Re: What size capacitor should I use? resistor or not ??

OK Scott and Wally, this a definate design point in which a person
with a hgh level of electical knoledge would be needed. Most of us
are hobbiests and not EE's.

If either of you guys want to make a spreadsheet and white paper on
how to impelment unregulated power supplies eq: bridge and cap
selection, that would be a really great resource.

From my watching the list, the discussion of the inrush surge being
either harmful or just un-nerving, came in to play as the
transformers got huge, not the 'average' 10 amps ones we see so often.

At what point would the resistor be needed to prevent damage ? at
what point would it help to prolong the life of the components, but
otherwise not be apparent it is needed. (no noise or whatever) and
when is it just not needed at all ?

As Wally mentioned, there was a circuit floating around that would
charge the caps upon inital power-on, then get out of the circuit
when the operating voltage is reached.

In other words, what advise and engineering can youse guyse offer to
help out the non-electronical types on the list ?

Dave





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

>
> 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



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> -- 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

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