Hello,

Need some help choosing diode modules to build a rectifier for a 225 amp AC welder. I've been researching this for a while and seen people use everything from antique Russian stud diodes of unknown capacity to nice brand new components that cost twice as much as buying a DC welder.

Output specs on the welder are:

225A/25V/20% duty cycle, 79V max OCV

The cleanest setup I've seen was using two IXYS MDD95-22N1B dual rectifier modules that are rated at 2200V, 180A each.

I understand wanting to give yourself some head room so using two modules totaling 360A makes sense, but is the 2200V necessary or can I go with something rated a little lower?

What would be the best balance between reasonable packaging (would like to get it inside the case of the welder), price, specs, and availability?

Oh, and one more question while I'm at it...would powdered iron packed inside an iron pipe make for a reasonable inductor/choke core? I think I'm going to need all the help I can get in that area.

Thanks for any help.

 

The diodes are rated at 120Amp avg, 180A peak , the voltage is the maximum reverse voltage , you only need 200v diodes rated at the current you need. Choose the ones that are within your budget.

 

Thanks for the replay.

Can you explain how you came up with that 200v number? Teach a man to fish and all that...

Is the current at 120A average suitable? I will rarely use the welder at max output; it will spend way more time at 75-150A than the 225 that it's capable of.

 

Reverse voltage is usually twice the maximum supply so 180v, i would use diodes bigger than the maximum current that can be used to be safe.

 

Is this just a one time purchase? Surplus source ok?

Full bridge?

SCR + diode bridge better?

 

I have worked on welder power supply systems for years the typical industry standards that you would find in any decent 225 amp stick welder.

250 - 300 amp rated diodes with at least 600 volts PIV rating.

Granted your open circuit voltage is not so high but you are working with what is essentially a controlled short circuit that is loaded with harmonics and very high voltage transients which is why you need a fairly high voltage rating.

Inductor design wise the simplest and most effective design would be to take a old automotive battery charger transformer from a charger that had a similar peak amp rating to what your welder has and just use its low voltage high current secondary coils and it core for your inductor.

Most battery chargers use a center tapped secondary so its fairly easy to rework that into two parallel windings to get a much higher DC current capacity without overheating the windings when being used as a welding current stabilizer.

 

Edit: I have one more question... When using these diode modules like the IXYS MDD to create a full bridge, does the rating double like I've been led to believe? Stated another way, if I use two of these modules rated at 142A, do I end up at 284A? Is PIV the same?

To answer some of the questions, this is just a personal, one-time project. I'm converting a Lincoln AC-225 welder to AC/DC so I can stick weld DC and do basic tig welding. I could just go buy a cheap Chinese tig welder and be done with it, but this is more fun and probably more reliable - certainly easier and cheaper to repair if it breaks.

There is a fair bit of information on the web about this, but in many cases someone has just thrown something together without much thought or any real knowledge of the subject. I don't want "I think it should work" when dealing with this amount of juice, and I have no problem admitting ignorance and asking for help.

As far as SCRs, I've come across them in my searching but don't really understand the difference. I need to read up on them. So many options, so little knowledge.

The battery charger idea is interesting. I've actually got a huge old microwave oven that I thought about pulling the transformer out of and rewinding with welding cable, but I don't think it's big enough to do me much good. I'll look into the battery charger...I need to find a local electronics scrap yard or something.

Thanks for the help, y'all.

 

http://www.amazon.com/Bridge-Rectif...=1403864361&sr=8-1&keywords=Bridge+diode+300a

$55 + $5 shipping (USA)
500 amps (on datasheet even though ad says 300)
3 phase but single phase works fine too (de-rate by 1/3 if using as single phase)

datasheet

 

I'd had come across those, didn't know you could use them for single phase. Nice.

 

Yes, in general, the ratings double when half bridge or individual rectifiers are connected in full bridge.

Diodes share the average current and two are always connected in series.

There may be reasons why they wouldn't share equally, I suppose.

 

The thing I see is given what you are describing of what you want to do an old Lincoln 225 AC stick welder is a pretty poor choice for a base power supply to make a DC welder out of. Especially for TIG work.

That and to be honest I have doubts that you would ever be running any weld work at the full 225 amp range. I had one of those welders for years and I can't recall ever needing anything much past the 160 amp range for welding.

 

Scratch-start tig will work fine for the shop projects I plan on doing. It's admittedly old school, but I'm just going to use it to build work benches and weld patch panels on an old International Scout.

I bought the welder with 50 lbs of electrodes for $100, and for another $100-$150 I can have an indestructible AC/DC stick welder and basic tig.