Wouter van Ooijen wrote:
>>> Divide 3.5V by that current and you've got the base resistor.
>>
>> 3.5V?
>
> rough guess: 5V - pic drop - Vbe.

Vbe is pretty reliably limited to about 700mV.  That leaves only the PIC
drop.  Unless you are getting near the 25mA max limit, it's not going to be
much.  5mA assuming minimum guaranteed beta of 50 allows for 250mA relay
coil current.  That's going to cover most relays driven right from a PIC.
If you need much more than that, you should probably consider using a FET
instead of a NPN anyway, and you've got other power issues to think about.


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