# QW-MS305D 30V 5A Bench PSU repair

#### Lumenosity

Joined Mar 1, 2017
559

Just stumbled across this thread. I laughed because you sound like me. I have the exact same power supply and I also have 3 other better power supplies but I want this one fixed.

You've probably moved on maybe tossed the unit.....but I'm still working on mine.
I can confirm I am reading the exact same things you are on yours. The 2-3-2 pin side of the transformer, all the pins are shorted together. so now I assume from your post that that is normal for this transformer. I was also getting the shorted power rails like you.

I messed mine up by accidentally connecting it reverse voltage to a 12v battery I was going to charge with it.

I checked nearly every component on the entire board and I found just a few things.
1). There was a burnt out section of track located on the bottom side of the board under C31 near the large coil inductor.
2). P75N75 Mosfet was blown
3). The two black NTC thermistors RT1 and RT2 both had very high resistance above specs so they were replaced.
4). Both IRF840's had high On State drain to source resistances and were replaced.

Everything else seemed ok. I'm still waiting on a few parts to finish it.
I'm wondering if either or both of the IC chips were damaged (TL494CN and TL084CN)
To that end, I'm building a test circuit for them so that I can test that they are both ok.
I'm wondering if the short we are both experiencing could be in those IC chips?

I'm doing this as a learning project. It has nothing to do with the value of the Power Supply as a power supply.
I have already repaired 2 Meanwell power supplies and a 48v ebike battery charger so it's all in the name of learning.

On my bench now is a battery pack from a Bissell Car Vacuum, a Power Inverter and a hard drive SMD controller board.
Starting to practice with replacing SMD components.

Anyway, if you fixed your 30v 5A PSU I'd sure like to hear about it. If it's still on the list of to do's maybe we could share info and help each other?

Moderator Edit: New thread created from here:

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#### MisterBill2

Joined Jan 23, 2018
8,990
Of all of the components in a switching mode power supply the transformers are the least likely to fail, unless the connections are poorly soldered. So really, the very first thing to do is check all of the semiconductor parts, and then check the capacitors for shorts and leakage, and last check the resistors for the correct value.
The funny thing is that I was given a supply looking like the big brother of this one, previously sold at a discount for $600. The complaint wa that it lit up but no output. The actual failure was that they did not read the manual. Both the current limit and the voltage limit controls must be set above zero for the output to be possible. So maybe this supply had the same problem. Really, the transformers seldom fail unless the thing is dropped and the core gets broken. Thread Starter #### Lumenosity Joined Mar 1, 2017 559 Cannot get schematics anywhere . Of all of the components in a switching mode power supply the transformers are the least likely to fail, unless the connections are poorly soldered. So really, the very first thing to do is check all of the semiconductor parts, and then check the capacitors for shorts and leakage, and last check the resistors for the correct value. The funny thing is that I was given a supply looking like the big brother of this one, previously sold at a discount for$600. The complaint wa that it lit up but no output. The actual failure was that they did not read the manual. Both the current limit and the voltage limit controls must be set above zero for the output to be possible. So maybe this supply had the same problem.
Really, the transformers seldom fail unless the thing is dropped and the core gets broken.
I dropped the stupid thing yesterday......now it may really be bad

I guess I better invest in an LCR meter. Thing is, since all 7 of the primary leads seem to be shorted together, I have no idea how to connect an LCR meter.

#### MisterBill2

Joined Jan 23, 2018
8,990
Once again, I saw a solder blob between pins 3 and 4 in the photograph.
And depending on the meter used to check resistance, even ten ohms may look like a short circuit. And often some of the windings ae just a very few turns and so the reistance is very low.

And even for an item that does not function there are usually useful parts, including mechanical piece like screws and nuts, and knobs.

#### Lumenosity

Joined Mar 1, 2017
559
Once again, I saw a solder blob between pins 3 and 4 in the photograph.
And depending on the meter used to check resistance, even ten ohms may look like a short circuit. And often some of the windings ae just a very few turns and so the reistance is very low.

And even for an item that does not function there are usually useful parts, including mechanical piece like screws and nuts, and knobs.
Thanks.

So where would the leads of an LCR meter be connected? Pins 3 and 4 ?

#### Lumenosity

Joined Mar 1, 2017
559
I am practicing / learning with a 30V 5A power supply that I shorted by connecting it reverse voltage to a 12v car battery.
I've been taking it apart, testing and replacing components, repairing pads etc.

Today it finally fully powered up and all the lights came on and I thought I had it. Then I started to turn up the current knob and an ES2G diode erupted in flames and smoke. It is seen as the missing Diode 21 (D21) in the photos below.

The thing is, right next to it is another ES2G in parallel I assume to increase the current handling capability. But the other diode seems to be OK?
(Meaning it didn't go up in flames and appears ok visually. I haven't removed it yet.

I had removed diode 21 (D21) about a week ago then reinstalled it. I had a bit of trouble since it's SMD and I don't have much practice with SMD.
It's likely I overheated it.

Question:
Did I possibly damage D21 and that caused it to release the magic smoke (since the parallel diode next to it didn't seem to be affected, or is it likely something else on the board caused the blow out of D21 ?

Opinions welcomed.

Thanks.

Here is a Zoomed in shot of D19 and D21.

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#### Lumenosity

Joined Mar 1, 2017
559
This last post had nothing to do with the other person's repair. it just happens to be a similar power supply. Different circumstance.
The OP (not me) started the thread some time ago. My concern is recent as in today.
I am not addressing his repair questions with my post.

A moderator moved it here.
Moving it here means it will never get an answer.
I spent valuable time taking the photos, editing them and posting the content....to have it arbitrarily moved out of view.

Is this to say that anytime someone posts about any item that has been discussed previously all those new posts should be tacked on to the older threds concerning the same or similar devices?

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#### MrChips

Joined Oct 2, 2009
23,869

#### Lumenosity

Joined Mar 1, 2017
559
That schematic is very close to the one for the PSU in question.

I may post my photos and question about the pair of twin diodes over at EEEBlog.

#### Lumenosity

Joined Mar 1, 2017
559
See Post #6

Question:

Did I possibly damage D21 during soldering and that caused it to release the magic smoke (since the parallel diode next to it didn't seem to be affected), or is it likely something else on the board caused the blow out of D21 ?

Opinions welcomed.

Thanks.

#### MrChips

Joined Oct 2, 2009
23,869
See Post #6

Question:

Did I possibly damage D21 during soldering and that caused it to release the magic smoke (since the parallel diode next to it didn't seem to be affected), or is it likely something else on the board caused the blow out of D21 ?

Opinions welcomed.

Thanks.
If D19 and D21 are two rectifiers in parallel I can see two possibilities.

1) D21 was weakened and got blown from high reverse voltage.

2) D19 is dead and an external fault took out D21.

#### Lumenosity

Joined Mar 1, 2017
559
If D19 and D21 are two rectifiers in parallel I can see two possibilities.

1) D21 was weakened and got blown from high reverse voltage.

2) D19 is dead and an external fault took out D21.
Thank you Mr. Chips.
I was afraid I get no responses due to the question being moved here.

I think at this point I need to think about short circuits.
My plan is to go ahead and replace the fried D21.
Previously, it powered up fine even with the blown D21. It was only when I attempted to apply current to a connected battery that it blew.

Hopefully I can come up with a strategy to check the board at strategic points for shorts, unexpected voltages etc.
I might try to disconnect the input side from the output side and then power up each side independently and measure the results.

I do have an isolation transformer on hand as well as a battery operated oscilloscope.

#### MrChips

Joined Oct 2, 2009
23,869
If you are going to replace D21 then replace D19 at the same time.

#### MrChips

Joined Oct 2, 2009
23,869
D19 and D21 are across the output rails.
Putting a battery across the output could blow both of these.
The PSU should still function with D19 and D21 not installed.

#### Lumenosity

Joined Mar 1, 2017
559
I'm wondering if I could have inadvertently installed D21 backwards.
Doubt it...but...I know I got it pretty hot reinstalling it.
D19 and D21 are across the output rails.
Putting a battery across the output could blow both of these.
The PSU should still function with D19 and D21 not installed.
Yes, the PSU will still power on, but it will not pass current to the outputs without D1 and D2 in place...at least that's what I thought.

I had the Outputs connected to a 12v battery. It was fine...until I turned the CURRENT knob to start passing current to the connected battery. I frequently use Power Supplies to charge batteries.

You're not saying that connecting the outputs to a 12v battery would be a problem are you?

#### MrChips

Joined Oct 2, 2009
23,869
What I meant to say was if you had inadvertently connected a 12V battery in reverse it should have blown both D19 and D21.

I see in the circuit diagram L2 and C42. I do not see these on your board on the photo posted in post #6.

#### Lumenosity

Joined Mar 1, 2017
559
What I meant to say was if you had inadvertently connected a 12V battery in reverse it should have blown both D19 and D21.

I see in the circuit diagram L2 and C42. I do not see these on your board on the photo posted in post #6.
Hello,

What originally caused the problem was just that. I connected the outputs to a 12v battery in reverse. Magic smoke released.
There was a part of the positive power track that was blown out, MOSFET P75N75 was shorted, both IRF840's had very high on state resistance so they were replaced. Other than that I couldn't find any other issues. IIRC, I checked every other diode except D19 and found none blown surprisingly.
However, I had previously tested D21 and it tested good showing .495vf and OF when reverse connected. But I'm thinking I overheated it
when trying to reinstall it and damaged it. I never removed D19 and that is why it survived (I think).

I am finding that while the schematic from eevblog is very similar, it is also VERY different than the board I have.
I was just checking the connections to the TL494CN chip and most of the pinouts go to places that are different than on the schematic you are referring to. I also notice that schematic does not use SMD components and my board is loaded with them.
Finally, I belive it is for a 60v PSU and I have a 30v unit.

I don't think we can use that schematic in reference to my board.

One other thing.....
I did remove D19 and found it to be in perfect condition. To me, this means that since it was not even damaged, then it points to D21 as having some kind of internal problem. Or at least I hope so because that might mean replacing it could put my PSU back in service. If there was a problem with the board, it seems to me both D19 and D21 would have been damaged.

I'm very optimistically thinking replacing D21 might do it. I've been over this board and the components with a fine toothed comb. Fairly confident it has no other issues. FLW

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#### Lumenosity

Joined Mar 1, 2017
559
Replaced the blown ES2G.......
A picture is worth how many words ?

I'll be modifying it to include a reverse voltage protection module.