Help on rectifier diodes

[Eurotorch]

Hi Everyone,

My first post as a new member, I have a Migatronic Automig 180MXE, it has a blown bridge rectifier unit, two of the 16 diode had gone s/c.
My problem is finding out what the diode are?, no marking except the anode, cathode marking, some of the diode have been replaced with motorola tr231r, but yet again I can;t cross reference them, they are all the press in stud type with a flying lead.
I have contacted Migatronic and I am waiting for a reply, I expect no information only that the will gladly sell me a replacement unit for x amount.
The machine is getting on abit but is a beautifull welder and performs lovely, when working that is.
If I had the specs of the diodes I could construct my own unit with better diodes than those used at the time of manufacture,
Does anyone have one the same??
Hope someone can help?
cheers John.

 

[Hitch]

Migatronics are great machines, parts are readily available, but as you say, usually you will replace the board rather than the individual component.

Have you tried the RS components technical support line, they are VERY helpfull...even with minimal info.

 

[Eurotorch]

My Head hurts

No I have'nt tried RS, but I will. Further on the problem, I have replaced the two bad diodes with a similar 50amp type made by International Semiconductors, on testing the rectifier on the bench there was a small AC leakage, that was with a 12volts ac psu, i'm not clued up about migs but electronical a bridge should only give out dc and I presume a welder that has ac on the output would be bad, and one which I would not be prepaired to try out properly??
Thank for the advice, I will goto there site next.

cheers John.

 

[Hitch]

I dont know much about the guts of them tbh. Theres a chap here that seems to know quite a bit, cant remeber his name though....?

The mig output should be DC (if i remember!!)

 

[malcolm]

Can't be of any help with electrics, but did find Bill the Arcstarter's website The site is a lot of fun, and he's built his own rectifier out of bits and pieces which might be worth a look (in the DIY welding shop section).

 

[Eurotorch]

Hi Everyone, thanks for all your replies.
I have had a response from Migatronic, and it was like I thought, no diodes available separatley, but I can buy the bridge rectifier unit for £106-00p.
So do I buy it or do I build one?? How much is the welder worth?? to build one would cost a fraction of a new unit, but my time is limited because of other commitments, and it was'nt long since I paid £40 for a new euro torch from them, so scrapping it is not the answer, there is also the fact that I have repaired the control unit on several occasions, and if that goes completely I might not be able to repair it again, don't know how much a new control unit will be, but I have email them to ask.
On the other hand how much would a new welder cost me of the same specs.
I'm sure the rest of the machine is sound, I look after it and give it a good clean each year inside and out.
What would you guys do??

 

[Eurotorch]

got the prices

Hi Everyone,

Finally got the prices for the control unit, out of six different units mine is the oldest, yet does the most as regards spot stitch current control etc, but is the dearest £176 so with the bridge also it would nearly cost me £300 to repair...Ouch!!!

 

[Hitch]

You can pick up recon'ed migatronics from about £300, or new units id guess were around the £600 mark.

Esab AristoMIG or Murex tradesmig, all similar machines.

 

[Highway_Star]

Rectifiers! My speciality!

If you can ID the diodes, they should be replaceable, unless they are those awful swaged in types used in vehicle alternators. If you can get a good close up digital photo and a note of any marking on them, I'll have a go at finding them. I've got a couple of suppliers who specialist in unusual semiconductors. These people are among the best http://www.caspiansemiconductors.com/

Now AC content. bear in mind that even a full wave rectifier does not produce true DC, it will always have some 50Hz ripple in it, only a power supply with decent smoothing capacitors and chokes can come close to true DC. Typically an arc lamp rectifier power supply running at 24v DC@75A will have a ripple in the order of 2-5%, that's with smoothing and chokes, and these beasties are rated for constant duty cycle. Some of the 3 phase designs can achieve that kind of ripple without smoothing, but they are ludicrously inneficient.

It's easy to see why a wee welder with zero smoothing and a fairly low duty cycle will ripple a bit.

 

[Eurotorch]

Hi, Highway star,

The bridge has 16 diodes in total, the origionals have no markings except the anode or cathode polarity, they are the press-in stud type with a isolated wire lead, the bridge produces the dc with the expected 100hz ac ripple, but there is also ac content present when zeroing the output on the scope, after taking further readings of the bridge i find that the bridge is un-balanced, which is not good, I get .348v on one side and .450v on the other, this maybe due to the fact that there are non original diode in the bridge from previous repairs, these diodes are Motorola tr231r I think very hard to make out as the cases are corroded and the others are IR 8AF4 3A which I think are 50amp rating, I think I have seen this number before, any way I will take a picture and post it soon and thankyou for the link and your help.

 

[Highway_Star]

Hence why we always replace all diodes in a bridge rectifier in one hit, even if only one has failed.

Do they look like the 9AF package on this page? http://www.irf.com/package/pkdstdrec.html

The International Rectifier type you quote is obsolete now, in that package at any rate, however there's no reason you couldn't use a diode with the same specs, but in a more readily availabe package, but replace them all together and the imbalance should dissapear. (It's probably that imbalance that has killed the other diodes btw). They aren't going to be cheap unfortunately, especially as there are 16 of the blighters.

 

[Eurotorch]

Nope they are the 8af package, but these are replacements, there are originals still in the bridge,same style package, but they have no markings and migatronic aint telling what they are, won't even tell me the power rating of the bridge.
They have sent me the circuit diagram/s which will now allow me to test everything out, and guess what? the bridge has no part numbers, everything else has except that and the control unit.
Yes they aint gonna be cheap and migatronic want £106 for a new unit, doesent sound too bad, when you consider replacing all the diodes and then making a new heatsink etc.
I have the spec on the IR package, Max forward current is 50amps, I have some old motorola diode data books, but can't find them, totally cheesed off with it, should have been repaired by now.
Anyway I will contact there engineer when he is back, maybe he will throw some light on it. Thank you for you efforts
regards John.

 

[Highway_Star]

If they are asking £106 for the complete bridge and heatsink, that's not bad at all. Recently I replace all 6 diodes on a big IREM arc lamp supply, the dioes are in the order of £30 each to us, so a bit more than that to the customer, plus my time, fuel etc.