measuring current from a 12v dc motor

[dewingong]

To all,
Please i have a problem measuring the current from a 12v motor. I want to pass the measured value through an ADC to be able to monitor the motor current on my computer.

 

[picbits]

Put a resistor in the line between ground and the motor negative and measure the voltage across it.

 

[BravoKilo]

measuring current

Dewingong:

If you have a tester, analogue or digital, you can set it to measure current. The setting is in milliamps(MA) or in amps(A). Better to use the Amps setting first. Current is electrons running through the wires so you have to series the tester problems, meaning, the current is made to go through your tester. It does not matter when you series the tester, on the + side or the - side. Just watch out if you are using an analogue tester cuz the needle might swing backwards. reverse the probe. No problemwith digitals, it will show a - sign.
Now, if you only have a voltmeter, you can series a 1 ohm resistor but better use a 5 watt wirewound if you are running a big motor. The voltage accross the ends of this resistor is numeracally the same in ampere. So, if you get 3 volts, then it is 3 amps. Ohms law is Voltage=Current x resistance, so current is Voltage/resistance. When the resistor is 1 ohm, no need to divide. If you have a different resistor, then divide the voltage by the ohmsto get current.

Good Luck.

BK