Kemppi Mastertig 3500 burned diode

[Dan89]

My Kemppi welder needs repair.
I'm not really that experienced with electronics, but have worked on it before. Just need some guidance
The HF tig function suddenly stopped working, but scratch tig worked.
After inspecting the unit I could see that a wire going to a diode was burned off, so I think I need to just swap this diode.

**broken link removed**

The diode is marked with 9750144. Google didnt show any results for this.
Talked to Kemppi service and they said they no longer manufacture these diodes, so they could not help me. They did give me some information though.
The diodes are used on the secondary rectifier. They could only say it is a very fast 60-70A / 600V diode. But no spec on how fast.
They said I could try to swap the diode if i find a similar, but that it probably would not work due to the current passing through would need to be exactly the same on all the diodes, if not they will burn up. Apparently kemppi has some super diode testing and sorting machine for this.. I don't

So my question is.. There are 8 diodes. Looks like there are 2 sets with 4 diodes each going to a bus of some sort?
Can I just swap the 4 diodes going to the same bus, or do I need to swap all 8?
Do you think it is possible at all to swap the diodes and make it work?
Any other things I should check?

I found these on ebay: **broken link removed**
PDF with specs attached to message below.

Regards
Dan

 

[debe]

Personaly I would remove the burnt wire & the blob of molten solder on top of the diode, & the surrounding ash. Then test the diode it may be ok, the wire may have over heated due to poor soldering joint on the diode. HF current can cause crystalisation of the solder.

 

[Dan89]

I actually didn't think of testing it, just assumed it was broken. And it is. Beeps both ways when I use ohmmeter so I guess it's shorted.

 

[debe]

Sounds like its shorted. But at that price those diodes are worth a try.

 

[Tony Stewart]

IRF also makes them but DK had no stock.

The key is 1.5V@60A at 25'C for matching <10% ok they come in 300 or 600v ratings.

Mismatch may cause DC magnetization on transformer and saturation causes runaway failures.

 

[Tony Stewart]

Cost about $6 (1)

Matching is easily testable with a low voltage 3 V PC PSU DC supply but needs experience to wire two and not burn out in a pulse test.

 

[Dan89]

Thanks for replies.

Do you think I should swap all 8 diodes or just the 4 diodes in the set with the broken diode?

IRF also makes them but DK had no stock.

The key is 1.5V@60A at 25'C for matching <10% ok they come in 300 or 600v ratings.

Mismatch may cause DC magnetization on transformer and saturation causes runaway failures.

Which ones are you referring to, do you have a link to a dealer with stock?

 

[Dan89]

Found this schematic on a polish forum.

No idea how this works. I guess I should just swap all 8 for new ones.

Can I use these https://www.digikey.com/product-detail/en/VS-70HFL60S02/VS-70HFL60S02GI-ND/357559 ?

 

[MikeMl]

I am sooo glad I bought a Miller...

 

[Tony Stewart]

Thanks for replies.

Do you think I should swap all 8 diodes or just the 4 diodes in the set with the broken diode?




Which ones are you referring to, do you have a link to a dealer with stock?

I would buy 4 and replace 1 and test with 100mA ~1A for Vf match then replace 4 or the best match, if much different.

I didn't find nor spend much time looking for stock.

 

[Dan89]

What about speed of the diodes, is faster better - found some cheap 200ns on ebay, can they work?

 

[Tony Stewart]

How cheap? depends on V/I slope at operating point and VI power dissipation and overall Thermal resistance Rja.

Arc welders are exceptional hard on all parts with huge avalanche currents ( fast)
So diodes dont need to be as fast as the caps pick up the transient with exceptional low ESR demanded but the transformer design to regulate current.... so depends...

https://www.ebay.ca/itm/2PCS-X-60EP...071?pt=LH_DefaultDomain_0&hash=item419514a3c7

These may work,

The thermal resistance and ESR are both critical so that thermal runaway does not occur.
As one string shares current with the other, the one which takes more current drops in voltage, thus drawing more current...into thermal runaway .. poof.

They must both be thermally matched to the same sink and exceptional low thermal resistance. and matched ESR or Rt at the operating point not 140'C or 120'C where they all match.

It is solved by Ohm's Law for ESR and Thermal resistance and NTC coefficient for diodes and load transient duration.

 

[Tony Stewart]

BTW if the ESR of the Caps degrades, it puts more stress on those Diodes causing them to fail instead. SO ensure you know how to check ESR.

The fundamentals are simple with Ohm's law and a voltage divider with high current injected pulses to determine the ESR and a high speed DSO or fancy peak detector and hold circuit to measure drop from a 100A pulse. THen you have to be aware of EMI pickup in measurements etc. Simple solutions but easy to overlook anything.

Also 6 are better than 4 if heat becomes an issue. But locating a bad cap when all are in circuit will be hard unless separated or you know how to decouple with some ferrite.

 

[Dan89]

Thanks for info Tony.
Cant use those diodes you linked to there, I need them with a m6 stud..

So, I should measure the caps to make sure they are not the problem that the diode blew in the first place.
I know nothing about measuring resistance in caps, but google and wikipedia can solve this problem.
Can this do the job?
**broken link removed**

 

[Tony Stewart]

The caps should be > 30% of their rated ESR. If you have a test set or a scope a Cap dump from a super low ESR film caps at low voltage with known SMD size resistance or short nichrome wire. 10mΩ, 100 mΩ 1 Ω . It must be low ESL.
Whichever comes closest to have an impulse peak voltage @ 50% into a 50 Ohm loaded coax to high speed DSO scope Charge voltage can be 1V as long as scope is fast enough to to see the step size before charge transfer . THis step size becomes a burden to those 4 diodes. THus why rise time , ESR and -Vrating and Rjc are all important.

 

[Tobias Möglich]

Hello

If this thread is still valid, I want to reply.
I had recently the same problem with a bigger machine (PRO 5000). The saw the labeling 9750144 as well on the diodes. It is not a device number. To replace the diode I found one from the distributor Farnell. It should be fast or better very fast. Unfortunately I don't have the ordering code here at home. I can tell you next week when I am at work again.
Indeed there are 2 types of diodes on there. 4 of them have the anode at the tip. The other 4 diodes have the andoe at the bottom (bolt / thread).
BUT first of all: I would find out if the diode(s) is really broken. The burned cable does not look good. But you still have to disconnect the diode on one side in order to measure if the diode is short circuited or not. It might be connected via an output coil (so called secondary choke). Not sure at the moment.
BUT for me it looks as if you rather have problems with control card A002 which is responsible for the TIG-HF or the HF transformer. The burned cables of the output diodes I would replace anyway. This is not a big thing. You just should have a strong soldering iron.
So - FIRST I would check the suspect diodes if they are really short circuited. If some are are indeed short circuited they have to get replaced.
SECOND. The burned cables I would replaced.
THIRD: still you might have problems with TIG-HF and this is ratther a problem of the control card A002. But this one might be obsolete as well. Or the switch MMA/TIG-HF/TIG-scratch is faulty. This would be the simplest fault. Kemppi has this one still as a spare part. It would be a try. But I guess it is not the cause of the problem.

Regards, Tobias

 

[Dan89]

Hi Tobias.
Thanks for input.

I forgot about this thread. I solved the issue by just replacing every cap on each board.. Now it works fine

As for the diode, I never replaced it since the diodes need to have the same speed or they will burn.
It only limits the max power output of the welder, I usually weld at 1 maybe 2/3 of max, and never for long periods of time, so I think I will be fine.

 

[Tobias Möglich]

Cool. Yep this is an solution. Just disconnect the faulty ones. I guess it is good to disconnect even amounts of each branch. As you can see in the main circuit there are 2 branches with 4 diodes in parallel. When one burns I reckon also 1 diode of the other branch should be disconnected. Anyway, it works.
I repair Kemppi machines for 5 years. Hadn't heard the the HF stops working in case the caps are faulty. Will take it too my notes. Normally I make my own service manuals / trouble shootings since the official ones are note so detailed.

Greetings