Tim, I wopuld put a 10K ohm resistor from base to Emitter, and change the 4.7K base resistor to 1K. The 10K will pull the base high unless it's driven low. This will tend to hold the transistor off. When the PIC pin is taken low, the 1K resistor will allow 5 mA of base current (Approx) which should be more than enough to drive the transistor on. As an aside, what is the HFE of the transistor you are using. Is it greater than 100? My guess is around 150 to 200. If this is so, then the resistor values I suggest should work fine. Let us know if this helps you out. Regards, Jim ----- Original Message ----- From: Tim Thompson To: Sent: Tuesday, June 12, 2001 7:59 PM Subject: [PIC]: Problems using PIC to controll small fan > 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 > > Thanks in advance, > Tim Thompson > > > > - > Remember, 'kill' doesn't kill processes, users kill processes. > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.