How to measure very low resistances?

Thread Starter

MikeA

Joined Jan 20, 2013
227
I have an unmarked current shunt that's I'd like to identify.

I have an oscilloscope, multimeter, adjustable bench power supply, etc.

Can I leverage generic electric test equipment (and generic electric components) to measure something in the 0.01-0.001 ohm range with fairly good accuracy?
 

SamR

Joined Mar 19, 2019
2,604
See if you can find the manual or PDF for you multimeter and look for it's sensitivity. Most digital meters, on their lowest scale, should give 2 decimal places accuracy. Depends on the meter. Or, using the power supply, put a voltage across the component and measure it's amperage and calculate the resistance.
 

Dodgydave

Joined Jun 22, 2012
9,482
Put a Constant current of 1mA through the load and read off the Voltage across it , the Voltage will be equal to the resistance.
 

Thread Starter

MikeA

Joined Jan 20, 2013
227
Put a Constant current of 1mA through the load and read off the Voltage across it , the Voltage will be equal to the resistance.
I think this suggesting is just like everyone before it, but I don't think that it will work give the inputs (generic test equipment).

I mean, yes, it works in theory, but to measure voltage across a very low resistance I'd need a very high constant current source. Or a very accurate low voltage voltmeter.

Say the current shunt is 50mA @ 50A, which would mean it is 0.001 ohms.

With 1mA across that shunt I'll see 0.000001v. So I'd need a voltmeter with a couple more decimal points of resolution.
 

Thread Starter

MikeA

Joined Jan 20, 2013
227
I was thinking of some more complex solution. Like say taking a large capacitor, measuring its specs to figure out what kind of high current pulse it can provide. Then shorting it across the shunt and looking at the pulse with an oscope.
 

Dodgydave

Joined Jun 22, 2012
9,482
I think this suggesting is just like everyone before it, but I don't think that it will work give the inputs (generic test equipment).

I mean, yes, it works in theory, but to measure voltage across a very low resistance I'd need a very high constant current source. Or a very accurate low voltage voltmeter.

Say the current shunt is 50mA @ 50A, which would mean it is 0.001 ohms.

With 1mA across that shunt I'll see 0.000001v. So I'd need a voltmeter with a couple more decimal points of resolution.
Then put 10mA in and divide the result by 10.
 

Thread Starter

MikeA

Joined Jan 20, 2013
227
How much current can your supply, er, supply?
What is the rated current of the shunt(s)?
The bench supply is 6A.

The shunt is unmarked. I would guess in the 50A range.

I have a large iron transformer, a Variac, and an AC current clamp meter. I think I can rig up a very high "constant" current source with those 3 parts.

But I thought maybe there is a more elegant solution.
 

Ylli

Joined Nov 13, 2015
978
The bench supply is 6A.

The shunt is unmarked. I would guess in the 50A range.

I have a large iron transformer, a Variac, and an AC current clamp meter. I think I can rig up a very high "constant" current source with those 3 parts.

But I thought maybe there is a more elegant solution.
Ohm's law isn't elegant enough? Put a know current through it and measure the voltage across it. It's that simple, and that is the best way to do it. Use DC if you want resistance, and use your 60 Hz if you want impedance at 60 Hz, which may be different than the DC resistance.
 

Thread Starter

MikeA

Joined Jan 20, 2013
227
Ohm's law isn't elegant enough? Put a know current through it and measure the voltage across it. It's that simple, and that is the best way to do it.
It is. But the constraint is that I'm working with consumer grade test equipment.

If I could pulse 5000A for 100uS through the shunt and get 5V across it and measure it with an oscope with good resolution, I'd do that. :p

Use DC if you want resistance, and use your 60 Hz if you want impedance at 60 Hz, which may be different than the DC resistance.
That I was not aware of. What would cause a shunt to have a different resistance at DC vs 60Hz?
 
A shunt has 4 terminals. The outer ones get the current. You measure the voltage across the inner terminals. the outer terminals are usually quite big.

A 10M input Z meter in parallel with a sub-ohm resistor means negligible error.

You have to measure the current through the resistor and the voltage across it. It really doesn't have to be controlled.

Your DC voltmeter needs enough resolution.

You do need some standard. e.g. a 0.1% resistor, preferably 4-terminal and a load. e.g. car head lamp and a power supply.

Measure the current through and the voltage across the resistor.
 

SamR

Joined Mar 19, 2019
2,604
Can I leverage generic electric test equipment (and generic electric components) to measure something in the 0.01-0.001 ohm range with fairly good accuracy?
My Fluke handheld will give me one decimal place resolution for Ω. My Fluke and Vici bench meters will give me 2 decimal place resolution. 3 decimal places? I think the only way to get there is by calculating Volts and Amps. My bench meters will give me mA to at least 2 decimal places so I am into uAs resolution and against a 3 decimal place V measurement for say 10.000V the calculation should be pretty darn close. But you going to need accurate bench meter to get there. A handheld won't do it.
EDIT: and you can buy a good used bench meter off of fleaBay for what a new handheld would cost or even less. My Fluke 8050A was $40.
 

Thread Starter

MikeA

Joined Jan 20, 2013
227
But you going to need accurate bench meter to get there. A handheld won't do it.
I have a 0.05% handheld, not much worse than the 0.04% 8050. Should be good for that side of the measurement.:cool:

Finding an accurate high current source is the trick.

How do those specialized handheld low ohm range meters work? The ones that can do 4 or 5 decimal places resolution. They certainly don't have a very high current source inside.

Is it just some high quality calibrated op-amp inside to get micronano-volt readings?
 

Ian0

Joined Aug 7, 2020
535
That I was not aware of. What would cause a shunt to have a different resistance at DC vs 60Hz?
You'd be surprised!
A 60mV/60A shunt has a resistance of 1m ohm.
You only need 1.2uH of inductance to be 10% out at 60Hz.

To be perfectly pedantic, it won't be a different resistance, it will be a different impedance.
 
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