At 02:02 PM 8/7/02 -0700, you wrote:

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

Well, you can figure it from Ohm's law. If it should drop 0.1V at 20A, then
it would be 5 milliohms. Obviously, care is required in layout with this sort
of thing.

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

Power = I^2*R, so at 20A , a 5 milliohm resistor would dissipate 2W

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

The back-EMF is 0, but the motor resistance is the winding resistance.
Your controller may limit the current to some maximum as well.

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

                               .... *through* this resistor.

>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, but the parts must withstand whatever you limit the current to for
some period of time. If there is no limiting and the motor can be stalled,
then they must stand up to the LRA rating of the motor at the output
voltage of the H-bridge.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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