Re: [CAD_CAM_EDM_DRO] Power Supply Guidance Needed

On 3/14/06, Larry Olson <larry@...> wrote:

>
> Oh and I understand we are talking about 2 different circuits here. Any
> feedback on the bleed resistors as far as their value and size?

Depends on how fast and why you want to drain your capacitors. I need it
only for general safety, and also to prevent arcs, loud bangs and molten
copper when I disconnect/reconnect power supply, which does not happen very
often at all. In other words, discharge time is not critical to me as long
as things do get drained eventually. So, for my purposes, a 33K
0.5Wresistor does the job.

Thanks Again,

> Larry Olson
>
> -----Original Message-----
> From: R Rogers [mailto:rogersmach@...]
> Sent: Tuesday, March 14, 2006 3:45 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: RE: [CAD_CAM_EDM_DRO] Power Supply Guidance Needed
>
> Larry,
>
> Yes, 20w 7ohm's for a dump resistor. Not a resistor that is connected
> and energized continually like a bleed resistor would be. Would be a
> great place to warm up lunch :-)
>
> Ron
>
> Larry Olson <larry@...> wrote:
> Ron,
> Would those resistor values be good for the 100-120VDC 30A PS
> that I'm planning? Thanks!
>
> Larry Olson
>
>
> -----Original Message-----
> From: R Rogers [mailto:rogersmach@...]
> Sent: Tuesday, March 14, 2006 3:25 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: RE: [CAD_CAM_EDM_DRO] Power Supply Guidance Needed
>
> Larry,
>
> The "bleed" resistors you refer to are, If I understand correctly, to
> drain the caps at shut down. Some even advise against doing this as it
> lessens the life of the caps (who knows). At any rate they are usually
> very small.
>
> You might consider using the E-stop/dump circuit that I use consisting
> a DPDT relay. One relay position fires up the power supply and it's
> "clamped" through the Estop button. The other position of the relay (in
> Estop) Dumps the Caps through a 20W 5-7 ohm resistor set at 7 ohms. This
> works really well and takes the Estop out of the logic loop. Drains the
> caps in a few seconds for safety. Without it when the Estop is hit the
> servos will continue moving until the caps are drained. Caps are
> essentially batteries.
>
> Ron
>
> Larry Olson wrote:
> I was going to reinstall the original bleed resistors and figured if I
> was tripling the VA of the AC side and doubling the Caps than I should
> double the Cap bleed. I have no clue what value would be best suited for
> the application. I was just going by what Bridgeport had done with the
> original PS.
> Any help, guidance, or opinions are always much appreciated as I'm in
> the learning process on CNC construction.
>
> Larry Olson
>
> -----Original Message-----
> From: Vlad Krupin [mailto:vlad.cnc@...]
> Sent: Tuesday, March 14, 2006 3:00 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] Power Supply Guidance Needed
>
> On 3/13/06, Roy J. Tellason 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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