Tim Thompson wrote:
>Hello,
>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.
>(-) 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..but it dosnt apear the
>pic can source enough to turn it off...what am I doing wrong? Probly
>something stupid..anyway
>

Tim,

You aren't accidentally setting the pin to "input" rather than to
"hi", are you? Does you code possibly have STATUS.RP0 = 1 ?

When you pull up the base of a pnp wired like this, you are
actually turning off the transistor, so the base current goes to
zero. So, source current is not a problem.

Actually I would have thought you might have a problem with sink
current instead. Transistor current gain decreases for higher
collector currents, and may not be 100 for this case. You may not
be able to pull a full 90 mA of collector current with only
~ 4v/4.7K = .8mA of base current. May need a smaller Rbase,
maybe 2.2K, in this case.

- dan michaels
======================

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