Tenma 72-10480 Bench PSU - 5mA offset?

[jewelie]

Hi

I've just bought a commonly recommended starter/basic bench Tenma 72-10480 30V 3A Bench PSU direct from CPC Farnell ( https://cpc.farnell.com/tenma/72-10480/power-supply-1ch-30v-3a-adjustable/dp/IN06822 )

In testing, the current reading on the display seems to be offset low by 5mA+, all the way down, e.g.

• 180mA (on PSU) = 185.5mA (on 2 DMMs)
• 90mA (on PSU) = 95.5mA (on 2 DMMs)
• 10mA (on PSU) = 15.1mA (on 2 DMMs)

This is my first bench PSU, I'm struggling to understand which accuracy ref to compare these to in the datasheet ( https://www.farnell.com/datasheets/2241024.pdf ) but I think it might just be within spec? 

I'm a little frustrated with this 5mA offset.

My options, as I see them are-

• If it's actually out of spec, get it replaced.
• User recalibrate it - if this is realistically possible. (Anyone know?)
• Put my big girl knickers on and deal with the fact that it's not quite accurate. (Meh, I bought a bench PSU because I didn't want to keep relying on fudging powering things.)

Thoughts?

- Julie

PS  For reference, I'm a hobbyist with an ancient CS+EE degree, so I messily straddle the line between beginner and not entirely. I'm relatively aware of the dangers of 250V (which is what the "230V" is around here) and tend to be very cautious etc, but I'm also aware that there's soooo much I don't know.

[Andy Watson]

Probably option 3, keeping in mind that these units are obviously built down to cost - you're still getting a lot of bang-for-buck if you can live with its foibles.
If you want to explore option 2, search-out details of the Korad KA3003 - the un-  or re-badged version of the Tenma. 

Or there's the second-hand TTi offerings for a similar price - but that's another can of worms

[mqsaharan]

I think its a nice unit for the price.
Please do perform some basic testing (and may be some intense testing) of the unit before committing to it and if you find anything wrong with it better to get it exchanged under warranty while you can or if you would like you can improve/repair it to your liking.
The early units had some serious problems with the design of output transistor board but I think that has been solved. Check out Dave's videos #315 (last five minutes) and #404 for it. #314 is the teardown video.
Take a look at the following links. These may not be exact models as yours but may give some useful information.
Some people have reported problem with rotary encoder. I don't know if it has been solved. Read the comments as well.



For repair, this also has schematics, calibration procedure:
https://www.eevblog.com/forum/repair/korad-kd3005p-problem-with-current-metter/

[ledtester]

fwiw, here are the specifications for the Korad series. The Tenma looks like a rebranded KA3005:

https://static.eleshop.nl/mage/media/downloads/KA3005P_datasheet.pdf

Note the Setup Accuracy and Read Back Temp Coefficient.

[J-R]

You could try the calibration steps listed here:
https://www.eevblog.com/forum/testgear/korad-3005p-calibration/

I ran through this on my KA6003P and while I wasn't particularly impressed, it's still reasonable for this class of device I suppose.

Mine is basically 20mV low all the time except for the upper setpoint, which is spot on.  I can adjust that high or low and doesn't seem to matter - it's always 20mV low everywhere else.
For current I was able to reach ~3mA setpoint accuracy and ~1mA readback accuracy.

[modoran]

If on 15mA real current it shows 10mA the error is 50%. No way this should be "in spec" by any means.

[alm]

If on 15mA real current it shows 10mA the error is 50%. No way this should be "in spec" by any means.

That's not how specifications work. They are composed of a fraction of the actual value (0.1% for setup accuracy), an offset value (3mA for setup accuracy) and sometimes a temperature coefficient in %/degree Celsius. You have to multiply these with the right value and add them all up to get the total uncertainty. This video touches on that: https://youtu.be/U4JFeU-o2kc.

In the case of the Korad, the readback temperature coefficient is a quite confusing 100 ppm + 5 mA. Does that mean if the supply is reading 10 mA, and the temperature changes from 20°C to 19°C, the reading might change with 100 ppm/°C * 1°C * 10 mA + 5 mA? Or did the labels get messed up, and is what they call "Read Back Accuracy" on the data sheet ledtester shows actually readback resolution? And is readback temperature coefficient actually readback accuracy?

In general, if a unit has a resolution of 1 mA, then I don't expect great performance at 10 mA. So 5 mA offset would be barely acceptable to me, although I might see if I can adjust it to be better.

[John B]

I have the 3005 (and a 6005) and did notice the same behaviour. I didn't check the full range but it will only read above 0mA once the actual current is >6mA, and then there is approximately a 5mA offset like you describe although it looked like that error was reducing above 20mA, like I said I didn't check the full range. I haven't checked the 6005, but I expect it to be worse.

They're OK for the price. The current limit is more of a safety feature than a precision source.

[coromonadalix]

Unless there is a calibration procedure to null thoses errors,  it could be hardware related

It could be less than perfect, because of the dac and adc errors, technology used,  slight loss in the control circuits  etc ...  design related ??

You would need maybe more advanced electronics / cost wise    to have a more precise psu ??


I have some BK  variable smps at my job,  same behaviour here, they are not perfect.

And sadly   Specs sheet are a thing, they are not absolute


OR maybe like me  i did bought an kikusui PAD serie psu (75v 5 amp),  buck boost, phase controlled, "semi linear psu"   BUT  there are trimpots to null out DC voltages and DC currents, offsets, meters  etc ...

And you can do it by yourself ...

if it still bothers you,  change for a more advanced one    with the needed adjustements, or known calibration procedures ?

[J-R]

Experimented a bit more with the zero setpoints and was able to get to under 10mV setpoint & readback accuracy.

[Kean]

Attached is some official Tenma 72-10480 calibration instructions I filed away.
They also apply to similar units like the Tenma 72-10495 dual 30V 5A model.

[coromonadalix]

thks  now thats better ...

[jewelie]

I decided try the calibration procedure, with much anxiety and trepidation but also ending in success, and now I'm really happy.   

Thank you so much everyone (especially Kean!)




Calibration:  The story

1. Dilemma: what to use for the calibration?

I have two meters, one newish generic Vici VC97 and I'm also very lucky to have a (admittedly 30-year old and only version 1) Fluke 89. 

Voltage?  For voltage it didn't really matter, both meters pretty much agree and voltage measurements even in cheap meters generally tend to be pretty accurate anyway.  It might be old, but I was previously able to calibrate it against my 2.5V ref, and I have a copy of the relevant service manual so was able to run through the various tests, and I knew can turn on the slow-but-precise option, and it's served me well, so I stuck with the Fluke.

Current?  Hmmm. This was harder. I do not have a current reference, nor do I have a relevant resistance reference.  Both meters mostly agree in the mA range, but slightly less so on the A.  Obviously the shunts in a new Fluke would have been made to a greater accuracy than a no-name meter, but what about it's age?  I did a few experiments measuring resistances and measuring the voltage across them under load (whilst trying to avoid starting a fire) and it looked like the Fluke was probably giving the most consistent results. I guessed the mA range was still good as the mA range fuse test procedure measures the resistance of the mA shunt and the fuse, which came to a nice round figure offset by only the smallest amount aka the fuse. Reflecting, I imagine A range shunt in the Fluke won't have suffered too much from age anyway, especially as it didn't get much use anyway until recent years?  I knew it's voltage readings were good too, so I settled on the Fluke again.

2. Doing the calibration!

At first I wondered why the display wasn't changing, then realised that was by design and it'll be the figures on the meter moving, doh.

The at the end, I did have some drama doing the full-scale current calibration!  I was trying to use a 3.3ohm load resistor in series with the meter, rather than actually following the instructions, panicked and pressed M1 (meaning the current presently being read is now gonna be considered 3A by the PSU going forwards.) So, I'd now completely messed the current measurement up by about a factor of 2! Aargh.

Hmmm, time to actually follow what it says in the calibration procedure and remove the load resister and just drop the meter set-up in A mode directly across the PSU and trust it wasn't going to blow the idiot-tax in the meter. I did briefly get a little startled sorting it out as the values on the PSU were now changing  But soon it was done.

Mission success!   

- Julie