Re: What size capacitor should I use?

Your analysis is way too simple. It assumes values, what type
of drivers you are using, etc, and most importantly, it assumes
that you are
throwing a dead short across the power supply at some chopper
rate and fully discharging the capacitors. In reality this is not
what is happening. I stand by my statement that good power supply design
practice would place a small value resistor between the bridge and
the caps. This is a general statement and as with all electronic
design, it needs to be refined to the particular circuit. One
may conclude that no resistor is needed, or useful, and that the
transformer winding resistance is sufficient. This can only be
determined by thorough circuit analysis and/or emperical design.

Scott














-- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me" <cnc4me@g...> 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