Driving relay with transistor question

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jdege 20 years ago

What I think I understand:

Picture trying to drive a relay with a Basic Stamp microcontroller.

The output pin on the Stamp won't sink or source enough current to power the coil on the relay. So we add an NPN transistor.

We wire a diode parallel to one of the relay coil terminals, to truncate the analog junk that the coils will generate. We wire the other coil terminal to the collector of the NPN transistor. The emitter of the transistor is wired to ground, and the output pin of the Stamp is wired to the base. When the output pin goes high, it saturates the transistor, current flows through the relay coils, and the relay throws.

But what if I want to add an LED indicating that the relay is thrown?

Driving an LED from an output pin is easy - LED + current-limiting resistor in series between the pin and ground.

But driving an LED and a relay from the same pin?

Where do I put the LED?

In series between the output pin and the base of the transistor?

In series with the relay coil?

In series between the emitter of the transistor and ground?

Between the output pin and ground, parallel with the transistor?

Seems to me that any of these would work.

What's the most usual practice?

And why?

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J

John Fields 20 years ago

No, since that would force you to push the current to operate the LED into the base of the transistor from the output pin of the BS

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J

Jeff Dege 20 years ago

In parallel to the coil?

Set R so that the LED gets only the current it needs. Depending upon +V and the internal resistance of the coil, might also need a resister in series with the coil to ensure it draws no more than it should.

Seems simple enough.

Thanks.

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B

Bob Monsen 20 years ago

A PIC will source 20mA or more from out output. That will really saturate the NPN. You can probably cut that back to less than 10mA using a resistor between output and base, and still maintain saturation. If you do this, you can just put the LED in series here.

Regards, Bob Monsen PATRICIA PAIWONSKI gave Ben Caxton the all-out kiss of brotherhood before he knew what hit him.

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R

Rich Grise 20 years ago

That's the one. :-)

No, you buy a relay that has a coil rated for +V volts. The resistance of the coil (and the applied voltage) is what determines its current.

Yup!

Cheers! Rich

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J

Jasen Betts 20 years ago

may reduce the drive takes drive current to the base of the transistor, will work if that's not critical.

will be passing all the relays coil current, could destroy the led,

will be passing all the relays current, may reduce VBE

may reduce the drive available to the transistor,

in series with a resistor, both parallel with the relay coil

it's least likely to make a critical difference.

putting it before the transistor limits the avalailable current or voltage putting it in series with the relay (or emitter) limits you to relays with low current coils.

in parallel with the relay it makes little difference as the relay is probably consuming more current than the LED etc...

Bye. Jasen

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Driving relay with transistor question

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