Re: [CAD_CAM_EDM_DRO] Power Supply Guidance Needed

On 3/13/06, Roy J. Tellason <rtellason@...> wrote:

>
> On Monday 13 March 2006 02:44 am, Larry Olson wrote:
> > Ron,
> > Thanks for the input. I have e-mailed the good people over at
> > Acme to get the white papers on the transformers I have. I should know
> > more tomorrow. If they put out 70V and are rated for 120V input than I
> > am in business. I understand you can parallel the output from 2 of the
> > same transformers if the impedance is within a few percent of each
> > other. Do you have any idea how to measure it? I was thinking of
> > paralleling 2 of the 3 transformers that I have because I think they are
> > around 1000VA or so.

You can probably use two identical transformers in parallel. If you try to
use more transformers, you may find some difficulties. Since no two
transformers are 100% identical, there will be some variations in output,
and those variations will become quite noticeable.


If the transformers in question are not identical, then the one with the

> higher voltage output will fight with the other one, not a good
> situation...



> I have 6 5400uF capacitors on hand that are larger than a soda
> > can that I would like to use. I figure about 4 of them if I'm thinking
> > correctly. Also there was a 50W 750ohm bleed resistor across each set of
> > caps (3 sets of 2ea.). Do you think one will be enough or should I put 2
> > in series to handle the extra current?
>
> Putting resistors in series will make them have less current, not
> more. If
> you put them in parallel, then they'll handle more current. Figuring out
> what you need here is simply a matter of using Ohm's law -- take the
> maximum
> voltage that you expect to see and divide it by the resistance and that'll
> give you the current flow. Volts times amps will give you the power
> dissipated by the resistor. Putting two of them in parallel will draw
> twice
> the current.

I think he meant 'handle the extra current' by reducing that current.
Putting two resistors in parallel will not really change the current on each
resistor. I think he does not really care about what the current really is,
as long as there is some measurable current, and the resistors do not blow
up. Putting resistors in series is the right solution to acheive what he
wants.

That said, are you really sure you want to use 750 ohm 50W resistors? I
thought bleed resistors are generally relatively small, 'bleeding'
insignificant current. 50W is a bit large to be used as a bleed resistor.
Not that there is anything wrong with that, but If you use the formulas that
Roy has given, you'll find that your bleed resistor will be dissipating as
much heat as my soldering iron. I think I'd choose something quite a bit
larger than 750 ohm, and much smaller than 50W.

> One more question. If I have a 2500KVA transformer (or 2 smaller
> > ones) what kind of amp draw will it have on 120V?
>
> Take the transformer's secondary voltage and figure out the ratio to the
> primary side -- the current draw on the primary side is the same ratio,
> only
> the other way around. For example (picking easy numbers out of the air)
> if I
> had a 12V transformer with 10A output then on the 120V side it'd have only
> 1A
> of draw (plus transformer losses, but I'm ignoring those for the moment).
>
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