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Old 21st April 2005, 01:45 AM   #1
gengis is offline gengis  United States
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Question Caps for PSU

I need some help for picking out PSU capacitors, in the process of collecting parts for an amp. Have picked one 1000VA transformer, 60V AC, 8.33A supply per channel; amp rail +/-90VDC.

Capacitor options:

1.) 100V, 18000uF, Max ESR=17.5, Max Ripple current=10.5 A
2.) 100V, 18000uF, Max ESR=0.018, Max Ripple current =10.4A
3.) 100V, 22000uF, Max ESR=0.015, Max Ripple current =12.5A
4.) 150V, 22000uF, Max ESR=15.4, Max Ripple current =15.6A

I am only going to pick one style of the caps (4 per channel), please also explain the reasons for picking the particular cap.
Thanks.
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Old 21st April 2005, 03:24 AM   #2
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All of them should be acceptable, though there's no need for the excess rating (150V) and expense of the last one.

Hmm, 8.3A is a lot of DC, won't ripple be up around say 20-30A? Then you'll need to get smaller (say 5-10mF range) caps and parallel a few.

Or you could get a higher voltage transformer and choke filter it (ripple current will be almost negligible), but chokes of a suitable spec are unfortunately hard to come by.

Tim
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Old 21st April 2005, 08:06 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
you're obviously planning a big amp and this will entail high ripple currents.
I second last recomendation to use multiple caps on each rail to increase the total ripple capacity. It has the other benefit of reducing ESR, provided you keep cable short.
The ESR for the first & last are suspiciously high, could they be milliohm?
You may find that 4 * 4.7mF & 2 *10mF are cheaper than 18mF.
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Old 21st April 2005, 08:15 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi again,
nominal voltage of transformer is 60Vac
unloaded voltage assuming 3% regulation will be 61.8Vac
rectified voltage will be 87.4Vdc less 0.3 for lo current diode loss = 87.1Vdc
Plus 6% incoming mains voltage will raise your unloaded voltage to about 92.3Vdc.
That leaves about 7% spare as a safety factor. Capacitor life should be OK if you keep them cool, another reason to reduce ripple current and case temperature.
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Old 21st April 2005, 09:38 PM   #5
gengis is offline gengis  United States
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Thanks for the reply.

It is E. Holton's 800W Mosfet amp.
I ordered the boards and collecting parts now.

He recommend the third choice:
3.) 100V, 22000uF, Max ESR=0.015, Max Ripple current =12.5A

I planned to have 4 of them or 88000uF per channel.
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