Brooken diodes - Cem Tri Mig 190

[bacanc]

I'm new here at the forum and needs some advice on how to change brooken diodes on my Cem Tri Mig 190. The machine got four diodes per phase(3 phases) with two in each direction. Only two diodes seems broken on one phase. The diodes aren't marked with any name. As the model name indicate, I guess the highest amp is 190.

1. How to know what to replace them with?
2. Do I need to replace more then the two brooken?

 

[gaz1]

hi and welcome to the forums

its best to have a picture that you can post up

2. its wise to check the others at the same time
1, replace them with same or higher values but not always

3, also check in line for further damage caused by the diodes failing

 

[skotl]

Welcome to the forum

As Gaz says - you should check all of them. How do you know that two are broken - did you remove them and check them with a multimeter, or are they physically damaged?

As Gaz also says, you should replace them with same or higher value but you really need to know what these values are - if it's an inverter type machine then the diodes might be rated at 12,000 volts or higher... I'd be surprised if the diodes had absolutely no markings (the boards typically have the values stamped on them, too). Any chance of some pictures of the diodes and board?

 

[bacanc]

Hi! Thanks for fast reply and intro :-)

I have not figured out how to attach documents/pics as described in forum guidelines yet :-S, but as soon as I do, I will show some pictures.

The two diodes are physicalled damaged /detached from wire. Multimeter indicates that the other pairs are working (can only measure in pair unless physicall cutting of leads)

 

[skotl]

The easiest way to test without damaging them is to desolder them, however if they read infinity in one direction and .6 in the other while on the board then they're likely to be ok.

I'd probably still suggest de-soldering one of the "good" ones so that you can check for markings.

For uploading pics the easiest way is to open the pic in something (paint, photo viewer or whatever) and then copy it to the clipboard. Then when you're typing a response back over here you can just CTRL-V / right-click&paste the pic straight into the response.

 

[bacanc]

 

[skotl]

Oh wow! So there probably are markings, just underneath all the crud

Can we get a wider pic of the board? And can you desolder that knackered one and give it a good massage to see what's underneath the dust?

P.S. I wonder if that's a mechanical failure from vibration...?

 

[bacanc]

Yes, I managed to upload now :-)

 

[bacanc]

I'll try to take away the dust if that can reveal anything like you adviced.

 

[skotl]

Well, the good news is that I'm assuming that this is a dirty great transformer based welder, not an inverter one, so the diodes are probably relatively low voltage (50-230V)

Can you measure the AC volt output from the transformer? It looks like they're expecting to carry some heavy amps, which I would be guessing would be the rated output of your welder (i.e. a 150A welder would be pushing a minimum of 150A through these diodes) - but that's all a semi-educated guess on my part. Someone like @mike os would be able to advise more accurately.

 

[bacanc]

Could not find any info behind the dust :-S I discovered that there where actually 5 diodes on each phase, 3 which deliver to the negative output and 2 for the positive output. A total of 15 diodes.

Yes, it is an old transformer based welder. It gave between 16 to 26 DC volt with only two phases working depending on power setting on frontside if thats reveal anything. The AC voltage is at highest 24 V between two phases what I can see.

 

[gaz1]

there your selector plate diodes to control your voltages on different settings also used to refine or smooth the voltages

rough idea you need something like this

http://www.brweldingsupplies.com/century-diode-for-mig-welder-m9661-32r-diode/

http://www.instructables.com/id/Mig-Welder-upgrade-diode-replacement-in-bridge-rec/

http://www.mig-welding.co.uk/forum/threads/mig-welder-rectifer-upgrade-project.21454/

your replacing the diodes within the switchable bridge rectifier

 

[Appyasme1]

Once you have found the value of the Diodes,dont just change the faulty Diodes,you need to change them all.The reason is that the faulty units will "pull down"the good Diodes

 

[Robotstar5]

Once you have found the value of the Diodes,dont just change the faulty Diodes,you need to change them all.The reason is that the faulty units will "pull down"the good Diodes

Good advice, but unlikely in this case as the diodes seem to have failed due to mechanical damage not electrical.

 

[bacanc]

Ok. I think I now understand why I can't just change the faulty ones. How do I calculate the specification needed for the diodes? I believe the welder was intended to deliver 190 Amp. Should every phase be able to be loaded with the maximum Amp? Could e.g. three 70 amp diodes in parallell deliver 210 Amp?

 

[bacanc]

Good advice, but unlikely in this case as the diodes seem to have failed due to mechanical damage not electrical.

I can't tell if it is mechanical or heating or something else that have brooken the diodes. The welder does not make much vibration I think. Measuring the broken diodes gives faulty result so there is not possible to try to fix them.

 

[bacanc]

Once you have found the value of the Diodes,dont just change the faulty Diodes,you need to change them all.The reason is that the faulty units will "pull down"the good Diodes

I guess I only need to change the ones within the faulty phase? The rest ( 10 diodes) are not working in paralell ??

 

[skotl]

http://www.brweldingsupplies.com/century-diode-for-mig-welder-m9661-32r-diode/

$110 a pop

 

[gaz1]

there is these depends on where you want to get them from lol

http://www.ebay.co.uk/sch/i.html?_from=R40&_sacat=0&_nkw=stud+mount+rectifier

just get right voltage and amps to what application that your needing

at 55.00 each one depends on is it worth getting another machine instead when the diodes can be suspect though the ones on this page is mechanical failure compared to electrical failure

 

[skotl]

...at 55.00 each one...

The ones you linked to are much cheaper, tho. If it was me (and I could afford to wait four weeks for delivery) then I'd probably take a punt at a handful of the £9 100A ones, running them in pairs to take the 190A, or wait for more of the £23 300A ones to become available (seller says he only has one left).