Help with calibration of a digital lab PSU

Yann Guidon / YGDESYann Guidon / YGDES wrote 07/27/2017 at 00:23 • 1 min read • Like

I just found out that my new shiny cheap lab PSU is 5% to 10% off !

It has a nice resolution but the readings in both V and A don't match my measurements with an external multimeter...

The model AX6002P from Axiomet is similar to the better known Velleman or KORAD KA6002P so it might be a known issue....

Update 20170730:

My "golden tester"'s battery is a bit low (8.5V) so I changed with a fresh one (curious since I changed it less than a year ago).

I tested the measurements against other multimeters, checked the power supplies and ... all values are coherent within the respective accuracies.

Bad battery. Case closed. Sorry for the alarm. But the question is still good and it would be cool to be able to calibrate the device "if something happened" :-)



Dr. Cockroach wrote 07/30/2017 at 08:13 point

Been there with a bad batt before. Bought a 9 volt that was dated for 2025 +- and my DMM did not like it ;-)

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Yann Guidon / YGDES wrote 07/30/2017 at 08:16 point

2025 ??

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Ted Yapo wrote 07/30/2017 at 14:30 point

I got fed up with 9V meters a while ago and started putting lithium 9V's in there.  They last longer, have amazing shelf lives, very flat discharge curves, and you see less of this kind of thing.

The best thing though are the newer DMMs that take AAs.  They run for many weeks straight (I've used ones with serial ports for long-term data logging).  Again, I put lithium AAs in there - same benefits apply, and you get a 20-year shelf life. The ones I have in the battery drawer now show a 2036 date code,

You pay more for the cells, but you have to balance against the cost of your time and accurate results :-)

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Dr. Cockroach wrote 07/31/2017 at 00:36 point

Yup, The local Wal-Mart has brand name batteries that are dated way out. Think that is just a selling point and not intended for the likes of us ;-)

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Elliot Williams wrote 07/28/2017 at 21:26 point

Do you really care?  I like my PS to be noise-free, but I don't mind if it puts out 3.5 V or 3.6 V.

If you do, does it have trimmer resistors inside?  Can you calibrate it?  I'm guessing yes, even if the manufacturer doesn't say you can.

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Yann Guidon / YGDES wrote 07/28/2017 at 21:30 point

Of course that I care...

If I recharge a Lithium based battery for example ?

I intended to use this digital PSU as a part of an automated test rig...

It seems I'll have to open the box :-/

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K.C. Lee wrote 07/29/2017 at 00:13 point

May be collect a set of data points to figure out if it is a scaling/offset/linearity issue(s)?

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Yann Guidon / YGDES wrote 07/30/2017 at 04:52 point

nah... a PEBKAC problem.

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Yann Guidon / YGDES wrote 07/28/2017 at 06:39 point

You have a point :-)

I should find my decade of voltage references and change the batteries of the multimeter, but I did that not that long ago... Anyway I have "a certain number" of multimeters, I'll cross-check.

If my PSU is indeed "off by 10%" then I don't know what to do, as I don't want to mess with it. I find curious that the voltage AND the current are off by approximately the same ratio, so there might be a drift in an internal common reference part.

Daaaamn many of my recent measurements are wrong then ?

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Ted Yapo wrote 07/28/2017 at 13:20 point

I have never really trusted meters in power supplies - I know some of mine are off for sure!  In fact, once you start using any significant current, any voltmeter built into the supply itself is guaranteed to be off somewhat because of the resistance in the power leads (unless you have a 4-wire power supply with separate sense wires, which I'm guessing is not the case).

I usually put together an external 4-wire setup with a trusted DMM (voltage) connected right at the load, with another DMM (current) in the line from the supply.  The only significant error source is the current drawn by the voltage DMM, but with a 10M impedance meter, this is tiny, and only affects measurements in the uA range.  You can subtract it off, too, which I end up doing for single-digit uA currents.

Like the two meters on the right side here (ignore the rest):

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Richard Winkelman wrote 07/28/2017 at 02:22 point

I would get ahold of a fluke meter. They are known for their quality and accuracy.  Use that to calibrate your power supply. There should be some blue dash pots that are used for adjustment. 

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Ted Yapo wrote 07/27/2017 at 12:01 point

I'm just going to play devil's advocate for a moment :-)

How do you know your external multimeter is accurate?

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davedarko wrote 07/27/2017 at 22:18 point

Ted has a point! I've been there...

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Yann Guidon / YGDES wrote 07/30/2017 at 04:52 point

and he was right :-D

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