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.

    Russell Hedges

Tony Goetz wrote:

>Okay, for my first real do-it-myself PIC project I decided to make an
>electronic speed controller for my scratchbuilt r/c boat. Got it working with
>my first piece of code! Alright, it's only forward/stop/reverse, but it still
>works. After I get the whole system working I'll go back and do some PWM
>stuff.
>
>Well, about the best way to control a DC motor from a PIC is an H-bridge. So
>that's what I set about doing and finally tracked down some P channel mosfets
>to complete it. *however.* I tried an h-bridge circuit I found online, but it
>doesn't put out enough power to even turn over the motor. I know I'm not
>trying to pull too much current - the motor only uses around 200mA and the P
>and N mosfets are rated at 2.0 and 4.0 amps, repectively. Using only one of
>the mosfets, switching in only one direction, I have no problem running the
>motor. The h-bridge is only allowing 50mA. As if that's not confusing enough
>to me (probably only to me), LED's hooked up alone across the outputs can
>pull 100mA with no problem. The P-Mosfets are NTE 2381's and the N-Mosfets
>are IFR510's. The 2381 datasheet says it's "complementary to the 2380,"
>another N-channel mosfet. Do I have to use these two kinds together, meaning
>another trip to the nerd candy store? Fine by me, as long as it works! But if
>I can save money by using what I have, please tell me so.
>
>This is the circuit I'm using: <A
>HREF="http://www.cs.uwa.edu.au/~mafm/robot/blanch-h-bridge.html">MOSFET
>H-Bridge Schematic & Theory of Operation</A>
>
>I'm so close to finally having this thing in the water again (the boat, that
>is) and need only to solve this problem. Unfortunately, I'm only beginning to
>understand electronics so I'm really not good at troubleshooting. If anyone
>knows of a different, good, reliable H-bridge circuit out there I'd like to
>take a look. I'm sorry for my long winded posting......I'm just descriptive I
>guess. Or maybe just long winded. Thank you for your help.
>
>
>-Tony
>
>"In the beginning the Universe was created. This has made a lot of people
>very angry and been widely regarded as a bad move." -Douglas Adams, 'The
>Hitch Hiker's Guide to the Universe'
>
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