Re: [CAD_CAM_EDM_DRO] Re: What size capacitor should I use?

That assumes that the capacitor was fully discharged each
cycle (or half cycle). Since you are driving an inductor(motor)
with the supply, the design gets a little more complicated than
that. So much power supply design is over kill 'just to make
sure'. I've never put a resistor between the rectifier and the
capacitor so don't know what the purpose of it is. I do keep
a bleed off resistor across the capacitor of maybe a meg just
to make sure it is dead. It has been years since I used Time
Constants, but I seem to remember that in one time constant
you charge to .707 of the applied voltage. Discharge circuits
work the same way to discharge a capacitor.

bob mcknight

On 27 Jan 2005 at 2:05, cnc_4_me wrote:



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





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