Author Topic: Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]  (Read 1989 times)

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Offline HighVoltage

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Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]
« on: November 01, 2017, 04:08:14 AM »
I was looking for a way to calibrate / check the calibration of my current probes and thought that this will be very easy. Well, it seems I was wrong.

As a current source I am using my Clark-Hess Model 828 calibrator with a 100mA or a 1A output at 50, 60 or 400 Hz. The AC output is verified by a Keysight 34470A.

As an amplifier I am using a 50 turns or a 100 turns loop of 0.5 mm insulated copper wire.
In theory I should get 10.00A or 100.0A output at the scope from the current probe with the 100 loop system.

But the problem is:

At an input of 100mA, the output is 10.4A with 100 turns, so about 4% high.
No matter what I do, I am getting an error of 4 to 5%

As you can see in the attached pictures, I tried a few different arrangements of the windings but it makes no difference. I tried different current probes and scopes and it also does not make any difference. The reading is always significantly 4% to 5% high.

The big question I have is why do I have a 5% error?
What is it that I am overlooking?

Thanks for any advise.
« Last Edit: November 02, 2017, 02:28:22 AM by HighVoltage »
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Online Andreas

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #1 on: November 01, 2017, 04:57:42 AM »
Hmm,

the last time I had this the battery from the current clamp was near empty.

with best regards

Andreas
 

Offline HighVoltage

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #2 on: November 01, 2017, 05:04:18 AM »
Hello Andreas,

Thanks, but I tried three different current probes, all with new batteries.
Always the same result, about 5% high and repeatable by all 3 probes.



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Online Damianos

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #3 on: November 01, 2017, 05:23:07 AM »
Just some thoughts:
Did you tried to measure the output of the probe with a voltmeter?
Did you tried to measure the signal with the probe without "amplification" or with a few turns, like 5 for example?
Has the signal distortion, that is not easy to see on the screen? An FFT may show harmonics...
What measures the scope if the probe is replaced with a shunt resistor?
 

Offline capt bullshot

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #4 on: November 01, 2017, 06:00:29 AM »
Hmm, it's not clearly to see, but I think you've got some DC offset on the scope. This will result in a higher RMS value calculated by the scope than expected.
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Offline khs

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #5 on: November 01, 2017, 06:40:13 AM »
Maybe there are different ranges / gains of the internal amplifier of the probe.

I would vary the current from 1 to 100 mA.

Maybe you can see some nonlinearities.
 

Offline fcb

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #6 on: November 01, 2017, 06:54:05 AM »
Current clamp probes are sensitive to the absolute position of the wire through the centre of the probe (video below) - that might account for a few %. Another possibility (unlikely!) is that you have too many coils - probably as likely as your 34470A being off..

 

Online Andy Watson

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #7 on: November 01, 2017, 09:29:49 AM »
Have you taken steps to limit the bandwidth of the signal from your probe? The meter probably gives up at 300kHz but the scope goes up to 350Mhz - that's a lot of extra bandwidth for noise to be added to your signal.

 

Offline HighVoltage

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #8 on: November 01, 2017, 09:53:25 AM »
Thank you for all the replies.

I tried the following so far:

- Slower scope (100 MHz and 200 MHz): Same results
- DMM measuring AC voltage output from current probe instead of scope: same results
- Used completely different brand current probe: same result
- The input signal is confirmed with another 34410A in series with the 34470A
- Moving the probe around all the copper wires does not make a difference.

Hmm, DC on the output of the calibrator, overlaying the AC is a possibility, I will check that tomorrow.
I will also try only 10 windings

So far, only if I reduce the windings from 100 to 96, will I get an ok reading, confirming again about 4 to 5 % error.

It seems I am overlooking something obvious, really weird.
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Offline dl1640

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #9 on: November 01, 2017, 11:08:48 AM »
try another gauge of wire?
 

Offline VintageNut

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #10 on: November 02, 2017, 01:30:23 AM »
Core saturation?
working instruments :Keithley 260,261,2750,7708, 2000 (calibrated), 2015, 236, 237, 238, 147, 220,  Rigol DG1032  PAR Model 128 Lock-In amplifier, Fluke 332A, Gen Res 4107 KVD, 4107D KVD, Fluke 731B X2 (calibrated), Fluke 5450A (calibrated)
 

Offline BNElecEng

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #11 on: November 02, 2017, 02:09:48 AM »
I'd second the DC offset suggestion. The pictures of the sine waves show approx. 3.875 Divisions to the top and 3.625 Divisions to the bottom, so there's a difference of about 0.25 Divisions. Assuming it is offset, then taking half that value multiplied by the actual Amps per division gives the DC offset value. In the case of the 5.3A rms measurement, it's 2A per Div which gives an offset of 0.25 A rms. That works out to 5% error right there.

I hope this helps.
 

Offline HighVoltage

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #12 on: November 02, 2017, 02:28:00 AM »
Thanks for all the great feedback.

OK, problem solved !

I tried 1mm copper wire instead of 0.5mm and no difference
DC offset, measured with a 34470A was about 3 mV DC, so not really a problem.
The scope was AC coupled anyways and the bad readings were confirmed by a DMM

All three of these current probes that I had tested are calibrated by an official lab, therefore I did not expect a problem with the probes, especially since they are very close to each other on the readout.

BUT ... I was so wrong with this assumption.

In the pictures below, I am testing with 10 loops of copper wire (1 mm diameter) and all 4 probes are hooked up on the same loop:

- Channel 1: Tektronix A622, 1.047A (4.7% Error)
- Channel 2: Keysight N2893A, 1.006A (0.6% Error)
- Channel 3: Tektronix A622, 1.043A (4.3% Error)
- Channel 1: FLUKE 80i-110s, 1.029A (2.9% Error)

So, obviously it has nothing to do with the wires and setup, just the probes are out of calibration.

Now I can continue this project and make a few nice loop arrangements.

I am planning to make a 5, 10, 50 and 100 loop gadget and this way I would be able to test up to 1000A with a 10A input.





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Offline SZA263

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #13 on: November 02, 2017, 02:38:32 AM »
There is something weird in your measurements. The crest factor for a sinewave is 2 * sqrt(2) ~= 2.82843. In your 100 turn photo the peak/rms values are 29,21/10,39A, Crest Factor is ~2,821; In the 50 turn photo they are 14,96/5.299A, Crest Factor is ~2,823. So I think of an high distortion of the calibrator on this high inductive load. Can you make an FFT on the output signal, to see the harmonics? I dont believe on the DC offset (how can a DC cross a transformer?), but an high value of the even harmonics can be seen as an offset.
I hope this helps.

Regards,
Adriano
 

Offline RobK_NL

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Re: Current Probe Calibration, DIY 1:100 Amplifier not working right
« Reply #14 on: November 02, 2017, 03:16:22 AM »
There is something weird in your measurements.
No, there isn't. A scope is not a precision instrument and the differences you point out are in the order of 0.25%. That's about as close as you can get!
Tell us what problem you want to solve, not what solution you're having problems with
 

Offline HighVoltage

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Re: Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]
« Reply #15 on: November 02, 2017, 03:31:16 AM »
Ok, it is confirmed that the problem is with the probes.

Two original Agilent / Keysight probes are almost spot on with 0.1% and 0.8% error.
And the tektronix and Fluke probes are 4.9 and 3.4 % off

I guess, this means: "You get what you paid for" although the Tektronix and Fluke probes were not cheap.
Somehow, I just did not expect such a large error.


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Offline capt bullshot

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Re: Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]
« Reply #16 on: November 02, 2017, 03:35:00 AM »
Two original Agilent / Keysight probes are almost spot on with 0.1% and 0.8% error.
And the tektronix and Fluke probes are 4.9 and 3.4 % off
They are all rebadged:
The larger ones are made by Chauvin Arnoux, rather lower end stuff,
The small ones are Hioki AFAIK, high end stuff. You can get similar ones from Yokogawa, also rebadged.

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Offline HighVoltage

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Re: Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]
« Reply #17 on: November 02, 2017, 04:09:22 AM »
Two original Agilent / Keysight probes are almost spot on with 0.1% and 0.8% error.
And the tektronix and Fluke probes are 4.9 and 3.4 % off
They are all rebadged:
The larger ones are made by Chauvin Arnoux, rather lower end stuff,
The small ones are Hioki AFAIK, high end stuff. You can get similar ones from Yokogawa, also rebadged.

Interesting, thanks
Are you sure the larger one is originally from Chauvin Arnoux?
I always thought that they originated by AEMC, the Model SL261
http://aemc.com/products/html/moreinfo.asp?id=20201&dbname=products

Depending, who is offering the re-badging, they cost  between 400 and 800 Euro
Well, I should also have read the specs first, they are listed with a +/- 3% accuracy only!
And perform worst than specs!


The Hioki, Yokogawa/ Keysight high end stuff are almost 10 fold in price!
And the Keysight is listed with a +/- 1% accuracy.
And perform better than specs!

So, what do you do, if you need better than 1% accuracy in a clamp on current probe?
May be it has not been invented?
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Offline capt bullshot

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Re: Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]
« Reply #18 on: November 02, 2017, 05:50:34 AM »
Two original Agilent / Keysight probes are almost spot on with 0.1% and 0.8% error.
And the tektronix and Fluke probes are 4.9 and 3.4 % off
They are all rebadged:
The larger ones are made by Chauvin Arnoux, rather lower end stuff,
The small ones are Hioki AFAIK, high end stuff. You can get similar ones from Yokogawa, also rebadged.

Interesting, thanks
Are you sure the larger one is originally from Chauvin Arnoux?
I always thought that they originated by AEMC, the Model SL261
http://aemc.com/products/html/moreinfo.asp?id=20201&dbname=products

Depending, who is offering the re-badging, they cost  between 400 and 800 Euro
Well, I should also have read the specs first, they are listed with a +/- 3% accuracy only!
And perform worst than specs!


The Hioki, Yokogawa/ Keysight high end stuff are almost 10 fold in price!
And the Keysight is listed with a +/- 1% accuracy.
And perform better than specs!

So, what do you do, if you need better than 1% accuracy in a clamp on current probe?
May be it has not been invented?


I believe AEMC is Chauvin Arnoux or whatever, see the bottom of the page:
© 1999 - 2016, Chauvin Arnoux®, Inc. d.b.a. AEMC® Instruments

The Hioki ones are quite good, better than the famous Tektronix AM503 / A6302 and others. Those were the best you could get back then.
Be careful not to break the ferrite core, this hurts the low frequency accuracy. It's quite interesting, I've got one of these from work that was very abused, the clamping mechanism doesn't lock anymore and the core is visibly damaged - It's still performing well for higher (> a few kHz) frequencies but has some percent error (reading is too low) at 50Hz. A square wave looks peaking and then settles, but the settled line has too low amplitude, the height of the initial peak is the correct amplitude. At work nobody noticed until scheduled calibration failed ...
Seems for higher frequencies less core is required for these things to work. Internally they have a high frequency and a low frequency path, the low frequency path using a Hall element for zero flux compensation through the same winding on the core that is used for the high frequency path. See the Tek manuals for all the interesting details, the Hioki work exactly the same way.

I don't know if the CA ones have a winding on their core or use the Hall element as the only sensor. Otherwise, when having a compensation winding on the core, it's quite difficult to achieve such a large error. So I guess, they are rather open loop, no wonder there's such a large error.
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Offline dl1640

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Re: Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]
« Reply #19 on: November 02, 2017, 10:56:36 PM »
i believe the Agilent one is just a Hioki made, hall element current probe
 

Online NANDBlog

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Re: Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]
« Reply #20 on: November 03, 2017, 12:02:01 AM »
Fluke 80i-110s AC/DC Current Clamp
40 A to 80 A ±12% of reading +50 mA
80 to 100 A ±15% of reading
DC - 100 kHz

Tektronix A622:
No real spec. But it is going to be awful guess-o-meter type probe.

Keysight N2893A:
Accuracy (Probe Only)b ±1% of reading ±1mV (DC or 45 Hz to 66 Hz)

I dont see problems. They are all within spec. I think you are "just" expecting them to be better, while they are obviously the guess-o-meter category. Like 99% of current probes.

Try finding a probe which not only has good DC and AC specification, but good transient response. I placed a square wave on few of these probes. Some's output (fluke i400 or somethingsimilar) even started to go the opposite direction before changing it's mind and going the right way. Try investigating the transient response of your high current device with that.
 
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Offline HighVoltage

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Re: Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]
« Reply #21 on: November 03, 2017, 12:28:28 AM »
Hello NANDBlog

You are so right, I had much better expectations in the accuracy of these probes.
I had done some DC comparison before and they lined up right on spot to the Agilent probes.
Although 50 Hz or 400 Hz is not fast, it makes a huge difference already.

Good idea with to test the response on a square wave.
I will just make a setup where I can turn ON and OFF a 10A current through a precision resistor and see what the different clamp on probes will show.


 




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Online NANDBlog

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Re: Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]
« Reply #22 on: November 03, 2017, 12:42:41 AM »
Well, I was talking about more about the experience I had with them.
Try investigating the transient response of your high current device with that.

Good luck with that NANDBlog... Talking about the current probe that I had and the device I had to test.

I ended up building a test probe with a big shunt and a x100 or x1000 amplifier (Just an instrumentation amp).
But yes. If you want to see how bad they are, even a low frequency square wave will show it.
 

Online Kleinstein

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Re: Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]
« Reply #23 on: November 03, 2017, 03:54:36 AM »
If the probes use a different path for AC and DC, the cross over is likely rather low, as the DC path with a Hall sensor is not that accurate and prone to drift. So 50 Hz might very already be the AC part only.
 

Offline PTR_1275

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Re: Current Probe Calibration, DIY 1:100 Amplifier [SOLVED]
« Reply #24 on: November 23, 2017, 10:21:15 AM »
Has anyone made a current coil like the transmille or fluke ones for calibrating current clamps?

I only need DC, but am thinking of making something like the fluke one as it's design seems a lot simpler. Get some magnet wire, wrap it around some pvc piping, bend it around and epoxy in place with some terminals.

Or am I overlooking things?

Does anyone have a fluke cool that they can measure the copper diameter and coil diameter?

Cheers
-Chris
 


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