Controlling a motor with a transistor and a PIC

I had looked at a very similar diagram (similar to the most relevant one in that link) and tried the diode but it didn't help. What I had neglected to see is that I would need a resistor inbetween the chip and the transistor. The thought had actually crossed my mind earlier, but when I tried it I was using too large of a resistor so it didn't work. Can't get enough of those current limiting resistors.

Thank you, Brick

purple_stars wrote:

Your biggest problem is that the 2N3906 is the wrong polarity for the circuit shown in your link. You need an NPN not a PNP.

If it's a really small motor use a 2N3904 instead. Use a 1k base resistor and a 1N4001 diode across the motor. A 2N3904 can only handle about 200mA max, so if your motor draws more than that you'll probably need an NPN darlington power transistor.

You will be much better off to use a power mosfet as shown at the bottom of page 7 of the picaxe manual. The purpose of the diode is to conduct the current generated by the magnetic field in the motor as it colapses when the motor current is switched off. Without the diode a fairly high voltage is generated by the motor when it's switched off and that high voltage can damage the mosfet and maybe cause the pic to get all confused. The diode needs to be a small pwer type such as a

Any type of N channel logic level mosfet rated to handle the voltage and current requirments of the motor will work.

Logic level just means that the mosfet will be have it's lowest on resistance with only 5v between the the source and gate pins. If it's not a logic level mosfet it will require about 10V to full turn on.

I may be wrong, but the IRF530 shown being switched directly from the pic pin is poor choice because the IRF530 requires more than the 5v output from the pic to fully turn on.

It's also good practice to insert a low value resistor between the pic pin and the gate of the mosfet. A 10ohm or so would be fine. Locate the resistor close to the gate pin of the mosfet. The resistor is just to help prevent high freq oscillations as the mosfet switches and the circuit will work without it, but may generate rf interference as the mosfet switches. The point maked +6 can be any voltage needed for your particular motor.

Using either an NPN bipolar transistor or an N channel mosfet will mean that the motor will be turned on when the pic pin is high.

Mike

I think he said he's using 3 volts as a high - rules out the mosfet unless he level converts. A TIP 120 would be fine assuming a supply for the motor a couple volts higher than it needs. Vce sat is around 2 volts as I recall. If the motor draw is small enough, one of the hot zetex bipolars would work, maybe even a transistor scrounged from a disposable camera.

Ed

Oops, I missed that 3v thing, but yeah, something like the ZTX1051 should work up to a couple amps.

Mike