MIG welder rectifer upgrade project

[combivan]

Hi All
Has any one replaced the rectifier diodes in the rectifier unit in their mig?
It’s a simple thing to do but would like to check the maths first just to make sure that I am using the correct diodes.
Thats once you have worked out which way the diode is bolted in ie anode or cathode. Mine is anode..
There are 8 diodes, 4 per phase. They were placed like this to reduce the current and voltage flowing through one diode.
If the welder is a 150 Amp, 240volts, each diode could be 40Amp 100v, because four in parallel together would equal 160 Amp 400Volts..
Cheers

 

[rtbcomp]

No no.

The voltage rating increases as diodes are wired in series, the current rating increases as they are wired in parallel.

Personally I prefer to use single big diodes rated at more (perhaps twice) the rated current. If you have two in parallel and one pops the other will be overloaded and its demise will quickly follow.

Not quite sure what you mean by 4 per phase, do you have a bridge rectifier or a centre-tapped secondary on your transformer?

Welcome to the forum

 

[GeorgeB]

Hi All
Has any one replaced the rectifier diodes in the rectifier unit in their mig?
It’s a simple thing to do but would like to check the maths first just to make sure that I am using the correct diodes.
Thats once you have worked out which way the diode is bolted in ie anode or cathode. Mine is anode..
There are 8 diodes, 4 per phase. They were placed like this to reduce the current and voltage flowing through one diode.
If the welder is a 150 Amp, 240volts, each diode could be 40Amp 100v, because four in parallel together would equal 160 Amp 400Volts..

I wouldn't recommend replacing the factory fitted modules with your own diodes wired in parallel. If you can source 4 single diodes to make up a new rectifier, then that should be superior to the factory-fitted arrays but such diodes are expensive. I guess the MIG factories use the parallel-wired rectifier modules because it's cheaper for budget-priced MIG welders. The rectifier modules are manufactured by a specialist in Italy, I believe. They must know the secret of how to precisely match the diodes. Top professional machines use 4 single diodes.

For the rest of us, search google on diodes wired parallel. Most sources say it's a no-no. Unless you get precise matching, all the current will tend to go through one diode, with least resistance, and blow it, then work its way through all the others.

 

[SirDick Diodenob esquire]

You can get good high current diodes and thyristors from old machinery cheap enough for few pounds each but you need lots of time to trawl through ebay and the scrap yards . I have seen some absolutely crap rectifiers fitted to used mig welders I have bought recently and it would be very easy to improve on whats already fitted to most machines unless you are looking at a similar standard to a Murex in which case they are fitted with proper diodes to start with . The downside is that if you fit really good diodes then you have made that section one of the strongest links instead of the weakest link ( that should be the fuse but often its not ) . Have just managed to burn out the transformer on a heavy duty battery charger that I replaced the diodes in with really good ones , the 13 amp fuse did not blow nor did the crappy 50amp mcb on the lv side , the 20amp mcb in the consumer unit tripped eventually but too late for the transformer which produced a large amount of black smoke from the primary winding . I agree with the advice to fit four good ones rather than 19 stupid little diodes like I have in my Thermal Arc mig but make sure the circuit protection and thermal cut out is all working properly .

 

[R Kraft]

I think they use eight small diodes because they are cheaper than 4 large ones,and they are common because they are the same as are used in many automotive alternators.
You are mixing your diode ratings on the input/output voltage and amps,and series and parallel ratings.
If you use 4 diodes in a bridge.
In general.
The diodes would be at least 3 times the OC voltage, usually they use the 300v diodes.
The amperage for each diode should be at least the output max amps.
Something like this should work.:http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=3574598
But you would have to make your own heat sinks.
Robert