Driving a DC motor /w 16f84a

[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

[Lilmanmgf]

Yeah, it just needs to go forward and stop.

[ALFRED]

If you find MOSFETs confusing you may wish to use an H-Bridge IC. The SN754410 is a full H-bridge in a DIP package. I have successfuly used this chip to drive two motors forwards and backwards on a small robot. Heat dissipation has not been an issue. The operation of this chip is extremely easy to understand and the inputs can be driven directly from the PIC.
If you would prefer to put your own H-bridge together you should definantly use MOSFETs. If you are having trouble with the on gate voltage from the PIC being to low you may try to connect a pull-up resistor from the gate of the MOSFET to the positive supply (this is assuming you are still running off of 9v). The pull-up would bring the gate to a full 9v when the PIC signal goes high but when the PIC ouput goes low the gate would be driven low. You should test your ideas out on a breadboard and find what works best.
Hope this Helps.