Re: [CAD_CAM_EDM_DRO] cap question

Mike Snodgrass wrote:

> Hello, on my on-going power supply project, I am putting
> together a 25A unit. By using a rough estimate of 1000mf
> per A load, I need 25000mf. I have a 33000mf 60V cap. The output of my trans is 58V. Is there anyway to "trim"
> 8000mf? How much overfiltering can be tolerated?

I think I have 250,000 uF on my 10 A 69 V power supply. I think it is adequate,
but not overkill. Especially when we had a short power failure, the system
kept running right through it.

25,000 uF is an absolute MINIMUM value! This sets the ripple to a reasonable
value, assuming there are no slight dips in the power, clipped half-cycles, or
whatever. Unless you run it from an on-line UPS, you won't have power like
that. You need a LOT more uF if there is more than 8.3 mS between charging
pulses - like 1 second!

Let's see, 25 A will cause 33,000 uF to fall 758 Volts per second, so you have
a droop of .758 Volts per mS. In 8.3 mS (the time between charging pulses
at 120 Hz) you have a droop of 6.29 V. BUT, if you want to ride out line
disturbances, you need more energy storage capacity. (Of course, will this
power supply REALLY be delivering 25 A all the time? I really doubt it.)
Multiplying by 10 wouldn't hurt, but you certainly won't have a problem by
using 33,000 uF instead of 25,000.

Jon