At 12:34 PM 2/5/2006 -0500, you wrote:
>   I am using a 2n3904 to drive a 12V relay off a 16F628 pic at 5V input, I'm
>not sure how to figure out the resistor for the base?

Assuming the coil current is under about 40mA nominal (>300 ohm coil), a
base resistor of 1K or 2K should be okay. Use a better transistor if it's
a higher current coil, switching a relay coil is actually rather hard on the
transistor. Some automotive 12V types have VERY low resistance coils.

A diode across the coil has been suggested, and is the simplest way of
handling the coil inductance. A 1N4148 is fine for any relay you'd be
likely to use.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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