I'm considering doing part of How to Install an Ammeter in the Black & Decker CMM1200 Cordless Electric Mower but first with just a shunt resistor and voltmeter, versus hooking in a dial ammeter (may hook in a dial later).

But, when examining shunt resistors, I found What Is A Shunt which states: "This shunt is calibrated such that the voltage drop across it is 100mV when the current flowing through it is 100 Amps".

So, in order to acquire such a shunt resistor to directly read current as amps being equivalent to the voltage reading on a voltmeter (in mV), I would need a 100A 100mV current shunt resistor. But, when I search specifically for 100mV 100A current shunt resistors on Ebay all that is returned are 75mV ones (EDIT: A better search link).

What I want to know is why 75mV and not 100mV in those listings? What is significant about them having to be 75mV and not just 100mV?

up vote 3 down vote accepted

There's no special significance, shunts are available in different current ranges and different full scale output voltages. Some applications may not want the 100mV drop in voltage on a 100mV full scale shunt, others may want the higher signal. Some are designed to work with particular meters and therefore have whatever scaling factor that meter requires. If you need a 100mV shunt try Newark Electronics: www.newark.com

  • Thanks. I'm hence marking this as the answer to the "why 75mV and not 100mV in those listings" part of my question. It does leave me puzzled as to why there are many many 75mV shunts being sold on Ebay and not many 100mV at all. I guess there must be many more meters that expect the 75mV shunts and not the 100mV ones (and because of not wanting the full 100mV drop at 100A as you stated). – bgoodr Jun 7 '14 at 19:30
  • Ancient post I know, but I think the 75mA shunts are prevalent because they are what is required for the inexpensive and very common YB27 type digital ammeters. – user81248 Jul 17 '15 at 7:29

But, when examining shunt resistors, I found What Is A Shunt which states: "This shunt is calibrated such that the voltage drop across it is 100mV when the current flowing through it is 100 Amps".

What it actually says is this: -

enter image description here

It clearly says that this is an "example" and the picture is "typical". What is the problem? It's a typical example of a shunt resistor used to measure currents up to 100 amps; it has a resistance of 1 milli-ohm and for every amp that is passing thru the device it generates a terminal voltage of 1 milli-volt. Here's one made by murata: -

enter image description here

And here is a Farnell page that lists 40 different shunts ranging from 5 amps to 1 kA with accuracies from 1% down to 0.0025%. This is the 1 kA one: -

enter image description here

And this is a 5 amp one: -

enter image description here

I've got to say I like the look of the 1 kA one and I'm thinking where can I use it but it is £116 so maybe I'll forget it for now. I'm also thinking (having reviewed the prices and seeing that the cheapest one is about £20) that a hall effect current sensor may be a better bet. I'm seeing this 125 amp one (below) for about £10: -

enter image description here

Here's the Farnell page that I've limited to showing hall effect sensors up to about 200 amp in handling capacity

  • 1
    Thanks for the helpful info. There is no "problem" per se. Between your answer and the one prior, I have a better understanding. I realized I should have asked "What is so unique or special about the 75mV shunt leading to its prevalence on Ebay listings when searching for 100A shunts, versus some other value like 60mV or 85mV shunts?" to which I answer: Most meters are the ones expecting a 75mV max drop at 100A. – bgoodr Jun 7 '14 at 19:42

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