# DC shunt used to measure current on AC mains

Discussion in 'Test & Measurement Forum' started by van53, Apr 8, 2018.

Nov 27, 2011
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2. ### LesJones Well-Known Member

Jan 8, 2017
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You could use it but I think you will have problems finding an AC meter with 75 mV full scale deflection. (And no you can't just put a bridge rectifier between the shunt and the meter as a bridge made from silicon diodes does not conduct until there is about 1.4 volts on it's input.) You would normally use a current transformer instead of a shunt to measure AC current.

Les.

3. ### van53 Thread Starter Active Member

Nov 27, 2011
67
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Thank you. I plan to use a digital multimeter to measure the voltage drop across the shunt to determine the current. Why is it that these shunts are only advertised as DC such as these:

Is it because they are mainly used in parallel with DC analog panel meters ?

Would a current transformer be more or less accurate vs a shunt when measuring AC mains current?

Jul 18, 2013
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5. ### van53 Thread Starter Active Member

Nov 27, 2011
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For the shunt I had just linked, I noticed there was a comment from a customer which stated:

Questionable For AC Use?
we are using this in a high frequency setting, and on initial use, our calculations were not turning out as expected, it could be due to the frequencies.

I am guessing that for AC 60hz it is fine but at high frequencies there could be issues?

Jan 15, 2015
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How much AC current are you looking to measure? That's where it all begins. Can a current shunt like what you have looked at be used to measure AC current? Yes, but measuring the low level AC voltage dropped across the shunt becomes a little difficult. Depending on the current level you wish to measure there are other AC current options.

The higher the AC frequency the more problems that can arise on the measurement plane.

So how much AC current? I assume you just want a basic Average responding RMS indicating measurement? The difference depends on the load you have. Additionally if this is just a simple home type project then I doubt you need an expensive solution but if this is an industrial type project where accuracy is paramount then things change.

Ron

7. ### van53 Thread Starter Active Member

Nov 27, 2011
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I am looking to load the mains as close as possible to 15A without tripping the breaker (perhaps a 20A shunt may be better). I am assuming that since I am measuring 60hz sine-wave mains it wouldn't make any difference if I use an average responding meter vs a true rms meter?

This is just a simple home project. I want to record the voltage before and after the load (along with current draw) so that I can calculate the neutral to live loop impedance to obtain the prospective short circuit current for a given outlet.

Jan 15, 2015
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From our friends at Keysight:
Likely if you just want to measure US mains power an average responding setup will get you close enough. The load is what determines what you want but for a true sine wave general use an average responding RMS indicating system would be fine. When considering a shunt if you expect a 15 amp load then consider a 20 amp shunt, when run at their limits shunts, especially cheap shunts heat and as they heat their resistance changes and your measurement degrades.

For 120 Volt 60 Hz mains power I would not use a shunt. I would use either a CT (Current Transformer) or an AC Current Transducer with a DC out proportional to the AC current. The only times I used a resistive shunt to measure AC it was 1.0 Hz and below. They simply are not practical for AC current measurement but nothing says if you find a good scalable AC low voltage meter you can't use one.

Current transformers are popular for measuring AC current in less costly manners,

Here is a current transformer opened up, Your main current carrying conductor gets looped through the donut hole..and the output is from the secondary wires, black and white,

They make 5 amp input ac meters which can be scaled to the current transformer connected to it.

Finally they make AC Current Transducers which outout a DC voltage or current which is proportional to the measured current,

So there is a wide variety of choice dependig on what your piggy bank allows. One time test curcuit go cheap, long time monitoring and recording data then things can get pricey,

Ron

9. ### van53 Thread Starter Active Member

Nov 27, 2011
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I was planning to use my Fluke 8050A to measure the voltage across the shunt. The resolution on the 200mV AC range is 10uV with an accuracy of 1%+10 (45hz). I calculate the potential error when measuring 15A would translate into 150mA. Would using a current transformer or ac current transducer with dc out provide better accuracy?

Jan 15, 2015
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No, as long as you have a Fluke 8050 available I would just use a shunt and see what you get. You should be able to find a 50 Amp / 50 mV shunt as they are real common or other shunt with 1 mV / Amp output. I also see Amazon has 50 Amp 75 mV shunts for a few bucks. So you figure 1.5 mV per Amp.

Funny you mentioned that meter as the model number sounded familiar. I have one of those in a pile of old stuff in the basement.

I would start with just adding an inexpensive shunt and see what you get doing it that way. The CTs and AC current transducer solutions can get a little pricey. Also when using a shunt, if possible, place it in your neutral leg just in the best interest of safety. In a DC solution it would be called measuring the Low Side current.

Ron

11. ### MrAl AAC Fanatic!

Jun 17, 2014
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Hello,

Just a quick note as much has been said already that would help.

Choosing a 75mv shunt isnt that good of an idea unless you have to. The reason is because it is harder to convert voltage to current in your head when taking readings. Sure you can do it, but with a 50mv or 100mv shunt it is much easier.

For example, with a 75mv 50A shunt, when the voltage reads 15mv what is your current?
Now, with a 50mv 50A shunt, when the voltage reads 10mv what is your current?
And now with a 100mv 50A shunt, when the voltage reads 20mv what is your current?

Notice the 50mv 50A shunt is easiest to read without even thinking because whatever you read in millivolts, that is the current in amps present.

Meters that read down into the 10's of microvolts work better for shunts as you probably already figured out. It's also a good idea to use a small RC filter if you plan to use a scope with it.

Shunts are not perfect, so you might calibrate yours with a regular current meter before you start taking readings.

12. ### MisterBill2 Distinguished Member

Jan 23, 2018
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Using a shunt and an RMS ac voltmeter to measure AC current through the shunt will certainly work, but the math will be a bit tedious, depending on the exact shunt ratings. And for your application it can work to use heaters and light bulbs as the loads. Determining the source impedance of your electrical system will be an interesting exercise. Good luck with the project, please post the results that you find.

13. ### Lundwall_Paul Active Member

Oct 18, 2011
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To answer your question. The shunt on eBay they should drop the "DC" from the description. I have used similar shunts on both AC and DC. Never used them on mains and wouldn't use them on mains. I would go for a current transformer.

14. ### MisterBill2 Distinguished Member

Jan 23, 2018
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A resistive shunt will work well on mains circuits, but of course making everything safe can be a big chore. But I have quite a few shunts and only two current transformers. And only one nice ammeter to use with a shunt on AC.

15. ### van53 Thread Starter Active Member

Nov 27, 2011
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I ended up purchasing a 20A 50mV shunt +/-0.25% (data sheet linked here). I am curious on what is the best practice for taking measurements and making calculations using a shunt. As the shunt has a very low resistance its effect on my intended circuit (target 15A draw on mains using appliances which would be equivalent to an 8ohm load) would be minimal.

Lets say you read 37.5mV across the shunt does one use the shunts rating (i.e. 50mV@20A +/-0.25% [5mV per amp]) to determine the current in the circuit which in this case would be 15A? Or is the best practice to take into consideration the very minimal effect the shunt has on the circuit (its reduction on the current) and using the measuring voltage and measured resistance of the shunt calculate the current going through the circuit?

16. ### crutschow Expert

Mar 14, 2008
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You could, but it has what would generally be considered a negligible effect on the load current (perhaps 0.04% in a 120V circuit).

17. ### WBahn Moderator

Mar 31, 2012
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It's not the frequency that matters (up to a point), but rather the waveform. If your current waveform (not voltage waveform) is a sinusoid, then you can use either an true RMS or an average responding but RMS indicating meter and you will get the same results. But if your load draws a non-sinusoidal current, as many do, then your average responding meter may give nearly useless results.

18. ### van53 Thread Starter Active Member

Nov 27, 2011
67
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Thank you. I'll use the marked value (20A@50mv) in this case to calculate the current draw.

Based on some of the above suggestions to use a current transformer, I also picked up a 20:5 CT (model 189-020 data sheet here)
The markings on the CT is Class 0.6 B0.2 50-400Hz.

I am curious how the readings of the shunt vs the CT will compare. I am planning to use the shunt and CT together in series at the same time so I can compare readings at once.

From what I've read online in terms of CT markings is that my CT will have an accuracy of 0.6% when a burden resistor not greater than 0.2ohms is placed on the secondary, is this correct? The CT in question does not have a window. The data sheet indicates "Window Size: Wound Primary". Would my CT be considered a bar type or wound CT, or something else?

I'm planning to use a true RMS meter, however now I am wondering if the load (heater circuitry) draws a non-sinusoidal current then will it cause an issue for my CT as it is rated between 50-400hz ? The loads I was planning to use were oil based heater or micrathermic heater and various light bulbs to get close to 15 amps. In this case I suppose I could view the waveform on an oscilloscope across the shunt.

19. ### MisterBill2 Distinguished Member

Jan 23, 2018
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If you are setting the current in the circuit then the exact voltage does not matter. It is the current that develops the voltage drop that will allow you to figure the resistance in the line. I you chose to be redundant then you could use two shunts and calculate the current based on the readings from each of them. Because you are controlling the current the effect of the shunt resistance does not matter at all, unless it is so large that it limits the current.