Faulty Bench Power Supply

[acorn222]

Hi all,

This is my first post on the legendary EEVblog forum, I have brought a bench power supply from aliexpress (0-30v 0-5a), after I asked about it, they asked for a video showing it being falty and then the aliexpress moderator gave me a refund. The issue seems to be if the power supply goes under 11v, it can't hold anything and slowly dwindles down to 0v when there is no load and it seems to turn on when it get's to 11v. I have tested the potentiometers and they seem all good (3.3k 10 turn pots), something strange I realised was that it can go under 11v with a load and the voltage turned to 11v and the constant current being turned down? I'm fairly new to electronics but I am aware of the basics, is there anything that I should be looking at? I have taken the cover off to have a look and there seems to be nothing obvious at first look, is there anything that commonly goes wrong that I should check?

Here is the video I sent in: https://drive.google.com/open?id=1w_NCbxxNWce-cM29wWmKp1TNnS8x_FIy
Here is the link if anyone wanted: https://www.aliexpress.com/item/Mini-Digital-Power-Supply-Lab-Voltage-Regulator-Bench-Power-Source-Adjustable-Switch-DC-Bench-Power-Supply/32957898671.html?spm=a2g0s.9042311.0.0.59ed4c4deG3gqH

If there are any requests for me to take more pictures of the components or the insides, I can do so,

Thanks for reading this

[acorn222]

I just realised this isn't my first post but the question still stands 

[xavier60]

My hunch is that there is a problem with the secondary tap changing circuitry.
Maybe when the output goes below 11V, the lowest voltage tap is supposed to be selected, but nothing is being selected instead.

Edit: assuming it's not a SMPS.

[acorn222]

Yeah, that was my initial thinking too, however after I've taken a look inside, it does appear to be a SMPS (I think, not 100%) with the one mosfet (I think) on the heatsink with 3 1000uf capacitors on the output along with a fairly big inductor (all of this on the low voltage side of the divider), I'm really not sure what the problem is now

https://imgur.com/a/K2YTh8W

[PKTKS]

YES  by any means this is not one but probably 2 SMPS in one.

The photo is not clear enough  but I CAN SEE 2 SUPERVISORS:
- a classic TL494 
- a TNY277G

They both differ quite a lot

While TL494 usually drives half-bridge or full bridge topologies
it can be used as a pre driver sometimes

On the other hand TNY26x/TNY27x  are widely used in forward converters

I can see two "transistors"  and if they are in forward mode they are likely
to be using the classic 2 switch forward topology for optimum transformer
coupling

If the number is not TL494 so the SMPS has a good chance of being
a 2 switch forward converter with clamping diodes on primary side

and likely the 2 topologies should be opto coupled 
usually the TL431 schema is the mostly used

not a trivial thing to debug

Paul

[PKTKS]

I am curious with your video...

I see a multimeter which my guess would be the M890G
by design and rotary switch..

But... I see the scale marking VOLTS and actually the switch
in VOLTS..

but your probes are INSERTED IN AMPS and that thing WORKS ?!!!!
reading volts in  amps plugged in

WTF
PAUL

[bitseeker]

What do you have the power supply's current knob set to when experiencing this fault and conducting these tests? If you're artificially limiting the power supply's ability to deliver current to the load, then it has no choice but to reduce its output voltage to obey your command.

As PKTKS pointed out, you have a potential short circuit being created with the multimeter's incorrect setup. This may have led you to turn down the current limit on the power supply.

First, fix the multimeter by ensuring that the leads are inserted into the correct sockets for the type of measurement you wish to make (e.g., Com and V for voltage) and set the mode to match. Then, set the power supply to maximum current and the voltage to the desired level (e.g., 5 volts). Measure.

Next, set the power supply's voltage to what's needed by the load that you want to test. Keep the current setting at maximum. Connect the load and see if it works properly.

[SlowLogicBoy]

I have the same PSU, and mine is failing too, mine is failing on 15v can't go any lower, also if I try to adjust any lower I hear relay clicking in.
Pots are good, on first inspection haven't seen anything blown up.

However few things are different (attached photos)

I'm a novice at electronics, I wonder what should I check, to diagnose the problem and possibly fix.

[Refrigerator]

I am curious with your video...

I see a multimeter which my guess would be the M890G
by design and rotary switch..

But... I see the scale marking VOLTS and actually the switch
in VOLTS..

but your probes are INSERTED IN AMPS and that thing WORKS ?!!!!
reading volts in  amps plugged in

WTF
PAUL

Good catch.
Black lead should be in COM and instead it's in mA.
Same DMM:
https://www.stevenhoneyman.co.uk/2012/07/review-skytronics-mercury-600219.html

[xavier60]

I have the same PSU, and mine is failing too, mine is failing on 15v can't go any lower, also if I try to adjust any lower I hear relay clicking in.
Pots are good, on first inspection haven't seen anything blown up.

However few things are different (attached photos)

I'm a novice at electronics, I wonder what should I check, to diagnose the problem and possibly fix.

The IC at the top left corner, above J3, is unusual. Can you read its part number?

[narkeleptk]

I had to fix this same similar issued on one of mine not long ago. It started out just like yours, Slowing dropping voltage. Then finally blowing the fuse any time it was turned on.
I dont feel like typing it all out but you can see the post here on AAC
https://forum.allaboutcircuits.com/threads/repairing-30v-10a-dc-power-supply.178711/#post-1654731

[SlowLogicBoy]

I have the same PSU, and mine is failing too, mine is failing on 15v can't go any lower, also if I try to adjust any lower I hear relay clicking in.
Pots are good, on first inspection haven't seen anything blown up.

However few things are different (attached photos)

I'm a novice at electronics, I wonder what should I check, to diagnose the problem and possibly fix.

The IC at the top left corner, above J3, is unusual. Can you read its part number?

LP15R045S G2B1

Attached the image

I had to fix this same similar issued on one of mine not long ago. It started out just like yours, Slowing dropping voltage. Then finally blowing the fuse any time it was turned on.
I dont feel like typing it all out but you can see the post here on AAC
https://forum.allaboutcircuits.com/threads/repairing-30v-10a-dc-power-supply.178711/#post-1654731

Thanks.

[xavier60]

Measure the voltage on pins 2 and then 15 on the TL494 while adjusting the voltage setting to find the voltage set input.
Avoid touching pin 3.

[SlowLogicBoy]

Measure the voltage on pins 2 and then 15 on the TL494 while adjusting the voltage setting to find the voltage set input.
Avoid touching pin 3.

pin 15 always ~3.3v
pin 2:
when trying to get lower voltage then 15.7v I'm getting 0, if I slowly decrease from 15.7 to lower it goes from v2.35 straight to 0 (voltage slowly dropping on output)
on 15.7v - 2.353v
on 16.0v - 2.394v
on 20v - 2.993v
on 30v (max rated) - 4.490v.

also I noticed that pot should be half way when trying to get 15v but it's actually near zero, seems like it regulates from 15.7 to 30v just more precisely.

[xavier60]

Is there anything unusual in the way the voltage changes at the pot's wiper pin?
What sort of pot is it?

[SlowLogicBoy]

It's a 10k pot, in resistance mode it goes from almost 0 to almost 10k normally.
When it comes to voltage, the same story as TL494 2nd pin, just a different voltage, ir goes from 0 to ~2.6v immediately and after that gradually ~4.9v.

Trying to troubleshoot if it definitely not a pot problem.

[xavier60]

What's happening on the pot's other 2 pins? Is it a 10 turn pot?

[SlowLogicBoy]

Yes, it's a 10 turn pot. and yeah it's a crappy pot... one pin is bad.

Added different 10 turn pot works normally now.

[xavier60]

Sounds like this , https://www.eevblog.com/forum/chat/fake-bourns-10-turn-potentiometer-tested-to-failure/msg3095977/#msg3095977
Bochen also make other styles including the 3590 which I have not tried yet.

[SlowLogicBoy]

Yes, exactly.