> I'm attempting to have a PIC 16F877 in my project controll a small cooling
> fan. The fan is a 5V 0.09A dc brushless type, and is connected to a
> transistor which actualy drives the fan, it is connected like so:
>
> Transistor is a typical PNP 2N3906..
> Base through a 4.7K resistor to a PIC I/O pin,
> Emitter to +5,
> Collector to + side of fan, with a 4.7K resistor pulling to ground.

I see no reason for this resistor, although this isn't causing your problem.

> (-) side of fan to ground, small diode between the +/- side to block any
> return voltages.
>
> By setting this I/O pin (PORTE,2) low, the fan comes on as expected.
> However i cannot turn the fan off by setting the port high! If i jumper +5
> directly to the i/o pin manualy the fan goes off

Check the +5 supply.  Is the fan causing it to droop?  Glitching it?  Is
everything properly bypassed?  Keep in mind that the fan startup current
will be much higher than the continuous current of 90mA.

A properly operating PIC will have no problem driving a 4.7K resistor with
the other end anywhere from 0 to 5 volts.  What is the voltage at the PIC
pin when the fan won't go off?

Overall, I think this is either a power supply problem, or more likely a
software bug where the port latch is getting set low when you don't expect
it.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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