need help with PIC to Hexfet for stepper

hi,
I am trying to design a dirt cheap stepper driver for my unipolar steppers (for use in a repstrap). I want to use a PIC to control the stepping sequence and have it drive 4 hexfet, irf510. My question is will the irf510 work ok, my steppers pull 4V at 1.1A. I was going to run them from a 5V source. There was a similar discussion 'Cheapest motor drivers possible' where nophead mentioned having to use a small signal transistor or buffer chip between the PIC and hexfets. I don't know enough about electronics to properly design this setup. Could I get some advise. I just want to make sure I can turn the hexfets on properly and that they can drive my steppers ok.

The data sheet for the irf510 is here:
www.irf.com/product-info/datasheets/data/irf510.pdf

Thanks,
Alchemistdagger

If you look at fig 1 then with 5V gate drive you only get 1A at 25C, so they won't quite turn on properly. That will cause them to heat up and fig 2 shows they conduct more when hot so just about make it!

Even when fully on they have a resistance of 0.54 Ohm, which is about 100 times worse than a modern device. Better to use a more modern FET with a lower gate threshold.

I would use something like this [uk.farnell.com]

The gate threshold is 1.5V rather than 4.5V, so will be fully on with a logic drive. When it is on the resistance is 0.08 Ohm so it barely get warm with 1.1A.

If you want to use the IRF510 then you really need to amplify the output of the PIC up to 12V.

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[www.hydraraptor.blogspot.com]

thanks for the explanation. I took your recommended criteria regarding the low gate threshold and Drain-source resistance and found a couple of possible chips.

IRLZ24
[www.jameco.com]

IRF7303
[www.jameco.com]

The IRLZ24 is about the cheapest thru-hole mosfet I could find ($0.54) with decent resistance of 0.06R and a gate threshold of less than 2V. Its easier for me to deal with since it is in a TO220 package.

The IRF7303 has about the same properties of 0.05R and Gate threshold of 1V. It's a better value ($0.35) since there are 2 mosfets in one package but I havn't ever dealt with surf mount before. The 7303 has a lower wattage that it can handle compared to the TO220, less than 1 W but by my calculations I shouldn't be generating more than about 0.1 W per mosfet (I^2 * R = 1.1A^2 * 0.5 = .06W).

Do these two devices sound reasonable or am I still missing something.

Thanks,
AlchemistDagger

Yes they both seem suitable.

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[www.hydraraptor.blogspot.com]