Russell Hedges wrote:

<<     The on resistance of the IFR510, with a gate voltage of 10 volts, is
 .54 ohms.  What is the supply voltage?  In any case, I think you want an
 FET with lower on resistance.  All FET's with lower on resistances.  Low
 RDS-ON is the parameter you need to worry about most.  Higher current
 FET's (with much lower rds-on) should source/sink more current through
 your motor.  And they are amazingly inexpensive. >>


I hadn't paid much attention to the Rds(on) of the 2381 when I bought them -
I was in a hurry since the store was closing soon. I grabbed the only
P-channel mosfets I could find after a quick scan of the rack. Looking at the
datasheet now, they do have a high Rds(on). 6ohms, according to NTE. But
still, I could get them to switch without problem out of the circuit with
less power than the h-bridge was giving them. As it happens, late night
thinking last night solved the problem, barely an hour after posting the
problem that has been with me for days.

The problem seems to have been in the transistors that turned the mosfets on
from the PIC. When connected exactly as the schematic shows (link at bottom)
the mosfets aparently wouldn't turn on all the way. The design uses one
transistor to turn on the P-channel mosfet, which would then turn on the
corresponding N-channel mosfet. Perhaps I didn't have enough power running
through the bridge to completely turn on both mosfets, but after hooking up a
transistor to each mosfet, it worked fine. Thanks for your input.

http://www.cs.uwa.edu.au/~mafm/robot/blanch-h-bridge.html

-Tony

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