Re: Bleeder resistor for power supply size?

I know there are more than a few who feel the need to bleed the caps
on the power supply, but from the time I got my first unit running
until I changed it for larger steppers, I never opened the case so
never had to worry about being zapped. I don't put nylon protective
caps on my 110 recpticles and none of the AC leaks out.

Also, the Geckos will bleed the cap down after some time, but the
real question is why ?

I would not want to give up that much power to just making heat. If
the need were more of a concern, I'd add a relay on the 110 line to
hold the contact open until the 110 was disconnected, then close the
circuit on some device like a light bulb.

Is there some pressing reason to drain the power ?

Dave



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "John Guenther"
<j.guenther@v...> wrote:

> thanks Jon, your response is greatly appreciated. I will order up

a 500 ohm

> 25 Watt resistor AsAP.
>
> John Guenther
> 'Ye Olde Pen Maker'
> Sterling, Virginia
>
> > -----Original Message-----
> > From: Jon Elson [mailto:elson@p...]
> > Sent: Sunday, July 06, 2003 15:41
> > To: CAD_CAM_EDM_DRO@yahoogroups.com
> > Subject: Re: [CAD_CAM_EDM_DRO] Bleeder resistor for power supply

size?

> >
> >
> >
> >
> > John Guenther wrote:
> >
> > >Good morning everyone,
> > >
> > >I know this has been asked before, but I can't find the answer
> > so I will ask
> > >again.
> > >
> > >What is the proper size bleeder resistor for a power supply with
> > 48 volts AC
> > >at 10 AMPS into the bridge rectifier, using a 15,000 uF

capacitor? Your

> > >assistance will be greatly appreciated.
> > >
> > >
> > >
> > What is the purpose of the bleeder resistor? To discharge the cap
> > to a safe value in some time? What is the time?
> >
> > Let's try it for 10 seconds. Assuming a straight-line discharge,

which it

> > won't be (it will be an exponential decay) you get T=C/I, and

I=E/R,

> > so T= E/(R*C). So, R = E/CT
> > C=.015 F, E = 67, T=10, R=67/(.015 * 10) = 447 Ohms
> >
> > Assuming a 500 Ohm resistor, what power do we need?
> > P = E^2 / R = 67^2/500 = 9W. Use a 10, or better a 25 W resistor

to keep

> > it from getting real hot.
> >
> > For purists, the exponential decay of an RC circuit is
> > V = V0 * e ^ -(t/RC), and the capacitor will be discharged to
> > 33% of the original voltage in one "time constant" or RC.
> > The time constant in this case is .015 * 500 or 7.5 seconds.
> >
> > Jon
> >
> >
> > Addresses:
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