Re: Measuring mA (debate solution needed)


"James Lerch"

A friend and I are having a polite debate.

He's calculating the current in a circuit my measuring the voltage
drop across a 20ohm resistor. (Current is ~30mA range for refernce)

I think best approach is to put a mA meter in series with circuit and
read the meter. He disagrees and said his method of measuring the
voltage drop across the resistor is more accurate.

His claim is the mA meter adds resistance to the circuit and under
reports the results. I agree in theory with his statement, but
disagree on the magnitude of the error.

I claim the amount of error in both the ability to measure 0.6vdc and
the error in the certaitnty of his "known" resistance is of a larger
concern.



** Time you measured the internal resistance of a DMM when set to the 200
/ 300 mA range.

A typical 3.75 digit meter I have here exhibits a whooping 5.5 ohms !!!

The voltage drop at full reading = 1.8 volts !!

Most of it is due to the 500mA glass fuse employed on that range.

Your friend is right & you are 100% wrong.




........ Phil



.

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