RE: [CAD_CAM_EDM_DRO] Power Supply Guidance Needed

Hi Larry,

Bleeder resistors just discharge the stored energy in the caps after a power off cycle, and the number I've always used is "discharge the caps in one minute (60 secs)", because usually the caps are inside a box and you can't get to them to shock yourself in less time than that.

So, if you use the formula I = C (dv/dt), a good discharge amperage for a 24,000 uf cap with a 100 volt charge would be:

I = C (dv/dt)
I = 0.024000 farads ( 100 volts / 60 seconds )
I= 0.040 amps

And a proper resistor, by Ohms Law (R = E/I)

R = 100 volts / 0.040 amps
R = 2500 ohms

And the power rating of the resistor should be:

P = I * E
P = 0.040 amps * 100 volts
P = 4 watts (absolute minimum, 10 W recommended)

But 4 watts continuous dissipation just for a bleeder resistor seems too wastefull to me. Perhaps making it harder to access the capacitors and use a larger value resistor would be a better design.

Regards,

Bill Newell



-------------- Original message --------------
From: "Larry Olson" <larry@...>

> 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?
>
> 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 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).
> >
> > --
> > Member of the toughest, meanest, deadliest, most unrelenting -- and
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