Thread: Faulty Mig !!!

I have a 120A unimig which I had to change the diodes on after a couple failed. I got replacements from RS components.

http://australia.rs-online.com/web/search/searchBrowseAction.html?method=retrieveTfg&Ne=4294934505&Nr=AND%28avl%3Aau%2CsearchDiscon_au%3AN%29&N=4294624928+4294624256+4294832004&Ns=stockPolicy_au|1||new_au|1&binCount=2852&multiselectParam=4294624928%204294624256&selectSubRange=Press-Fit%20Diodes#breadCrumb

I did read somewhere they should be matched pairs but I just bought a normal packs of 5 and haven't had a problem. I ended up changing all the diodes, from memory they was eight all up. 4 wire to cathode and 4 wire to anode.

The only real specs you need to be too worry about apart from the polarity is the voltage and amp ratings. I used 200V but you could probably use ones with a higher voltage rating and it wouldn't make any differenence.

I would assume the current rating would be related to the maximum output of the welder and number of diodes installed. So 2 x 75 amp diodes for a 150 amp welder but because the duty cycle associated with welders its possible they could be smaller. In the end I only found 60A diodes in a matching size so didn't think about it too much. I was considering installing larger stud mounted diodes instead but they were quite a lot more expensive and I would have had to make some modifications to fit them.

Do you have spec sheets for the replacement diodes you have found?

I'm knew to this forum and for the life of me I cannot work out how to actually post a msg, the only way i can is by replying to one that is already posted... can you please help?

What I would like to do is post a msg seeking assitance in securing a copy of a Migomag 220 instruction manual, I've had the machine for 15 years and hardly used it. I can't remember what it will actually weld, i.e. stainless, copper, alluminium...

If you have to mount multiple diodes in parallel, they do need to be matched sets. There will be minor variations in the forward voltage drop and effective forward resistance of individual devices, and in the absence of something to control it (and waste power) the device with the lowest forward drop will carry the bulk of the current until it is destroyed, then the next device will take over. In this situation, a 0.1% variation in a parameter across a couple of devices could mean that one device is doing 99% of the work and suffering for it.

If the welder has multiple secondary windings with their own pair of diodes, the windings would control the current sharing due to there greater resistance, but for a single secondary winding and multiple diode sets, matched diodes would be the way to go. Without access to appropriate parameter measuring gear and a large supply of devices to test and match, the best approach would be to try and source OEM spares or a similar set from an equivalent machine.

Originally Posted by Thora Fabricati

I'm knew to this forum and for the life of me I cannot work out how to actually post a msg, the only way i can is by replying to one that is already posted... can you please help?

What I would like to do is post a msg seeking assitance in securing a copy of a Migomag 220 instruction manual, I've had the machine for 15 years and hardly used it. I can't remember what it will actually weld, i.e. stainless, copper, alluminium...

There is a New Thread button at the top of the topic list for each forum. Clicking it will open a window similar to the Reply window you used to make the post above, except that there is no existing discussion or title. The topic list is one level up from the thread discussion that you are in at present.

I have added a screen grab attachment with the New Thread button highlighted. Hope this helps.

Originally Posted by malb

If you have to mount multiple diodes in parallel, they do need to be matched sets. There will be minor variations in the forward voltage drop and effective forward resistance of individual devices, and in the absence of something to control it (and waste power) the device with the lowest forward drop will carry the bulk of the current until it is destroyed, then the next device will take over. In this situation, a 0.1% variation in a parameter across a couple of devices could mean that one device is doing 99% of the work and suffering for it.

If the welder has multiple secondary windings with their own pair of diodes, the windings would control the current sharing due to there greater resistance, but for a single secondary winding and multiple diode sets, matched diodes would be the way to go. Without access to appropriate parameter measuring gear and a large supply of devices to test and match, the best approach would be to try and source OEM spares or a similar set from an equivalent machine.

I'm sure your right about the imbalance if their not matched but I priced a new rectifier assembly at about $200 and I couldn't find matched diodes to suit seperately. The diodes from RS components were around $40 including postage. So far they have lasted 18 months and maybe 10kg of wire so have worked out OK.

Thanks Galleogs. Nothing like learing from experience...
Yes, I agree that matched pairs are best, but under the circumstances, I would also prefer to try the cheaper alternative. Who knows how they REALLY... manufacture the originals. Probably do what we do, knowing the item will fail in the future some time, and then we have to get another replacement from them. If it does fail in the future, we just replace the diodes again... no problem.
On another note, The leg of the diode is currently welded to a inner contact ring. Is it ok to solder the replacement or should I cut off the exsisting diode and leave it's leg connected, so that I can crimp connect the new diode to it? This would eliminate a soldered componect in the bridge.
What are your thoughts?
Thanks again to everybody for the feedback..... Ordering my diodes this week. Should have my welder up and running next week end....

I can't really remember how the diode legs were attached now. I think maybe eylet things that looked to be spot welded. I think I crimped and soldered some unisulated eyelets on but I could have been feeling dodgy and just soldered them. I think its better not to rely on solder to hold them though.

Hope these diodes work for you too. Because my welder only cost me $400 it was going to be a new welder if the cheap fix did't work.

A picture would help but. Crimping is generally better than soldering. You could crimp then solder if you like.
Hope it works out for you
Stuart

I went to RS in Sydney, and they only have the wire to anode press fit in stock. Apparently the wire to cathode is no longer in stock.
I am converting my exsisting rectifire bridge to a sealed unit. RS has a bridge rectifier that will do 107 amps peak average forward curent. I have drawn out the curent rectifier bridge and confirmed how it all connects to the welder. This should be an easy conversion that should be more durable than the last rectifier. I wil be getting the rectifier tommorow and hope to have it set up by the weekend. Here is the unit from RS:

IXYS | Semiconductors | Discretes | Bridge Rectifier | Single Phase - Screw Terminal |VBO105-12NO7

Here is the welder I am repairing.

EASYCRAFT 150

The bridge costs $128.00 but it wil be worth it if the welder is up and running again.

Finaly got the welder working last wekend.
Decided ultimately to build my own rectifier bridge, Used 70 amp 40 volt rectifiers from RS ( Stock No 2279450 Description 70HF40 and Stock No 2279472 Description 70HFR40) Cost about $70 including heat sink and aluminium bar. I now have higher rated duty cycle bridge.

Attachment 145573

Attachment 145574


All fitted well and is working prefectly. Already busy with my caravan rebuild again....

Thank you all for your assistance

Nice work
Don't melt your transformer
Stuart

Looks like youve done a really good job there... nice work.

Like stustoys said you can still over heat the transformer so be a bit careful. I notice that when my machine gets a bit hot the force of the arc seem to drop off, at which point I give it a rest.