Success!

Looks like my repair was successful as I was able to make some nice test cuts in 3mm steel, it really is like a knife through butter. Next I need to sort out a proper 30A connection so I can try cutting 12mm as an acid test.

 Amongst some welding practise you can see a little plasma cut face, there as a little slag on the cuts because of having to pierce but it knocks off.

 This is more like it, really nice cuts and a super finish, very little slag on the lower surface, with practise or better, CNC, this would be awesome.

Rectifiers in

My plan to install the new rectifiers in a twisted orientation to make up for the incorrect pin out worked. After cleaning the heatsink and the IGBT's with IPA to remove the heat transfer compound I applied fresh and reassembled. It turns on but makes a funny crackling sound, turns out this is normal according to some forum posts (one by a Hypertherm employee).

Tomorrow I hope to sort out a proper high amp socket so I can see if she cuts. Will also find out for sure if the softstart relays are switching because if not the NTC resistor is toast!

Almost there now, I hope!

Abort face palm

Looking at the underside of the PCB at a space where a rectifier should be made me think, hmm why don't I just stick a rectifier in there from this side, just to see if the beast will boot up.

So I did, and it works! The professional electronic engineer should avert his gaze now but for everyone else here is my temporary kludge:

I have a huge collection of these rectifiers, I think they are International Rectifier (oh yes it says on the side) and they will do about 25A. They were used in a project by Nottingham university on wind power, apparently the implementation was done by contractors in Scotland on site, it seems they did not realise that these devices are really designed for crimp terminals nor did they have a soldering iron big enough to solder them, so they just put the wire through the hole and soldered it back to itself with a little extra solder for good measure. Out of about 50 rectifiers I have not found one good joint! Not that this is a lot better, but....

It works!! At least it does not explode and not exploding is always good in electronic circles! I get the proper (yet wrong) rectifiers tomorrow and will mount them properly also replacing the thermal transfer compound on the IGBTs.

Thank F for that!!

Face palm

Put the board back with only one rectifier and no joy, then realised that they are not wired in parallel rather one for neutral and one for live, doh!

The plan

The original rectifiers sit off the board by quite a way and are then screwed to the heat-sink. This means there is a gap between the board and the rectifier. I'm hoping that I can bend the leads of another rectifier to fit. The original devices were Semikron SKB 51L16-W, which can do 29A forward current and are rated to 1600V.

The selection of leaded rectifiers available is not huge but I found a Vishay GBPC3512W which can handle 35A and 1200V. I lose a little on the voltage overhead but I suspect 1600V was overkill anyway. The package has a similar thickness but a different pin out. The following is what I plan:

It does not show that well but the general idea should be obvious, the twisted positions are the new rectifiers, twisting reduces the length of lead between the board and rectifier, much more and they probably would not be long enough, it still might be boarder line because of the stand off. I think I will turn some spacers to act as a jig when soldering and to ensure the right stand off. Will probably have to be split so I can remove them afterwards.

If this doesn't work then I may have to try and make some weird intermediate PCB from copper sheet, sounds like a recipe for disaster. While I wait for the rectifiers I'll check the rest of the board, I may even try "booting" from the remaining rectifier, if all else is well it should work and if it blows it shows the problem lies further in.

And in case it helps anyone this is the first draft of the first stages of the power supply where the problems have arisen (drawn in Diptrace).

RectiFIRE?

Making some progress still, I started off by emailing Hypertherm, I heard they had really good customer service so I thought I would ask for some tips from tech support. Initially they did not seem to understand my request, "the power board might be faulty". So I explain that I know it is faulty but wonder if they might have an idea of the likely culprit that could cause this fault. I got this back: "We are not aware of any problem with the power board. You will need to replace it." OK so there is nothing wrong with the board I just need to replace it. To be honest I am not suprised they are not massively helpful, most companies are not interested in assisting repair.

So I move on and look at the schematics which are laid out to match the board which is nice but resemble spaghetti. With some help from a friend I redrew the first stages of the circuit so I could actually understand it and determined that the cause of the high current that was cooking the resistor was probably either the rectifier that produces rectified DC to the power factor correction inverter or a capacitor on the rectifiers output. Some resistance testing suggested the former. So I took the board off and this is what I found:

These are two bridge rectifiers that are wired in parallel, each can handle 30A so they should be more than OK for the 30A max input currents. But notice the small hole at the bottom right, yep it's fried!!!!

And wow, it blew a small hole out of the aluminium heat sink, impressive!!! I've now taken the rectifier off the board and have discovered the first real problem. It is no longer made and worse, the pin out is non standard!! It sits a fair way off the board so I have to hope I can find a way to make some sort of adapter for a new rectifier, we shall see. And lastly, a few photos of the insides for context:

Plasma progress

Well I replaced the resistor thing but it blows the fuse. That in itself is not a bad thing because I am trying to get it to start on a 13amp fuse and it probably should be on a 30 amp circuit however the flash that appeared close to the resistor was more worrying, do I really want it on a 30 amp circuit at this stage I wonder? The resistor is part of a soft start circuit, once the power supply gets up to speed a relay bypasses the resistor. The idea is to limit the initial in rush currents. I suspect that for some reason the bypass relays have not done their job and there has been welding current going through the resistor causing it to explode. But having said that even after the brief turn on it was pretty hot, and I have to wonder where the flash came from. It was probably wishful thinking to thing this would be easy!

Hmm plasma

I have a super special secret idea for a CNC plasma cutter but my plasma cutter is a bit of a wimp, an ancient Hypertherm 380 which can only cut 3mm max properly. Looking at better machines today made me realise why I have such a wimpy machine. A Hypertherm Powermax45, which is the daddy, and will cut 12mm steel is a whopping £1756 inc VAT. So I had a look on Ebay, you know, just in case, and found one available that was not working, some disk shape part had blown and the supplier would only offer to change the board out for £500.  Well as the guy said he had it for sale elsewhere I gave him a call and we negotiated a price. Given the torch itself is a £350 spare I could not expect to pay peanuts but I think it was fair. I'll divulge the amount when (if?) I get it working! I could not resist going to pick it up straight away so had an enjoyable drive to Richmond in London where I met him at the edge of the Thames in the pitch black. This is not as bad as it sounds as he lives in the middle of the river on a boat, a very nice chap he was too!

So here it is:

Its actually smaller than my old one and possibly lighter, that's progress for you! And here is at least one of the reasons it is not working:

The supplier would not even look at replacing the parts on the board which is a shame because I put this into Google and found out immediately that it is a 10ohm circuit protection thermistor by Ametherm. I just ordered a couple from Farnell for around £3.50 each. If this is all that is wrong then I will be a very happy bunny indeed but we shall see, maybe tomorrow, bum it's Saturday tomorrow :(