30Amp amps clamp

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30Amp amps clamp

Postby leemack » Tue Dec 22, 2015 9:57 pm

Just recemtly purchased the 30A amps clamp and just want to clarify its zero shift. I've noticed after a while in use if you remove it from the cable (in my case main battery cable) then re-install it seems to read a higher current reading.

Check zero point and its shifted. I know its working as I compared it to in series current draw and its with in 4mA once re-calibrated and installed again.

I must admit I haven't read any documentation yet as I was in a hurry so not sure if its normal. It does though seem to work fine and fairly axcurate to about 20mA.

Thanks
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Re: 30Amp amps clamp

Postby FioranoCars » Fri Jan 15, 2016 3:29 pm

Sorry for the late reply!
sadly with all inductive clamps, they utilise a design which inherently suffers from drift.

First battery voltage will drop over time, and for our task when we learnt all of this we were doing 24 hour and longer parasitic drain tests, so 50-500ma range for long times. Tried using multi battery and larger battery configs but these all suffer from voltage drop over time, and this affects the output. Even tried a mains based stable output supply, and this reduced the drift but not entirely.

Second the "press button" zero style clamps seem to suffer from fast and more drift than the rotate knob user adjusted style, but regardless both still drift! It's in the design! (unless anyone can tell us all otherwise!)

Third we tried (and got inconclusive results, and bored!) using no zero, and using a maths channel to zero in software (although you might be able to do this in the probe config now?), but I'm sure in our tests we still saw drift.

We tried a 10 loop of thin wall cable to allow amplified current reading without amplifying the drift, so the drift would be 1/10 of normal and this worked well for some 4 hour based time frames.

For long time frame measurements of low current we moved to "volt drop across fuses" method (see another post here https://www.picoauto.com/support/topic12061.html ), which allows for long term measurement with high accuracy.

see also by Steve Smith:
https://www.picoauto.com/library/training/parasitic-drain

and VW technical bulletin 270711 with a downloadable PDF in the page here:
https://wiki.bentleypublishers.com/x/AoD3B

So basically your clamp is probably fine and behaving as they all do!

If you find any way to stabilise the clamp or another method please let me know

Best
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Re: 30Amp amps clamp

Postby PicoChris » Wed Jan 20, 2016 5:12 pm

Hi there

The other thing to remember is that all current clamps will also suffer from thermal drift, especially if doing long runs overnight where the garage temperature may change by several degrees during the period of the test. Unfortunately there is no easy way to compensate for this in the clamp without a lot of additional complexity.
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Re: 30Amp amps clamp

Postby FioranoCars » Wed Jan 20, 2016 11:28 pm

Hi Chris
Totally forgot about thermal issues!
Maybe you can help, what is the main internal issue that affects the "Press" zero and drift verse the turn knob zero. I'm sure someone at Pico gave me the full technical low down about 6-7years ago, but to be honest, I was a tad less able to comprehend the implications and must have stored it in the WOM section of my grey cells (for those under 40 ... it's the "Write Only Memory"!).

Thanks

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Re: 30 Amp amps clamp

Postby PicoChris » Thu Jan 21, 2016 3:27 pm

The difference is in how the clamp stores the required offset information.

With the "push the button" version an internal cap is charged by the correct amount to correct for the offset. The advantage of this method is that it is both quick and accurate. The problem is the charge on the cap will leak away over time causing the unit to settle back towards its original setting.

With the "turn the knob" zero then the offset value is corrected by simply adjusting a small pot to zero out what the customer is seeing. The downside of this method is you have to look at it and do the correction by hand. One the other hand the advantage is that you should get less long term drift as you do not suffer from the charge leakage problem of the push button version.

With all these concerns you are only likely to have issues when doing long term tests. When looking at typical signals over a few minutes you are unlikely to have any problems.
The key to remember with all these errors is that generally they all tend to generate more offset drift than gain drift which can be easier to allow for.



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Re: 30Amp amps clamp

Postby FioranoCars » Thu Jan 21, 2016 3:45 pm

thanks,
Nice detailed notes, but begs a few other question before this subject is almost flogged to death!

So would it be right to say that with the push button there is more drift if the button is pushed verse software offsetting and not using a push button at all (2-6 hour tests)?


Would inherently, all thing being equal doing a 2-6 hour test - the manual zero type, zero'd, be prone to less drift that an unzero'd push button?

I assume as it's a manual pot, there is no discernible difference in drift be it zero'd or not, as it's a manual pot?

Can the Pico software Zero the clamp in the probe settings (there is a ZERO function but not sure if this is the sort of intended use, or if it would be applicable/work at all?

Thanks
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Re: 30Amp amps clamp

Postby PicoChris » Thu Jan 21, 2016 4:14 pm

Regarding the push button I have to say I really don't know we haven't done enough testing to say with confidence how quickly the cap will discharge and the environmental conditions will have a significant effect (humidity, temp etc.)

The pot on the TA234 should be stable but is inherently a little more prone to thermal drift if there are any temperature changes

The Pico offset zero should be a stable offset but it relies on having a stable "zero“ for the Pico software to null out and so may not always work.

With all of these it is important to understand that thermal drift can easily be larger than offset null drift so a stable temperature is key for any long term runs


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