Re: What size capacitor should I use?

Attn CNC power supply people...Please DO NOT put a resistor between
your bridge rectifer and your capacitor...Because of the time
constant increase (T=RxC)x5, you will render the recovery of
capacitor voltage to be to long in time to be of any use to PWM
drives...

T = time in seconds
R = resistance in ohms
C = capacitance in farads
X 5 = 6 time constants for a full charge

example... Power supply has 30,000Uf and you add a .5 ohm resistor.
C = 30,000Uf is 10,000 to the minus 6 farad.
R = .5 ohm
T = C x R
T = .03farad x .5 ohms = .015 seconds for 1 time constant.
It takes 5 time constants to charge a capacitor...So..
5 x .015 seconds = .075 seconds for capacitor to recover...
.075 seconds = 75mS = 75000uS

We are PWMing around 22khz or 22,000 cps..At this frequency each
chopped wave can be as small as 1/22,000 = .0000454 seconds...Or..
.0454mS....Or 45.4uS

With resistor in circuit it takes 75mS to recover and we need a fully
charged cap in .045mS....Adding a .5 ohm resistor makes the system

75mS/.0454mS = 1670 times to slow.

And now you can see why many power supplys use buss bar or very heavy
wire....


Wally



--- In CAD_CAM_EDM_DRO@yahoogroups.com, polaraligned
<polaraligned@o...> wrote:

>
>
>
> >Huh? Larger capacitance will REDUCE voltage ripple (although it

may

> >increase
> >peak current from the rectifier).
>
> >Jon
>
>
> I think good design practice would be putting in a small value

resistor

> between
> the bridge rectifier and the capacitor. This would "soften up" the
> tremendous
> surge the cap sees at turn on.
>
> Scott