomas C. Sefranek]

"Alex Hunter" <alex.hunter@comcast.REMOVEnet> wrote in message
news:100120040318188198%alex.hunter@comcast.REMOVEnet...

Hello. I am building a circuit where a PNP transistor is used to switch
a 12v relay. I was using a general purpose transistor but the 12v was
enough to pass through the transistor even when the base was not
grounded.

My question is what kind of transistor should I use that will work with
12v? I know very little about different kinds of transistors, so any
help would be appreciated.

Thanks,
Alex Hunter

The classic way is to Ground the relay coil with an NPN transistor.

If you insist on a high side switch the PNP should have a pullup
Base to emitter resistor. You ARE using the emitter to the +12 supply
right?
All transistors have leakage, so if your relay coil is sensitive enough,
and the transistor is leaky enough, you can't shut it off!.

I prefer Power FETs, so a P channel FET would do this job as well.


--
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| __O Thomas C. Sefranek WA1RHP@ARRL.net
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[Watson A.Name - "Watt Sun]

In article <100120040318188198%alex.hunter@comcast.REMOVEnet>,
alex.hunter@comcast.REMOVEnet mentioned...

Hello. I am building a circuit where a PNP transistor is used to switch
a 12v relay. I was using a general purpose transistor but the 12v was
enough to pass through the transistor even when the base was not
grounded.

My question is what kind of transistor should I use that will work with
12v? I know very little about different kinds of transistors, so any
help would be appreciated.

Thanks,
Alex Hunter

If the 12V (hopefully this is DC, not AC) is passing thru the
transistor, even tho the base is not connected, then you may have the
transistor connected incorrectly, or else the transistor is breaking
down. Whatever the case, it shouldn't allow current without the base
being connected. I would remove it from the circuit, and not use it
but replace it with a new transistor. Chances are that it is damaged.

You should have a resistor in series with the base lead to limit the
current, and it's possible that if you did not have this, the
transistor may have been damaged by excessive current.

Also, you should have a 1N4002 rectifier across the relay coil, with
the cathode or banded end towards the positive supply. If you didn't,
then the relay may have generated high voltages that could damage the
transistor.

--
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