In article ,
mentioned...
"Watson A.Name - 'Watt Sun'" wrote:
I got tired of switching the leads of my DMM. Suddenly if dawned on
me that I can just set the power supply to 10.0V for exaample, and
read the current, and then divide the voltage by the current to find
the resistance. Like I put a resistance on the PS, it reads 10.0V and
the current is .018A, so 10 / .018 gives 555.6 ohms. Must be a 560
ohm resistor.
Dangerous and inaccurate. Inaccurate, because the resistor will warm up
and the resistance will increase.
Using a volt meter and ammeter is certainly cheap and practical for
measuring a very low resistance that the typical DMM can not.
Say you have a 25ft 12AWG extension cord and you want to know the total
resistance. The average DMM has a resolution down to 100mOhm. The
resolution is nowhere near what you need.
Connect it to ~12V DC(you don't want to use 120V, because high floating
voltage will throw off accuracy on the low voltage meter range) and
connect an ammeter in series.
Connect a load on the other end that takes about a few amps.
Or use a constant current PS and short the far end of the cord, and
measure across both conductors on the near end.
Measure the voltage across both ends of one conductor in mV range.
If you read 54.6mV and the ammeter reads 2.12A, you can figure out the
resistance by:
0.0546/2.12=0.02575
three sig dig=0.0258ohms=258mOhms x 2(to accomodate for return
path)=506mOhms
Or you can just look up the resistance in a wire table and find that
12 AWG has 1.59 milliohms per foot. Then multiply by twice the
cord length.
As for being "dangerous and inaccurate", electronics experimenting is
frought with danger, one being 'letting the smoke out.'
As for "Inaccurate, because the resistor will warm up and the
resistance will increase", you could have the same problem if you were
measuring a resistor with a DMM, and the equipmewnt with the resistor
had been powered on before you did the measurement. The resistor
could already be hot.
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