# Ammeters - shunt resistance

#### Coucou80

Joined Apr 7, 2018
78
Hello,

1. I have handy a 1000amp shunt resistor rated at 75mV per amp.
2.. I have a amp analog gauge that goes from -60 to +60 amp at 75mV per amp.

Will they get along in providing accurate measurements or should the gauge be also rated from -1000 to +1000 amp?

Best regards,

#### WBahn

Joined Mar 31, 2012
30,239
I'm not sure quite what you are asking. If your analog gauge is rated for -60 A to +60 A, don't try to put 1000 A through it!

What do you mean by "get along"? Get along how? What is it that you are trying to do?

#### crutschow

Joined Mar 14, 2008
34,680
I have handy a 1000amp shunt resistor rated at 75mV per amp.
I believe that is 75mV full-scale (1000A), since 75mV/amp would have it dissipating 75,000W @ 1000A.
Your gauge is likely rated the same way.

#### Coucou80

Joined Apr 7, 2018
78
I believe that is 75mV full-scale (1000A), since 75mV/amp would have it dissipating 75,000W @ 1000A.
Your gauge is likely rated the same way.
I dont arrive at the same result
75mV is 0.0075v
0.0075 * 1000 = 75 watts

#### Coucou80

Joined Apr 7, 2018
78
I'm not sure quite what you are asking. If your analog gauge is rated for -60 A to +60 A, don't try to put 1000 A through it!

What do you mean by "get along"? Get along how? What is it that you are trying to do?
Does the shunt resistor needs amp value needs to match the analogue guage max and min value ?

#### Coucou80

Joined Apr 7, 2018
78
Solved... I just tried the shunt resistor with analogue display and the DC hall effect clamp give same result.
So all in all as long as the analogue display and the shunt resistor voltage value match with the one from the analogue display it will work. In this case its is 75mV for both

#### crutschow

Joined Mar 14, 2008
34,680
I dont arrive at the same result
75mV is 0.0075v
Of course, because you didn't use the value you posted.
You stated its output was 75mV per amp which is 75V at 1000A.

#### Ramussons

Joined May 3, 2013
1,416
Hello,

1. I have handy a 1000amp shunt resistor rated at 75mV per amp.
2.. I have a amp analog gauge that goes from -60 to +60 amp at 75mV per amp.

Will they get along in providing accurate measurements or should the gauge be also rated from -1000 to +1000 amp?

Best regards,
That analog meter will work with the 100 Amp shunt. But the permitted current will still be limited to +- 60 A, the permitted voltage range across the meter.

#### Ian0

Joined Aug 7, 2020
10,006
Both shunts and meters are rated for the current at FSD, so a 60A/75mV meter connected to a 1000A/75mV shunt will read 60A when the current through the shunt is 1000A.

#### Coucou80

Joined Apr 7, 2018
78
Both shunts and meters are rated for the current at FSD, so a 60A/75mV meter connected to a 1000A/75mV shunt will read 60A when the current through the shunt is 1000A.
yup exactly what I thought. As long as shunt and meter are rated for 75mV the reading will be accurate up to the limit of the full scal definition of the gage limit.

#### Ramussons

Joined May 3, 2013
1,416
.....................
1. I have handy a 1000amp shunt resistor rated at 75mV per amp.
2.. I have a amp analog gauge that goes from -60 to +60 amp at
75mV per amp.
........................

Unless the above Highlighted is wrong, Post #8 is correct.

#### Ian0

Joined Aug 7, 2020
10,006
yup exactly what I thought. As long as shunt and meter are rated for 75mV the reading will be accurate up to the limit of the full scal definition of the gage limit.
If that's what you thought, then why did you say that it was rated 75mV per Amp? You confused everyone.

#### Coucou80

Joined Apr 7, 2018
78
If that's what you thought, then why did you say that it was rated 75mV per Amp? You confused everyone.
What I meant is that the voltage drop at that current (1000amp) is 75 mVolt.

edit: should I edit my initial post ? ... I dont want to confuse everyone indeed.

#### MrAl

Joined Jun 17, 2014
11,565
Hello,

1. I have handy a 1000amp shunt resistor rated at 75mV per amp.
2.. I have a amp analog gauge that goes from -60 to +60 amp at 75mV per amp.

Will they get along in providing accurate measurements or should the gauge be also rated from -1000 to +1000 amp?

Best regards,
Hi,

It looks as if you have posted some unusual specs here.

First, #1 is probably 100 amps at 75mv, period. It's not 1000 amps at 75mv per amp.
Second, #2 is a meter that has a range of 60 amps at 75mv period, since the shunts are not polarized it will also do -60 amps. The meter itself would have a scale that goes from -60 to 60 amps though. If you can change the face plate (and many have done this now due to the availability of printers used with computers to create hard copy graphic images) then you can change the calibration from -60 to 60 amps to -100 to 100 amps. That would make the meter read 100 amps when the shunt drops 75mv. You may even be able to find a graphic online you can use to print a new face plate.

100 amp, 75mv is a typical shunt spec, and also we would see 100 amp, 50mv which is another typical spec.
The meter, on the other hand, reads 60 amps when there is 75mv presented to it.
Because the two are matched for the 75mv spec but not matched for the full scale reading (100 versus 60) it will take 100 amps for the meter itself to indicate 60 amps. That means your readings will all be 0.60 times the actual current. For example, if the current is 10 amps the meter will read 6 amps.
To use this arrangement you would have to mentally convert the readings to actual current, which means multiply the reading by 1.667, and this would make a reading of 60 amps convert to 100 amps as needed.
A few other examples using this conversion factor of 1.667:
A reading of 30 amps times 1.667 gives us 50, which shows that the current is really 50 amps.
A reading of 15 amps times 1.667 gives us 25, so the current is really 25 amps.
A reading of 1 amp times 1.667 of course gives us 1.667 so the current is really 1.667 amps.

The most important part is that 75mv is not 0.0075v volts it is 0.075 volts, and it is the FULL SCALE voltage not the volts per amp spec.
To convert to volts per amp, divide 0.075/60, and that gives us 0.001250 volts per amp, which is also 1.25mv per amp. That's quite different.

A shunt that is 100amps with 50mv full scale is easier to deal with, but a shunt that is 50 amps with 50mv is even simpler because that's just 1mv per amp.
Usually the shunt is matched to a given meter movement though, so a 60amp with 75mv meter would be matched to a 60amp at 75mv shunt.

Total dissipation in the 100 amp 75mv shunt is 100*0.075 which of course equals 7.5 watts.
Total dissipation in a 100 amp 50mv shunt would only be 100*0.050 which equals 5 watts.
For a 50 amp 50mv shunt the power dissipation would only be 2.5 watts which is a lot lower than either of those above.

Lastly, not all shunts are very accurate. Sometimes you can buy two of the exact same shunt and see two readings that are significantly different. Sometimes there is a way to adjust them though. If you need good readings then you should check the shunt and meter with another current meter that is known to be accurate.

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#### Ian0

Joined Aug 7, 2020
10,006
A colleague of mine took a shunt out of stores and brought it to me "Someone's been cutting this shunt with a hacksaw".
. . . .
.. . . .
"That's how they are calibrated".

#### MrAl

Joined Jun 17, 2014
11,565
Hi,

Yes if the reading is too low you can cut a small notch and that will increase the resistance slightly. The notch cannot be too large of course or it creates a hot spot.

#### Coucou80

Joined Apr 7, 2018
78
So here is a tricky one.
How can we know how many volts it takes to bring the arrow to full scale definition.

...I guess without precision power supply equipment its difficult to say?
I would like to figure out what shunt resistor I would need with this gauge.

Rsh = Rm / ((I/Im)-1)

For Rm, can I just take a general purpose voltmeter and measure the ohmic value of the coil?

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#### Ian0

Joined Aug 7, 2020
10,006
So here is a tricky one.
How can we know how many volts it takes to bring the arrow to full scale definition.

...I guess without precision power supply equipment its difficult to say?
I would like to figure out what shunt resistor I would need with this gauge.

Rsh = Rm / ((I/Im)-1)

For Rm, can I just take a general purpose voltmeter and measure the ohmic value of the coil?
For ammeters, some need external shunts and some don't. It should say.
For those that don't, it is simply the FSD current multiplied by the resistance of the coil.
You might find a coil on a 100A ammeter rather hard to measure: at a guess it will be around 100μΩ.

#### MrAl

Joined Jun 17, 2014
11,565
So here is a tricky one.
How can we know how many volts it takes to bring the arrow to full scale definition.

...I guess without precision power supply equipment its difficult to say?
I would like to figure out what shunt resistor I would need with this gauge.

Rsh = Rm / ((I/Im)-1)

For Rm, can I just take a general purpose voltmeter and measure the ohmic value of the coil?
A way to do this is to place a series resistor in series with the meter, then apply a test voltage. Setting the voltage to full scale and then measuring the voltage across the meter Vm, you can use the voltage divider formula to calculate the resistance of the meter at full scale.
Vm=Vs*Rm/(Rs+Rm)

and solve that for Rm. That's the value usually sought after.

#### Coucou80

Joined Apr 7, 2018
78
A way to do this is to place a series resistor in series with the meter, then apply a test voltage. Setting the voltage to full scale and then measuring the voltage across the meter Vm, you can use the voltage divider formula to calculate the resistance of the meter at full scale.
Vm=Vs*Rm/(Rs+Rm)

and solve that for Rm. That's the value usually sought after.
I'm a bit confused now..
In this experiment, at this stage, you still don't know Rshunt?? Where:
Vm : voltage at meter
Vs : Voltage at shunt
Rm: unknowm
Rs: series resistance??