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Old 30th March 2009, 03:24 PM   #1
diyAudio Member
Join Date: Mar 2008
Default are these caps okay?

I'm constructing a power supply, consisting of toroidal trans with 30v secondaries, a 35a bridge rectifier, and 4 large 18000 uf, 50 v. caps I procured at a surplus store. I'm now concerned that the caps may not have enough "safety margin", as there will be (theoretically) 42v at the rectifier output. I'm also incorporating a soft-start circuit to minimize the "inrush" current.

Are these caps okay? I'd hate to have to find new ones, as the cost of even 10,000/63v caps is astronomical. I just don't want to have 4 hand grenades when I power up. Also, can I check output voltage from the bridge rec + and - terminals?
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Old 30th March 2009, 03:52 PM   #2
Mr Evil is offline Mr Evil  United Kingdom
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Join Date: Aug 2004
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That's 20% safety margin, which is just about enough, but if the transformer is specified as having a 30V output voltage then that's what it is at full rated current - it will be higher with no load.
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Old 30th March 2009, 03:57 PM   #3
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
measure the open circuit voltage of the transformer secondaries and also measure the mains voltage.

What is the maximum mains voltage your supplier says they will never exceed?

Calculate the maximum voltage that your secondaries will output if the supply matches your suppliers maximum voltage.

Now multiply by sqrt(2).
Subtract 1.2V if you have a bridge rectifier on each secondary
subtract 0.6V if you have a single bridge rectifier across the series pair of secondaries.
I expect your worst case maximum voltage will be getting close to the 50V rating of your caps, but still with a margin to spare.

Most manufacturers state the maximum working voltage. You should not need to add any margin above that.
You must check what you think your worst case maximum working voltage will be.
regards Andrew T.
Sent from my desktop computer using a keyboard
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