Paul,
Your schematic is a basic h-bridge and would work fine. One note is
that it has parallel 19amp p-channel mosfets on the high side (to
somewhat match the 50amp n-channel mosfets on the low side). You
could remove the extra p-channel mosfets as long as you stay within
the current ratings of the p-channel mosfets. About the current
ratings of the components. At first glance it would seem these
components are vastly overrated for the motors we want to drive. In
fact they may be underrated. The stated current ratings for mosfets
are usually the max rating for a period measured in milliseconds and
the manufacturer is assuming that's with sufficient heat sinking
installed. If you dig into the spec sheets of these componets you
will find the "real world" current ratings are always much less than
the advertised ratings. You will need to build the circuit to handle
the stall current of the motor you will be using. On the voltage, the
components you will use are marked with a maximum voltage. As long as
you stay under your lowest rated component you should be fine.
Mosfets are very sensitive to static and are easy to destroy just by
handling them improperly during assembly so order extra. You can also
assemble an h-bridge using dc solid state relays (SSR's) simulare to
what Joe Summer did with his controler on his Hetzer.
http://rctankcombat.com/articles/speed-control/084Hbridge.jpg
SSR' are nice because they are designed to be abused. Instead of
using the big Crydom SSR's you could use smaller pc mount SSR's like
these:
http://www.alltronics.com/cgi-bin/item/24B003/search/Grayhill-70M-ODC5-Relay
The only issue with the SSR's is that they weren't really designed
with high speed switching operations in mind so may not be able switch
at the frequency that the hacked servo is switching at. We know the
Crydom D1D40's will switch reliably to a certain frequency but that
was proved by buying
and testing them ourselves. If you want to try the Grayhill SSR's
listed above you'll be blazing a new path for the hobby.
I'm all about reliability in my combat vehicles. To me it isn't worth
saving a few bucks on a critical component just to have it fail on one
of the two weekends a year I really need it to work. So my preference
if I were to build this hack would be the commercial h-bridge, an SSR
h-bridge, and finally the homebuilt mosfet h-bridge. I'd also acquire/
build a second as a spare.
Steve Tyng
Concerning your voltage
On Mar 4, 1:37 am, "Paul Hilton" <[email protected]> wrote:
> OK Steve, sometimes I'm a bit 'thick" when it comes to electronics. I have
> zero experience with H-bridges and MOSFETs in general. Are you saying that
> this will do the job?
>
> http://130.94.182.150/servo.jpg
>
> Seems vastly overpowered for my app, but I'll give it a go. I assume that
> the ".2" beside the capacitor is .2 micro farads. I also wonder about
> substitutions for the MOSFETs. If I used smaller MOSFETs, would it all
> still work? What about 2 N-channel and 2 P-channel, rather than 2 and 4?
> And what of voltage, is this not an issue? Seems kind of important to me.
> And why build a 38 amp H-bridge, then call out "three good gearmotors for
> steering servos" none of which draws more than 750 mA, like these guys have
> done? Seems like a bit of overkill.
>
> Paul H.
>
> ----- Original Message -----
> From: "Steve Tyng" <[email protected]>
> To: "R/C Tank Combat" <[email protected]>
> Sent: Saturday, February 28, 2009 11:15 AM
> Subject: [TANKS] Re: High Power Servo Hack
>
> > Paul,
>
> > The Pololu board would not work to "amplify" the current capability of
> > a standard r/c servo. The Pololu board is a bit to intelligent in
> > that it has it's own processing capability and determines motor speed
> > and direction via separate PWM and direction signals. Though the
> > Pololu board has an h-bridge the board itself is not controlled like a
> > typical h-bridge circuit. It is meant to be controlled be a
> > microprocessor and not as something as crude as a hacked hobby servo.
>
> > The hack we are discussing replaces the internal and very basic h-
> > bridge circuit of the servo with an external higher capacity circuit.
> > This h-bridge circuit requires a PWM speed signal on one of either two
> > inputs unlike the Pololu board which has only one input for the PWM
> > speed signal and a second to determine direction.
>
> > Simple h-bridge boards a few and far between. The Tecel was one,
> > RobotPower simple h-bridge is the only other commercial h-bridge that
> > I can recall offhand. Most servo hacks off this type I've read about
> > usually involves making a custom h-bridge board, which actually isn't
> > very difficult to do. Here's a link for one such project.
>
> >http://130.94.182.150/mowers.htm(scroll down to "CONVERTING STANDARD
> > R/C SERVOS INTO MONSTER SERVOS")
>
> > Steve Tyng
>
>
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