Death of a PSU

Robin, can you see what the part number is of the device fitted to the metal bracket/heatsink?

Andrew is correct - the many resistors are a snubber, the current sense resistors are the ones at the burnt pcb track.

This PSU is a quasi-resonant topology, and for the power rating indicated by Robin, unlikely that there is an external FET switch.

It does appear that the arcing on the PCB may just have been due to poor layout - the tracks a little close at that point, and maybe some dirt developed that precipitated the arc.

If you want to perhaps waste some money and time....

The diode next to the arc point might have blown - I believe that is a zener regulating the the supply line to the VCC input the the 8pin chip. The current sense pin and the drain pin on the device are very close, made worse by the pcb track layout, and those two pins have full rectified line voltage between them.

So, if the zener diode is blown ( would probably be a short) , replace, and replace the 8pin device - available maybe from Digikey , Mouser, maybe even RS components, CLEAN the pcb well and give it a try!

Below is a reference circuit for the supply - this one is a 20volt output, but I am pretty sure the the circuit is close enough.

Interested to know what that device on the heatsink is..

Good luck!

Joe

My initial thought was that likely all the components on the board could be replaced for twenty quid - and even any tracks repaired on the board. But even doing that might not be successful....

I would likely do as Duncan suggests - screw a suitable power supply to the wall and run some curly lead across to the unit - unless a cheaper (reasonable quality) unit, that fitted the space, could be sourced.

Joe, it's not easy to see the markings on the component but as far as I can make out there are two lines - top LT 1117 and bottom MBRF2045CT.

Thanks for posting the schematic. If I have a go at repair, what voltage Zener would I want? Presumably the 22V in the schematic is to do with it being a 20V supply? I am

Ta for the suggestions of mounting the PSU externally - the unit I have is the chassis mount (ie not fully enclosed) type, and I was thinking that by the time I'd bought a suitable enclosure and associated gubbins it'd be getting close to the cost of a new unit. But on second thoughts, maybe I could lash something together from stuff I've already got.

Robin.

The enner voltage has nothing to do with the output voltage. It s tere to protect the IC from voltage spikes (If R3 was more than 0 ohms it could be a regulatrd supply for the IC). Note that the circuit cannot be totally correct for the supply you have as it is using multiple surface mount components that are not showm on the circuit.
The failure appears to be on the un-isolated part of the circuit. While I would normally encourage any one to attempt a repair, even if not really economic, for the learning experience, an mains SMPS is noot a safe place to start learing about electronics. No disrespect, but from the questions you are asking, you almost certainly do not have the knowledge or equipment to safely repair this power supply.

Robert G8RPI.

A 22volt zener is appropriate - As Robert said - it catches spikes on the chip supply line and will regulate or clamp them to 22V. Chances are that if the zener does not measure short circuit it will be ok. The output voltage is controlled by the Optocoupler and the TL431 in the circuit - the latter may differ from your PSU.

Note that I did not say the reference circuit I gave is the actual circuit - it is a REFERENCE circuit provided by the chip manufacturer to aid the designer - Robin's psu will be a close derivative.

The part number MBRF2045 is a schottky rectifier, so most likely the output rectifier ( place of D3 in the circuit), I am sure that device will be operational still.

I do go along with the general persuasion that you just ditch it and get a replacement, but if you are like me and cannot leave things alone...go ahead and try to fix it!

It's not magic and the important thing is to be safe in the process. If you have no experience in the subject, and wish to have a go at replacing parts and seeing what happens, ok, just beware of high voltages on that circuit.

WRT the reference circuit, EVEYTHING to the left of the transformer and optocoupler is a life-taking high voltage. There is NO safe earth or ground reference in that part of the circuit, so you cannot easily connect oscilloscope probe grounds, etc to measure anything.

This danger is easily seen in the circuit, but is magnified on your actual board, as you cannot draw a nice visual line separating the danger zones. High voltage can be present anywhere. Do not handle the board at all in a manner that may allow skin contact with any pcb conductor or component lead. Make sure that heatsink is actually grounded, else don't touch it. Don't touch the PSU with mains on! And when the mains is off and disconnected, BEWARE of the high voltage still remaining in the region of the input filter electrolytic capacitors - 'C3' in the circuit...

Just be careful and attentive..

Joe

When working on this type of power supply would it be safer to power it with an isolating transformer,like we used to do with AC/DC radios and tv’s ?

I'm with Robert on this one SMPSU's aren't easy to troubleshoot and repair.

While an isolating transformer will give protection from shocks to other grounds it won't help with the 350V DC on the PCB. Unless the whole workshop is isolated conneing test equipment like an oscilloscope removes the isolation.

Be carefull out there!

Robert G8RPI

Thank you for the warning Robert . There was actually quite a lot hidden under the the conditional 'if' in my last post. Such as (inter alia) 'if ever I feel I understand this well enough to proceed safely" - which, at the moment, I don't, as you have rightly observed. Doesn't mean I can't learn by asking naive questions though!

Stay safe, Robin.

Yes Maurice, you can do that - exactly how it IS done in development labs and production test.

You don't need isolation on the 'isolated' side of the isolation transformer - that is what it is there for - so with a proper isolation transformer in place, you can then ground ( to mains ground/earth) the negative lead of the rectified PSU 'mains' input side. You can then also connect your oscilloscope ground lead to that same ground, safely, and probe about. HOWEVER .. the high voltage still exists as before, nothing has changed, you only have a safe ground reference.

That's why I said - There is NO safe earth or ground reference in that part of the circuit, so you cannot easily connect oscilloscope probe grounds, etc to measure anything-

'you cannot easily connect ...' you need an isolation transformer and some Care..

That supply is so simple - no switching FET, etc, and generally they pop the controller - The voltage reference optocoupler and TL431 reference normally are well isolated and do not suffer damage. So it is not as daunting a repair as may seem - check the zener, check the snubber diode ( the diode on the PCB top side) , just above the burnt underside, and replace any damaged, as well as the control IC. If it does not work, or pops loudly again, ditch it...

Joe

As Robin has said he will use the other PSU he has or maybe fit a new one, maybe we could give the defunct unit a decent burial and move on?

Don't know - ask Robin?

I haven't read all the replies to this post, this maybe useful or not.

Seven pin I.C's (Offline regulators) are used in washing machines to battery chargers for drills etc.

It is not uncommon for them to fail.

**LINK**

Good luck, Bear..

Well, my answer to John if he were to ask me would be move on, don't read, that's up to you as an individual.

For me personally this has been a very interesting discussion - for the first time in my life I think I am on the brink of the threshold of understanding an electronic circuit. I could have just bought a replacement PSU - it wouldn't have left me starving. But I wanted to understand what had happened and if I can repair it it would give me a buzz, of the life-enhancing rather than lethal type I would hope!

Robin.

Edited By Robin Graham on 28/07/2020 02:59:04

Edited By Robin Graham on 28/07/2020 03:00:48

Edited By Robin Graham on 28/07/2020 03:01:28

Switched Mode Power Supplies are a technology that in theory are very simple, but in practice very tricky to get right. By right I mean stable and reliable. Much improved over the years as many devices have been by introduction of integrated control ICs. When I was a lad, most were controlled by discrete components and would go bang very frequently and often for no apparent reason. I used to work on early CAD terminals, which had a PSU where its main switching device device would fail short cicuit often. We had a big stock of replacements. TIPL755A, I can still remember the number after 35 years....