Driving a DC motor /w 16f84a

[Lilmanmgf]

FETs will improve the circuit. Choose "logic level" FETS with a low ON resistance. If you use an N-channel FET (recommended), put it in series with the GROUND side of the motor. You won't need a resistor between the PIC output pin and the FET gate, although a 1-10K resistor between the gate and 5V will improve the circuit because it will insure a full 5V on the gate when the PIC pin is high. The ON resistance of the FET can be as low as 20 milli-ohms. That means that if your motor draws 2A, then the voltage drop across the FET will be 40mV and the dissipation will be .040 * 2 = 80mW. It will barely get warm - even without a heatsink.

What FET would you recommend in this application?

[Charles Linquis]

Sorry, I'm an engineer, not a secretary.

[Charles Linquis]

But.. Just to insure you don't get it hooked up wrong.

Source to GROUND
Gate to PIC
Drain to motor (-)
Motor (+) to 5V

1-10K resistor from Gate to +5

[Lilmanmgf]

I have a TRF510, but I was unable to get it to work. I'm really not sure how to read mosfets. It says the gate threshold voltage is from 2v to 4v. I'm assuming this means at least 2 volts is required to turn the mosfet on. In my current setup the PIC is powered by a 3 volts, so shouldn't this be sufficient to turn it off? Perhaps I unknowingly fried it previously.

[skimask]

It says the gate threshold voltage is from 2v to 4v.

If we're both reading the same datasheet, then you may also notice that in this case, it says that the gate voltage (Vgs) is equal to the drain-source voltage (Vds) and the current (Id) is only 250uA.
In your case, you're probably trying to draw too much load without enough Vgs (see Figure 5,6,7 of the datasheet).
You would want to find a 'logic level' n-channel enhancement mode mosfet with enough current capability at low gate threshold voltages.
I use an STN2E10L (sot-223 package) in most of my stuff. I can pull over 5 amps thru it @ 5v using a 5v (which ends up being about 4.5v) PIC pin to drive it.

[Charles Linquis]

An International Rectifier IRF3706 would be a good choice.

[Archangel]

Hi Lilmanmgf,
This Sumobot you are building, Is it going to have reverse and forward, or only forward and stop? I ask because if you want it to have reverse, then an H-Bridge is what you are looking for. If it only goes forward, you might do as well or better with a fast acting relay, and lose the heatsink, MOSFETs and some of the confusion, you would likely still need a small switching transistor and a snubber diode. I know these bots are pretty small, and the space and weight saved might be useful for something else.
JS