Re: [CAD_CAM_EDM_DRO] Re: capacitor

Hi Mike;

Variacs are OK to feed transformers. Just remember that a variac is an
autotransformer, the output is not isolated from the line, unlike a
conventional transformer. The secondary and primary winding are all the
same winding in a variac.

If you connect capacitors in series to get a higher effective working
voltage, you will half the total capacitance. 2 1000uF 100V caps in
series have a working voltage of 200 volts, but an effective capacitance
of 500 uF. Additionally each capacitor should have a bleeder/equalizing
resistor across it, with a value of V applied X 100 ohms/volt.

The total filter capacitance required will be determined by the load
current draw and the allowable ripple in the circuit. Naturally a half
wave rectifier will need a lot more filtering than a full wave
rectifier. The trade off here is massive filtering results in high
inrush current when power is applied, although the variac will reduce
this effect provided you remember to turn it all the way down before
applying power.

Any power supply design book, or the ARRL handbook (often available at
the local library) will have all the formulae, design considerations,
and trade offs fairly well explained.

With all that said, from what I've seen of the Gecko docs, I'd be
inclined to build a nice bulk power supply with some spare capacity, and
save the Variac for some other project or sell it on eBay or at a
hamfest. High power Variacs are pretty pricey, and using one adds
several levels of complexity to your system, in design and use. I
believe the Gecko units have internal regulation, so the bulk supply
could be pretty simple.

Stan

Mike Snodgrass wrote:

>
> Could I use the variac as input to the transformer? I need wide control
> over output voltage. I thought that
> a variac was a variable transformer? Either way I would
> still have to know the cap value. I have tons of large caps
> but the highest V rating is 40V, I would need 80v with
> headroom.
>
> Thanks for the input
>
> ----- Original Message -----
> From: "mariss92705" <mariss92705@...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Tuesday, March 19, 2002 8:18 AM
> Subject: [CAD_CAM_EDM_DRO] Re: capacitor
>
> > DO NOT use a variac unless you are planning to kill yourself! Use a
> > transformer.
> >
> > Mariss
> >
> >
> > --- In CAD_CAM_EDM_DRO@y..., "Mike Snodgrass" <snarf@s...> wrote:
> > > Hello, I have a variac rated at 60A, and plan to rectify and
> > > filter the current at my driver enclosure, to power my geckos.
> > What value of cap would I need? I realize it would
> > > have to be rated at higher than required voltage. Also could I use
> > a AC cap ahead of the rectifier?
> > > Thanks
> > >
> > >
> > > [Non-text portions of this message have been removed]
> >
> >

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