# Help! High current DC welding bridge rectifier

Discussion in 'Circuit Help' started by Easymkay, Aug 3, 2012.

1. ### Easymkay

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Hello all...new to your forum...thanks in advance. Took/forgot physics, so I have a vague idea of AC/DC theory, but not its particulars/many caveats. I am working on a welding project and need some advice on some fairly high current switching of AC to DC. I am taking an AC arc (old fashioned "stick") welder, and converting it's output to DC. There are many reasons to do this...it boils down to being a smoother weld with more versatility too, with reversal of it's terminal polarity used for varying metals/thicknesses. It is supposed to put out 90 amps continuous, I am told RMS, at about 38 volts. I'm also informed that I ought calculate for peaks of double that 90 amp current. A bridge or 4 diodes at this high amperage gets expensive quick. My questions are these: 1) Anyone aware of a reason I could not wire two of these... http://www.sanrex.com/images/PDFs/BKR400AB10.pdf ...up, back to back, to form your classic, 200 amp bridge? There's a schematic lower on that page. They are \$13 each online as opposed to other solutions at >\$120. 2) Is being "shottky" a problem or advantage here? 3) From what I remember, a higher component voltage rating like this (100v rated component; 38 volts actually running) is not the problem that a too low current rating would be in practicality, correct?? Any input appreciated!

Easymkay, Aug 3, 2012

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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You would need a common anode as well as a common cathode version to create a bridge rectifier.

The other issue is that you would have to be sure that your open circuit voltage did not rise above 100V.

Strange things happen when the arc is struck and when it stops. You need to cater for the voltage spikes that will occur.

(*steve*), Aug 3, 2012

3. ### CDRIVEHauling 10' pipe on a Trek Shift3

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Have you investigated welding forums? Specialized forums like that will have a knowledge base of suppliers and components . You won't be the first welder that's wanted to do this.

CDRIVE, Aug 4, 2012
4. ### CDRIVEHauling 10' pipe on a Trek Shift3

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CDRIVE, Aug 4, 2012
5. ### Easymkay

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Thanks *steve*, I should have seen that obvious design flaw of mine with non-opposing [gate?] polarities. I guess from here I will better consider some variance for voltage as well as current. The components I see seem to jump from the 50-100 volt-range-rating, straight way up to 1 to 1.6 kilovolt ratings, with not very many rating ranges in between. I have been told on lower current/voltage projects that it's all OK as long as the individual component voltage rating exceeds what the circuit will experience. Do you see any exceptions to that "rule" for this welding application (other than being more bulky/over engineered)? On a tight budget I'm now looking at something like 2, in parallel, of these: http://www.ebay.com/itm/Bridge-Rect...926?pt=LH_DefaultDomain_0&hash=item1e67eb8fe6 ...Otherwise, I am down to this Asian model: http://www.ebay.com/itm/MFQ60A-1600...664?pt=LH_DefaultDomain_0&hash=item45feec8458 which professes to be a standard bridge, looks plain single phase, yet has a confusing wiring diagram and fifth diode?? Thanks for your input so far...further advice, of course, is appreciated.

Easymkay, Aug 4, 2012
6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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That's beyond the power ratings that I am comfortable advising you about.

CDRIVE's recommendation of asking on a specialised welding forum is a good one.

There may be people with practical experience who can give you "what works" advice.

(*steve*), Aug 4, 2012
7. ### duke37

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The arc is struck by shorting the output of the welder, developing a high current and then opening the contact and with the internal inductance gives a high voltage. Your diodes must withstand both of these conditions. Overvoltage could kill the diodes quickly but diodes can take an over current for a very short time if they are kept cool.

I would think Schottky diodes would not have sufficient voltage capability.

Miller describing their diodes as silicone does not inspire confidence.

duke37, Aug 4, 2012

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