I have a big problem!!
There is a rectifier (IN 380V ; OUT 20V 50A without a transf.) connected to an anodizing cube. when start to raise the voltage, the current strats to flow thru the bath when the voltage reaches +- 10V!
Now the problem is how to make the current flow thru the bath with less voltage??
I attached a file so that you can see what i think it will solution this problem, but please write your opinion!!!

Thank you all

Attached Thumbnails

 

 

It can't just be a rectifier, a rectifier doesn't work like that - and assuming it did, it would be dissipating 18,000 watts of heat!. It sounds FAR more likely to include a transformer.

Presumably this 'problem' is a chemical one?, the solution in the bath requires a certain voltage before it begins to pass enough current. Any potential solution is likely to involve the bath or the chemical solution, not an electronic fix at all.

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To get 50 amps thru 1 ohm, you need 50 volts but your power supply only provides 20 volts. I don't understand why you want to do that? Why do you want current flow at voltages less than 10 volts? If this is a plating operation, it depends on the current, not the voltage (as I understand it).

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Most plating operations involve a phase control circuit, then rectification via SCR's and or large diodes, in either full bridge or half bridge arrangement. As has been said above, its the current you need to control, not the voltage. With the correct bath solution, you will get conduction at low voltage, and then you need to have the curent available to drive what is nearly a short circuit.

Industrial plating / anodizing power supplies are generally operating from about 5 volts up to 25 volts, with well over 10,000 Amps available.

You may have a problem with you chemicals, or you sacrificial electrodes being contaminated.

 

I assume what you're saying is that the system will draw too much current if you dump 20V into it. Or will 20V not work with your reaction?

It would help if you could say exactly what voltage and current you need.

There are plenty of ways to drop 20V to something like 10V. Unfortunately, a resistor or simple transistor reg generates so much heat it may not be practical. A buck converter would need to have an enormous inductor at such currents. PWM sounds like the only thing practical, but this still means 20V will be applied to the bath, but just for a shorter period of time. If the instantaneous current doesn't work with the bath's chemistry or it draws enough to overheat the power supply, this may not work either.