Re: What size capacitor should I use? resistors....

You can use an inrush current limiter. This is a resistor that has a
negative temperature co-efficient; the hotter it gets, the less
resistance it has.

It is normally placed between the bridge and the bulk capacitor. When
power is first turned on, the resistance is high because it's cold.
As current passes thru it, it gets hot and the resistance drops. By
then the cap is charged up, the current having been limited to a
reasonable value during charge.

An example is Digikey part # KC003L-ND. Its "cold" resistance is 2.5
Ohms, "hot" resistance is 0.06 Ohms. Rated at 8 Amps.

Mariss



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

>
> > I understand the desire to limit the starting inrush current as
> well
> > as the need for the smallest time constant.
> >
> > would it not be prudent to charge the cap slowly up to some

value,

> > and at that point, switch over to the raw voltage directly ?
> >
> > It would seem a simple electromechanical relay would be a
> possibliity
> > to eleminate the hum of initial power on.
> >
> > Dave
>
> You are exactly right dave...And for a example Jon Elson used to

have

> on his site a schematic of what you just described (not sure if it

is

> still there). He used a time delay relay to slowly charge the

caps...

>
> This method also helps save the bridge rectifier from the huge
> inrush...I usally spec the bridge rectifier current rating 50 to

100%

> larger than desired current because of this huge inrush...
>
>
> Wally
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
> <dave_mucha@y...> wrote:
> >
> > --- 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
> >
> > I understand the desire to limit the starting inrush current as
> well
> > as the need for the smallest time constant.
> >
> > would it not be prudent to charge the cap slowly up to some

value,

> > and at that point, switch over to the raw voltage directly ?
> >
> > It would seem a simple electromechanical relay would be a
> possibliity
> > to eleminate the hum of initial power on.
> >
> > Dave