> My questions is how to I add current and thermal limiters to an H-bridge?

The easiest way to add current limiting is to add a small resistor in series
with the H bridge.  It should only drop a small fraction of the supply
voltage at full current.  Just enough to trigger a comparator to the
accuracy you need.  This goes into the PIC as an input, which causes it to
shut down the H bridge, go into error mode, or whatever.

Thermal limiting depends on what you want to thermally limit.  The H bridge
FETs or the load?  I think the best answer for the FETs is to design the
system to guarantee they won't overheat under the worst case load and drive
cases.

Thermally limiting the load could be done by outright sensing its
temperature, but there can be other options.  In one of my projects there
was also an H bridge running a motor, also controlled by a 16F876.  The
motor occasionally needed to produce a high torque for a short time, which
required more current thru it than it could tolerate long term.  The system
had to guarantee not to smoke the motor, by scaling back its input if
necessary.  We ended up modeling the motor temperature in the firmware.  The
simulated temperature went up linearly with power dumped into the motor, and
went down proportional to itself.  This system worked quite well.  We could
afford to be conservative because the motor power was usually quite low
except for the high torque spikes.  There was enough headroom so that the
thermal limiter never kicked in during normal operation.  You could make it
happen by clamping the motor and telling the system to move it.  The PID
control algorithm would produce a full scale response causing full voltage
applied to the motor continuously.  This would happen for a few seconds
until the thermal limiter kicked in and backed the voltage off automatically
to a sustainable level.  If you let it cool for a while by not telling the
motor to move, you could get a full pulse again for another few seconds.


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