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Old 03-18-2011, 03:08 PM   #4488 (permalink)
DJBecker
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The capacitors also have to do work. If they don't get warm at all, they probably aren't doing much good.

We've experimented with big surplus caps and found that they are mostly ineffective. They are optimized for filtering 120Hz ripple. At 15KHz they just slowly absorb a some of the energy and ring -- the ESR and inductance makes them a tank circuit. It only takes two smaller 330uF caps designed for switching power supplies to match the peak clipping of the 4,900uF and 18,000uF monsters we've tried, with much less (and higher frequency) ringing.

That said, those two caps work very hard. They get warm with just a minute or two of operation. You need more caps to spread the energy absorption. And with more caps, there is a lower combined ESR, therefore much less total heat, and much less ringing.

It does take a while to internalize that it's not about bulk capacitance. You do need enough capacitance to absorb the inductive spike, but it's really about how quickly you can push the energy into the cap.

Another concern with giant caps is the heat build-up. Besides having less surface area for their volume, the heat has much further to travel to get to the surface.

Last edited by DJBecker; 03-18-2011 at 03:17 PM..
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