Re: [CAD_CAM_EDM_DRO] Power Supply Question - a bit OT

Hi list

Before I go on to ask my own, somewhat bizarre question, here is a small
contribution to the 'on' the topic.

A variac is, at least in the UK, a 'continuously' variable auto transformer.
It has only one winding connected across the incoming supply. This winding
is placed around a toroidal core with the insulation removed on one face to
allow connection to a wiping contact which delivers the output voltage. This
winding usually has an extension at the high end allowing output voltages up
to 115%, or so, of the input.

The low potential connection of the input is directly connected to the low
potential connection of the output and the variable output connection has a
direct copper connection to the high potential connection of the input. No
isolation from the main supply is afforded by this device and you must be
very careful when using it.

I once accepted a kind offer, on site, to help me debug a system exhibiting
teething troubles. In my naivety, I connected my box of tricks to the guy's
home made power supply, (this turned out to be a variac arrangement), and I
then attached my scope, which grounded of course. A loud bang ensued!

The resemblance between watch hair springs and PCB tracks which have become
dissociated with their formerly intimate fibreglass substrates is
remarkable - even if you do have to wait a minute or two for all that white
smoke and confetti they put inside electrolytic capacitors to clear from the
air!

So - be careful, isolation is best, and if you need the variac benefits,
put an isolating transformer in front of it.

Now for my question!

I have a requirement to deliver 2000 Amps or so at about 30 or 40 volts AC
or DC into a load. The load is made of stainless steel wire and is basically
resistive with a 'smallish' positive temperature coefficient.

At this stage the precise current requirement is unknown, so the ability to
adjust the current is desirable. There may also be a need to control the
current / time profile, as the particular requirements of the process become
apparent.

My immediate solution to this is to take ten 200 amp, cheap, arc welding
units and connect their outputs in parallel. This is clearly not their
intended mode of operation, so I don't imagine that the manufacturers will
be prepared to comment on their suitability to this use.

Is anyone here willing to venture an opinion as to whether or not simple
(reluctance controlled - I assume) arc welders can be combined in parallel
to achieve high current outputs? Will they current share, current hog,
trip-out, explode?

TIA

Brian Walsh
Glasgow
Scotland

----- Original Message -----
From: "brian" <ka1bbg1@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Thursday, November 01, 2001 01:43
Subject: Re: [CAD_CAM_EDM_DRO] Power Supply Question


> Hi, well if you use a machine control transformer and a variac to control
> the input voltage would work. you also could use a boost/buck arrangement
> with 2 transformers to get the desired output. I am due in the next 3 days
> to change "isp" but will send you my new email addr. so i can help you off
> line. cul brian f.
> ----- Original Message -----
> From: "Dan Statman" <dan.statman@...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Wednesday, October 31, 2001 5:03 PM
> Subject: [CAD_CAM_EDM_DRO] Power Supply Question
>
>
> > This is unrelated to CNC, but you guys seem to know a lot about power
> > supplies. I am looking for a variable DC power supply capable of 3A and
> > 0-200 Volts. Does anybody have a surplus one sitting around? Anybody
> know
> > where some plans would be to build one? Any help appreciated. What
would
> > the cost of components be?
> >
> > I need this for a titanium anodizing tank that I would like to build.
> >
> > Thanks,
> >
> > Daniel J. Statman, Statman Designs
> > www.statmandesigns.com
> >
> >
> >
> >
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