Re: Sanity Check on Power Supply Capacitor Size

--- In CAD_CAM_EDM_DRO@yahoogroups.com, luke1027@p... wrote:

> Sanity Check
>
> Filter Capacitor Size for Power Supply based on:
> C=(80,000*I)/V
> Transformer (secondary tap) will be putting out (18V * 1.414=25.5V) with
> 8.3A*2=16.6A (since it is wired in parallel).
> With these specs I get C=(80,000 * 16.666)/25.452=52,386 Microfarads.
>
> So the filter capacitor size I need must be at least 52,386 Microfarads?
> Is this calculated correctly? Boy, that seem high … does it not?

Hi,

The calculation is a little off.

There is a 1 volt drop from the bridge, so the 18 becomes 17 before
you use the 1.414.

but, look at the wattage
raw = 16.6 amps x 18 V = 299 watts
then your DC voltage is 24.04 VDC
299 watts / 24.04 volts = 12.409 amps at 24.04 volts.

as you raise the voltage, you proportionally lower the amps.


But for caps, the higher voltage means lower cap size, lower voltage
means higher cap size.

But, it appears you are using the power supply and not the total motor
amps. That should make another difference.

Also, the 'huge' caps are pretty cheap and physically small. you only
need 25 volt rating caps.

Even with the corrected numbers, the calc comes out to about 42,000µF
but that is still not the proper number as it is not the total of the
motors.

Here is a fast pick, Digikey - P10007-ND $11.68
56,000µF and 25 volts. The diameter is only 35mm, or 1.4 inches.
pretty small actually.


Dave