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.