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Old 25th November 2004, 04:24 PM   #1
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Default What size Capacitors to use?

Im building a pre-amp PSU and i'm using 2 off 75VA laminated transformers with 24-0-24v secondaries and half wave rectification.
I have a couple of 15,000uf caps but would think that is to much for the relativly small transformers. I'm considering either a pair of 4700 or 6800uf caps, does this seem about right?
I've looked on the net to try and find any advice on transformer size in regard to capacitor size but didnt come up with anything.

TIA,
Paul.
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Old 25th November 2004, 04:45 PM   #2
markp is offline markp  United States
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You might want to try a few 1000uF caps in parallel as they are cheaper and have lower series resistance all together than single larger caps will. Also, bypass your diode bridge with small film caps in parallel with the diodes.
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Old 25th November 2004, 05:33 PM   #3
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Originally posted by markp
You might want to try a few 1000uF caps in parallel as they are cheaper and have lower series resistance all together than single larger caps will. Also, bypass your diode bridge with small film caps in parallel with the diodes.
Actually, you should not bypass those diodes but place a series RC filter across the rectifier bridge output. Please look at http://www.hagtech.com/pdf/snubber.pdf.
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Old 25th November 2004, 05:43 PM   #4
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Firstly I have to ask why you are only halfwave rectifying not full. Fullwave will give you less ripple for a given size cap.

As for cap size, it won't hurt to have as big a cap as you can get.
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Old 25th November 2004, 06:18 PM   #5
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I'm building a Naim style PSU, hence the halfwave rectification.

See here for layout.

If the output from the caps are identical I want to try hanging just the single reg from the caps to make a totaly mono supply.


Paul.
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Old 25th November 2004, 06:19 PM   #6
lpm76 is offline lpm76  Denmark
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Quote:
Originally posted by hlyytine


Actually, you should not bypass those diodes but place a series RC filter across the rectifier bridge output. Please look at http://www.hagtech.com/pdf/snubber.pdf.

Well, I for one can't figure out how to calculate the optimal snubber values. I get lost in the math every time I read that paper. Isn't there another place where it is explained in a better way?
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Old 25th November 2004, 06:42 PM   #7
markp is offline markp  United States
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Quote:
Originally posted by richie00boy
Firstly I have to ask why you are only halfwave rectifying not full. Fullwave will give you less ripple for a given size cap.

As for cap size, it won't hurt to have as big a cap as you can get.
A single big cap is not as good a few smaller caps that add up to the same value. The smaller caps will also add up to less money.
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Old 25th November 2004, 07:25 PM   #8
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Originally posted by lpm76



Well, I for one can't figure out how to calculate the optimal snubber values. I get lost in the math every time I read that paper. Isn't there another place where it is explained in a better way?
Forget the advanced math, page 9 gives you an example on how to calculate the exact values. Doing the maths is not that hard; measuring the leakage inductance of the transformer is more complex. I can't think of any method how to do this with a DVM (with the most expensive true-RMS-meters it could be possible). The oscilloscope helps a lot.
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Old 26th November 2004, 07:58 PM   #9
PRR is offline PRR  United States
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> 15,000uf caps but would think that is to much for the relativly small transformers.

Why?

Yes, it takes a while to fill 15,000uFd. But once filled, they don't impose any extra load on the transformer. I have a box with 15,000uFd fed from a 24VA core, has been running happy for 25 years.

And my motto has always been what Richie says: "it won't hurt to have as big a cap as you can get."

Over-capping does raise peak current. Peak current depends a little on load, but for low-ripple supplies it depends much more on series resistance, which depends on transformer rating. Rectifiers should never be less than transformer rated current, plus 2 to 5 multiplier safety margin. Your 1.5A windings suggest 3A to 7.5A rectifiers, and go higher when you have big caps. Since 15A is cheap in plain silicon, I'd go that high. Yes, even if you only really need 0.1A to the load. If you like exotic rectifiers, cost and reliability are something to ponder.

If you do like the idea of dozens of smaller caps, split them up as C-R-C. 3,000uFd, 1 ohm, 12,000uFd. This helps clean some of the rectifier spikes from the supply rails. While I understand the point Hagerman makes, and have seen it, personally I like brute-force cleaning rather than specific tricks, at least in my own DIY. But there are many ways to skin cats.

75VA seems like a lot for a preamp. I confess I've done similar over-kill.
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Old 29th November 2004, 10:16 AM   #10
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Thank you for the reply's, even if some of them were WAAAYYYYY over my head!!
I will use the pair of 15000uf caps that i've already got and see how I get on with them.
Have any of you had any experiance of pairing up the regs? Im using Avondale TPR's so it is possible to trim the outputs to match each other. The idea is that I can end up with a dedicated mono PSU for each rail, all be it at some cost in space and cash!

Paul.
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