RE: [CAD_CAM_EDM_DRO] Bleeder resistor for power supply size?

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@...]
> 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
>
>
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