Byron A Jeff wrote:
>
> After years of development ;-) I've finally have my prototype robotic lawnmower
> platform to a usable state. The most valuable lesson I learned is that
> everything in the drive path must be metal, clamped, and use lock washers.
>
> Next on the agenda is motor control for the drive motors. Bob Blick's
> discrete h-bridge is a natural for the project. But of course you end up
> with 8 power transistors, plus another one for the cutting motor.

And you end up wasting a lot of energy on the
forward losses of the bridge transistors, see below.

> I wanted to get some ideas of the best way to heatsink a bunch of power
> transistors.


Hi, great to see the robot lawnmower taking shape!
If you haven't put too much work into the drive motor
h-bridges, I would suggest using the more old fashioned
system, ONE transistor with PWM to drive the motor,
and a cheap relay to reverse the motor wires.

You don't really need h-bridge complexity or cost,
and with 5 amp motors you mentioned, the relay system
is much more efficient, relay contacts have very little
heat or waste. You can also optimise the ideal ONE
transistor or FET for the motor, with a very low on
voltage (Vce sat) and good base drive circuit.
This is harder to do with a h-bridge. With a battery
powered mower robot you probably want to keep it as
efficient as possible.

And you only need 2 large heatsinks, one for each
transistor. :o) You don't really need high speed
control of engine braking, which is one area that
the h-bridge might be better.
-Roman

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