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Please Help with DC Power Supply

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New Member
My Son and I are building a project for the science fair. We need 300 vdc at 3amps max. We bought a Variac that will give us from 0 to 300 vac and were told to just run it thru a bridge rectifier and we would be good to go. Now that part has turned into a big project because neither of us have that much experience.

We have a simple diagram (that I can't seem to get to post) that I'll describe here. From the Variac we go to a CoreCom 5K1 filter - We're told that this will remove 'noise'. from that we go to a doide bridge rectifier that should be rated at at least 300 volts and 50 amps just to be safe. From that we go to three capacitors (C1-1000uF / C2=10uF / C3=920uF) wired in parallel and we're done. My questions are these:

1) I have found the CoreCom but cannot locate a diode bridge rectifier rated above 40 amps and the specs are confusing - forward voltage & amps and reverse voltage & amps - I assume I'm only concerned with forward ? Could you recommend a brand/part#

2) The end result is to hook a cathode and anode up that are submerged in a chemical solution. Does it matter what the resistance is here - or is it okay to turn this ON with no resistance or does it really matter?

3) Is there a reason I can't just have one big capacitor instead of three - and what should the uF value be.

4) Only the Variac itself has a fuse - shouldn't there be some type of additional fusing somewhere?

Thanks so much for any help you can provide.
If there is an easier way to get the Variac A/C into the desired output voltage and amperage please feel free to suggest.



Well-Known Member
The main diode specs you care about are current rating and peak inverse voltage. To get 300VDC, you need around 220VAC input. That is about 300V (peak) and 600V (peak to peak). 300V diodes will not be safe to use. I recommend 400V diodes.

The diode (and AC transformer current) is always higher than the DC current which you say is 3A. I think the diodes need to be rated for at least 5A, higher is better. They will also need to be mounted to a very good heatsink. I am not sure why you need a 40A bridge? That would be huge.
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New Member
My son now tells me that he can get by with 250vdc at 1.5 amps. Sorry for the confusion but I would still appreciate any comments or help on this.
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