> This seems reasonable, but I have a few questions.  First, I assume a
small
> resistor is on the order of milli-ohms.

I think you said the max current was 30A.  Let's say you want a 250mV signal
at that current:

  ohms = volts / amps = 250mV / 30A = 8.33mOhms

That's just the right size and length of wire.  You have to make sure your
circuit truly measures the voltage accross this "resistor", and does not
include the resistance of the ground connection, etc.

> Secondly, given such a small
> resistor value what power rating does it require?

  power in resistor = Volts x Amps = 250mV * 30A = 7.5 Watts

> Theoretically, when power
> is first provided to the motor (i.e. the H-bridge is turned on) the motor
> resistance is 0.

No, it's not.  It will have roughly the same resistance and inductance as
when it's running.  There just won't be any back EMF to cancel out some of
your applied voltage.  This means the current will be the full applied
voltage divided by the resistance.

> Thus, there is a very large current across this resistor.

Current is thru a resistor, voltage accross it.

> Now, the time must be very small (or there would be a short to ground
> without the current sense resistor).  If I hope to run my controller at
24V
> and current limit at 20A, do I really need a sense resistor capable of
> 480W!?

No.  That is the maximum power available from 24V at 20A.  The power into
the current sense resistor will be small fraction of that because only a
small fraction of the voltage will be accross it.  See above.


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