> I am trying to build a motor controller to drive a robot from a PIC using
> an H-bridge, so I'm
> reading up on FETs now in The Art of Electronics.  They have a circuit
> which limits the current
> draw through a mosfet, which I'm trying to understand.
> It's Figure 3.72c, with a PNP transistor before the source of the FET with
> a 0.5 ohm resistor from
> emitter to base with the base wired to the source, and a 100k resistor
> from their +12V source to
> the collector of the PNP and also to the gate of the MOSFET.

Beware that this circuit, in real life, will not protect your MOSFET. You
will end up with a "power oscillator" that spends a lot of its time
switching. Your MOSFET will overheat, or exceed its safe area limits, and
die a horrible smoking death.

Since you are using a PIC, use the PIC to do the current limiting. I am
assuming you are trying to protect the MOSFET, not do variable or exact
current limiting (the H+H circuit does neither of those).

You can do cycle-by-cycle current limiting various ways, with or without
using the A/D converter. You can refer to this if you are interested in a
completed project(and if it is interesting, I can forward a complete
MOSFET schematic):

http://www.bobblick.com/techref/projects/sv2hb/sv2hb.html

Cheerful regards,

Bob

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