Welding rectifier more details

>
>> Occasionally day-dream about how I could make a stand-alone rectifier.
>> Practical advice appreciated.
>> Less and less need now with inverters getting better and better.
>> But no inverter I have yet met will run a 6010 "keyholing".
>>
>> Rich Smith
>
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> Start with the welder's maximum current, lets say 300A.
>
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The 300UR is reverse polarity. You need two electrically isolated
> heatsinks, the positive one for the two diodes whose threaded studs
> are cathodes and the negative one for the two that are
> anodes. Insulating the four diodes from a single grounded heatsink is
> possible but liable to hidden short circuits and the diodes will run > hotter.
>
>
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> Large finned aluminium heatsinks are common cheap electrical scrap, if
> you know where to look. You may have to experiment to find how large a > fan it needs.
>
> The hardest part may be finding and fabricating heat-resistant
> electrical insulation that will withstand rough handling. I'd try
> unperforated FR4 unclad circuit board material rather than the temping
> PVC pipe. The high current terminals can be brass bolts through the
> insulation. I've found threaded copper starting motor terminals at a
> Diesel electrical shop and lathe-turned them into the high current
> studs I needed for a current measuring shunt.
>
> The housing for it can be a welded cube of angle iron with flat sides,
> if you don't have equipment to bend sheetmetal. Be sure that at least
> one side can be closed without access to the interior, i.e. tapped
> instead of clearance holes for the screws.
Calling Jim, and anyone else who can help.
Jim - you said separate the four rectifiers of a bridge rectifier into
two pairs - because of their polarity? If I understand you rightly?
That would mean a practical welding rectifier could have two metallic
heat-sink assemblies separated in a glass-fibre (GRP) frame
(insulating)?
Like the idea...
Should it be air-cooled or oil-cooled?
The one I used (probably 1960's or 1970's vintage) was air-cooled.
Regards,
Rich Smith
Reply to
Richard Smith
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Calling Jim, and anyone else who can help.
Jim - you said separate the four rectifiers of a bridge rectifier into two pairs - because of their polarity? If I understand you rightly?
That would mean a practical welding rectifier could have two metallic heat-sink assemblies separated in a glass-fibre (GRP) frame (insulating)?
Like the idea...
Should it be air-cooled or oil-cooled? The one I used (probably 1960's or 1970's vintage) was air-cooled.
Regards, Rich Smith
================================= Having built a machine that used heat transfer fluid, I wouldn't choose it for a home project. That machine temperature-cycled GM HEI ignition modules between [above boiling] and [well below freezing] to confirm that they would withstand the thermal shocks of for example starting cold and immediately plowing snow in Alaska. The heatsinks were channels formed on a press brake from 1/4" copper plate with tubing for the fluid soldered into the inside corners using large amounts of 95/5 solder.
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This style of heatsink is convenient to work with:
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the thermal calculator the power in Watts is the diode voltage drop times the welding current.
One heatsink is positive, the other negative, so the welding leads can connect directly to them, which saves fuss and expense of high current terminal blocks. If instead you choose four identical diodes they need to mount on four separate heatsinks of the same total size since each diode conducts half the time. I would enclose the heatsinks in air ducts made of GRP (FRP here) to confine the air flow within the fins and protect you when testing it with the external covers off.
An electrically isolated module like this might be a simpler solution if you have or can machine a flat enough contact surface on the heatsink. It's less work to build, more to test that it won't overheat and burn out.
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I didn't have machine tools available I've fitted an overheating device to its heatsink by scraping like the old time gunsmiths.
I haven't mentioned methods that require special test equipment like high voltage insulation testers.
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When I design a home project I determine the essential component specs such as diode voltage and current, and buy whatever usually surplus parts I find that meet them. The wiring and mounting details can be worked out later. A good drill press is essential, a milling machine very helpful.
Reply to
Jim Wilkins
Air cooled should be fine. If you are using silicone diodes the frop is only about 0.7 volts If you have a 135 amp welder that is only just over 80 watts dissipation.a few hundred square inches of heat sink fins can handle that with no problem - particularly with a fan. I have a set of 600 amp diodes on a finned heatsink of 288 square inches, more or less. They are a 3/4" stud mount with 3/8" bolt on "buss bar" type terminals
Reply to
Clare Snyder
My Maxarc rectifier unit is air cooled and no fan either, the cooling of the heatsinks and inductor being by natural convection. I could ask my mate who has it at the moment to pull to cover and takes some pics if necessary.
Reply to
David Billington
Air cooled should be fine. If you are using silicone diodes the frop is only about 0.7 volts If you have a 135 amp welder that is only just over 80 watts dissipation.a few hundred square inches of heat sink fins can handle that with no problem - particularly with a fan. I have a set of 600 amp diodes on a finned heatsink of 288 square inches, more or less. They are a 3/4" stud mount with 3/8" bolt on "buss bar" type terminals
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0.7V is a good number to remember for general low power circuits but it's not strictly accurate. The forward drop is around 0.62V at microamps and it increases as the logarithm of the current, plus resistively from the bulk silicon and leads etc. OTOH the drop decreases as they warm up.
The forward drops are as much as 1.65V for these devices:
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Reply to
Jim Wilkins
This 400A rectifier module is electrically insulated from the heatsink it mounts on, which can be grounded to simplify construction.
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I didn't find a current rating this high when the thread was active.
Reply to
Jim Wilkins
I can't make out its size, but those terminals don't look huge, and I'm thinking "Wow is that all that's needed?" (??!!!). Given the size of a welding cable for 400A ... Plus the amount of heat one assumes it would have to dissipate (a few percent of the input power?) If that allowed you to keyhole with a 6010 from a "tombstone" welding machine that would be amazing. Given the cost is rather affordable if it gave that outcome...
Jim - I'll post about welding conditions on s.e.j.w. Bit of science in there. covid19 lockdown restricting us quite a lot - not welded for months and getting by as construction site labourer.
Reply to
Richard Smith
I can't make out its size, but those terminals don't look huge, and I'm thinking "Wow is that all that's needed?" (??!!!). Given the size of a welding cable for 400A ... Plus the amount of heat one assumes it would have to dissipate (a few percent of the input power?) If that allowed you to keyhole with a 6010 from a "tombstone" welding machine that would be amazing. Given the cost is rather affordable if it gave that outcome...
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Using the closest match on an architect's scale the 110mm dimension measures 100 and the pad widths are 17+ and 20, so 19mm and 22mm. The screws just clamp the cable lugs into contact with the pads. If they are steel they don't have to conduct much of the current.
If you figure on 1.5V drop per diode the power loss is 1200W, or less if ITM(?) is the peak forward voltage. For a rough comparison my 1500W electric heater has a 13 Watt, 5" diameter fan that blows 7(?) MPH at the outer edge.
Reply to
Jim Wilkins

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