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how to make a shunt (or maybe why its not easy) >> Posted by JimBlake [Email] ![]() ![]() ![]() In Reply to: Any improvisations/substitutes for big ammeter?, Dorsal1, Tue, 3 Sep 2002 18:22:18 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I know a motor is an inductive load, but you can pretend that ohm's law applies (V=I*R). The "resistance" of the motor has to be around 0.5 ohm for the current to be around 24 amps. You don't want to use a resistance much bigger than about 0.02 ohm (20 milli-ohm) otherwise it will drop the fan speed just like the resistor pack that the fan speed control uses. (That resistor lives under the LH speaker.) Then you'd measure 0.56v at a current of 28 amps.
But power is (I^2)*R = (28^2)*.02 or about 16 watts using the highest guess of 28 amps. Good luck finding a 20 milli-ohm resistor capable of dissipating 16 watts (or more); but you could check out electronics shops. That's all a shunt is, its an appropriate resistor for the particular measurement. You could use a long coil of wire to make up 0.02 ohms, or part of a hair-dryer heater element. But the trouble is your multimeter probably can't accurately measure a resistance that low. Can't use a light bulb because when the filament heats up it probably changes its resistance too much.
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