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Old 22nd November 2001, 11:24 AM   #1
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I've decided to not use these transformers I found and decided to buy a pair of 500VA (or maybe 1000VA) transformers from RS -- 2 x 40Vac secondaries (which should return about 56V raw DC for the PSU regulator board) -- the only thing is I have 8 x 40V Aerovox caps lying about here but I really think I should be using 63Vdc+ caps -- can you think of any problems running 56Vdc into 85 degree 40Vdc caps? A new set of 8 x BHC Aerovoxes giving 88,000uF per side will set me back serious money!

colin
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Old 22nd November 2001, 11:52 AM   #2
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Seriously you might not want to try it..running 56V into a 40V cap. Consider a new set of 8 x BHC Aerovoxes cheaper than a pair of eyes. Look for surplus if you don't have the cash rolling for a new set.
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Old 22nd November 2001, 11:55 AM   #3
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Yep, thanks I had a read on the net and saw the problems...

Will grit my teeth and geta new set... I can always use these in a DIY PSU project elsewhere I guess...

thanks for the warning...

colin
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Old 22nd November 2001, 01:07 PM   #4
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there might be another way ...... depending on how much capacitance you want/need and how much money you can afford to spend.........

if you put 2 of your 40V caps in series, you will get the equivalent of an 80V cap with twice the esr of the 40V cap and half the capacitance ...... so you could run 4 of these in parellel (ie. 8 x 40V caps) and get the capacitance of 2 of the original caps ..... so this leaves you with 1/4 the capacitance you would started with but with a usable voltage rating and if you make a capacitance multiplier power supply this is probably good enough and you haven't had to spend the money on all new caps ......

so there you have it.

good luck.
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Old 22nd November 2001, 01:17 PM   #5
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If you series caps, you <i>must</i> put resistors in parallel with each cap (creating a voltage divider) to ensure that each cap sees only half the voltage across the pair. Elseways, the voltage will tend to drift proportionate to the actual capacitance of each unit--possibly exceeding the voltage rating of the cap.

Grey
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Old 22nd November 2001, 01:22 PM   #6
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Thanks Grey,

yeah it works great without the resistors if the caps are practically identical but with the +/-20% rating that many electolytic caps carry i wouldnt be holding my breath the resistors kinda slipped my memory but yeah they really are important with most electo's used in this manner.
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Old 22nd November 2001, 09:31 PM   #7
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Thanks for that. I'd thought about running them in series but wasn't really sure it would be such a good idea in a Power amp so I decided to just get the right caps and I can always use these in a lower power PSU project I'm also working on...

Thanks again, I really appreciated it.
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Old 22nd November 2001, 11:07 PM   #8
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It's always best to condition your capacitors before puting them into service, most manufacturers recommend it as it lowers the leakage current and increases capacitor life.
It's also safer, I have had one explode on me, an experience you NEVER forget, (it was marked with the incorrect polarity, the whole batch was)!

Regards WALKER

[Edited by walker on 11-22-2001 at 06:12 PM]
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