WOLF Heater PCB fried by technician - need to identify Diode (resolved)

[markusdd]

Hi Guys.

Also shared this on X and pinged Dave, but I think here it will be more effective.
Could not find my old account, so here we go with a new one

Friend had his WOLF heater control PCB fried by a technician who (I suppose) connected the mains on wrong pins  after maintenance or shorted them out to a non-AC-pin. So the magic smoke escaped...
I need to order parts and I assume as per the picture the diode of the clamping circuit was fried. The switching IC of the AC/DC converter is a Power Integrations TNY279PN (output 12V, 12W max). According to their datasheet a 1N4007 should be fine, but the circuit on the heater PCB is not exactly according to the datasheet and they have used a slightly different one it seems.

According to power integrations datasheet it should be the red circled diode that is broken, the resistor above it  going to the top should be a 1kOhm, which according to the rings fits with the picture of the WOLF heater PCB.

Any thoughts? You think a 1N4007 is fine or can someone identify the original Diode? Maybe there is even a schematic of this board somewhere?

EDIT: Also would like to know the value of the resistor right next to the Diode. It seems to be an 18 Ohm when I get the rings correct, but that seems weird to me. Any input on this?

[markusdd]

*lil boost*

meanwhile I ordered some replacements for the bridge rectifier, the TNY IC and the resistors and diodes, but still I am a bit stuck on what exactly that diode is and if I got the resistor valkues correct.

Any input from someone?

And also: some ideas how this might have happened...this diode is not exactly at the input so it seems kind of strange.

[MathWizard]

What wires enter the heater ? A live and a neutral, is there also a common GND wire ? So you say there's a full-wave rectifier that got fried too? Yeah I wonder what order stuff blew up in ?

What was the technician supposed to be doing ? Was it in a shop or anything ? I'm just remembering some PCB's I worked on lately that had a lot of solder balls stuck all over the place. Flux residue kept most of them in place, but not all.


It looks like the diode was part of a snubber circuit, to handle the inductor currents when the mosfet's turn off in the chip. So if it was a 1N4007, I would assume the only easy way to destroy it would be from overcurrent, but yeah IDK for sure what voltage might be seen when it's blocking and the inductor is doing stuff.

Did it blow up the moment it was plugged in  ??

[Jwillis]

That diode looks like it was shorted. You can see the arc spots on the lead as if high voltage was touched there. If that's the case then the TINY could have taken a shock. I would also give the transformer primary a check as well. That zener most certainly would be a victim.

[markusdd]

I was not there when it happened and I just know the board, not the whole heater. But the board definitely has mains coming into it.
I also only have the photos right now, I'll get a chance to look at the thing in person Wednesday or Thursday this week.

My friend just said when the maintenance guy plugged everything back in the circuit breaker tripped and something smoked (surely that diode). As an intermediate solution he now has a new board which works, so the rest of the heater system is fine. But of course he doesn't wanna pay for this, it's over 300€.

And yes, that diode should be the one in the snubber circuit. I suspect a 1n4007 could work, still it does not look as if this is what was originally installed. Also: which Zener diode are you referring to?

I meanwhile found a photo of the back and there are huge exposed mains traces there. My theory is that the service guy shorted them out against something causing this damage.

The bridge rectifier and the input Elko I just ordered for contingency, I have no indication that they are broken right now.

Also would appreciate help regarding the resistors. Is it 18ohm and 1k what we are looking at? I have some trouble reading the rings on these.

As for the primary winding: very good point, I will check this as soon as I can. And I surely will change the TNY even if it now checks out ok, I don't trust it as the event directly happened next to one of it's pins.

[markusdd]

Ok...haha guys, the story just got better: I requested a few more photos. Backside is clean, but from a different angle it became clear that it is not the diode that is broken:

Mr. TNY himself spit out his guts:



I have 2 of these in my order. So maybe the Diode isn't even broken, it more looks like the regulator 'sprayed' it's neighbors with his inerts 

So maybe the internal  FET was already weakended and it just did not survive the ramp from zero when turned on again. Or it was an ESD event that killed it.

Will keep you posted, will receive the Board on Thursday.

[markusdd]

So guys, I still owe you the resolution:

As lined out in the previous post it turned out it was the Power Integrations TNY IC that broker and sprayed it's inters over the 400V cap, as well as the snubber diode and resistor.

All of those were completely fine and just had some crud stuck to them which cleaned up with some IPA.

When measuring the TNY had about 300 ohms left across S and D, normal is a value in the megaohms. So yeah, that was obviously the problem apart from the obvious visuals.

After I replaced the TNY IC I wired myself 2 helper lines to the net filter main capacitors that I did not have to perform some sketchy manual stuff with live mains wiring.

I then set the board oin an isolated surface and plugged it in, and tada, the yellow small LED above the microcontroller then came up yellow and I started to hear some relay clicking every few seconds. Great  stuff.

I then measured the output cap of the AC/DC converter and I got some 22.5V volts. A bit odd, but ok. I then proceeded to measure the smaller downstream DC/DC and got 5V bang-on.

So I consider that fixed and I gave it back to my buddy, let's see if it also works when put back into the heater.

Hope this was interesting and thanks for the suggestions above.