Bob Blick wrote:
>
> > > http://www.bobblick.com/bob/projects/hbridge/index.html
> >
> > The thing that concerned my about that circuit is the 4V voltage drop.
> >
> > I was hoping to get a circuit that has a minimal drop of the supply voltage
> > because I'm driving 12V motors with a 12V battery.
>
> Hi Byron,
>
> It only drops 4 volts when it's at full current. At lower current your
> drop is more like 2 volts. If you think about how a DC motor works, you'll
> see that there is no way to operate a 12 volt motor with 12 volts and 5
> amps at high speed. If you had a motor like this, it would draw much more
> than 5 amps at low speed, and destroy the H-bridge. At low speeds, it
> doesn't matter how much voltage gets to the motor, since you are torqueing
> around and the current is what matters. So in fact, dropping 4 volts at
> full current makes the bridge a little sloppy, but limits the current.
>
> Be sure to heatsink it well, though :-)
>
> It is easy enough to build one and test it, you really should since it
> does work pretty well and is tougher than a mosfet one. I would not
> consider building a mosfet hbridge for use with motors unless there was
> fast current limiting.
>
> Cheerful regards,
> Bob

You can also change the Darlingtons to BJT's. I'm doing this primarily
because there are far more choices in surface mount packages for BJT's
than there are for darlingtons. This has the added benefit of a lower
voltage drop. OTOH, you'll require a larger base drive... Sorta typical
when you exchange BJT's and Darlingtons.

If you choose MOSFET's you always are confronted with driving the high
side. I'm not sure if this is on the web or not, but Motorola's AN-913
"Designing with TMOS Power MOSFETs" describes several ways to drive
MOSFETs. In a recent design that had similar switching characteristics
to a PWM H-bridge servo thingy, I chose to implement an isolated gate
drive: I used a "gate-drive transformer" as a power supply and an
Hewlitt-Packard HCPL-3100, an optically-isloated gate drive IC, as the
driver. The primary side of the transformer was driven by a simple
74AC04 relaxation oscillator while the secondary was full-wave rectified
and filtered to provide the power for the HCPL-3100. This works well for
frequencies upto 20khz or so. Beyond that you might as well just use the
gate drive transformer directly.