Find resistance of 0.1Ω resistor

crutschow

Joined Mar 14, 2008
34,285
Ideally, you want R1 and R2 to be about the same order of magnitude.
Why is that?
The accuracy should be the same if you use a larger resistor to limit the current from a higher voltage source as long as there's enough voltage to give a good reading across the UUT (say 100mV) on a standard multimeter to give good precision.
As noted, you just have to be mindful of the power dissipated in the larger resistor.
 

Thread Starter

k1ng 1337

Joined Sep 11, 2020
940
As pointed out, you can do this using the 4-wire measurement method.

Here is a similar technique along the same idea using a battery, resistor and your DMM.
Firstly, you need a known low resistance. For example, suppose you have a 1Ω ±10% resistor.
You create a circuit with your battery in series with the 1Ω resistor R1 and your 0.1Ω DUT R2 (device under test).

View attachment 234037

With your DMM in voltage range, measure the voltage V1 across R1 and V2 across R2.
By Ohm's Law analysis:

I = V1 / R1 = V2 / R2

R2 = R1 x V2 / V1

independent of the voltage of the battery.
While trying this method I realized my multimeter is no good, the resistance values are all over the map and the slightest touch or repositioning of components gives wildly different results.. I almost certainly ruined the guts of the thing by accidently measuring the resistance of voltage sources (haha) including having bypassed the fuse months ago :)

thanks everyone for the replies, I am still surprised there isn't a simple Arduino solution with all the bells and whistles on the thing.
 
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Thread Starter

k1ng 1337

Joined Sep 11, 2020
940
Safety mechanisms are there for a reason. Bypass safety mechanisms at your own peril.

Live and learn.
I only did it because the fuse blew and I was planning to get a 10A meter, normally I'm a safety nut but I read a lot on the issue that suggested that the voltage I'm working with will at most ruin the meter which is what has come to pass..

I would never connect an unfused low impedance device to mains voltage.. at this point in my studies I'm trying to work with low DC voltages (<10) to minimize my exposure. My calculations reveal that this is not enough energy to even pulse my heart while my body is dry, but I am interested in more information on the issue, I've seen enough YouTube to know electricity is unpredictable and dangerous to the would be investigator.

I don't even like going near the utility panel in my house.. while I trust the engineering I don't trust the installer and I've seen some monkey wiring in my day.
 
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