Re: How to calculate power supply bleeder resistor

Hi,

First off, 40VDC or 80VDC is not lethal. It gets a little more
interesting at 150VDC or higher. When I was an electronic tech during
summers working for an electronics company between school years at
OSU, we would amuse ourselves with a Tektronix curve tracer.

This piece of equipment was meant to measure power transistor
parameters, consequently voltage and current were exquisitely
adjustable. The purpose we put the machine to at lunch time was, I'm
sure, different than Tektronix had intended.

The "victim" would hold the test leads while someone else would
adjust the peak voltage and current. Ones' progress could be seen on
the CRT reading. I generally cried uncle at about 180VDC and 50mA.
Then you would trade places and administer the same or better to the
other guy.

Regards to discharging a cap. What drives you have attached will
discharge it faster than any resistor. If you must discharge it
quickly, connect a 115VAC coil relay across the primary and a low
ohms resistor in series with the N.O. contacts, the whole mess across
the DC buss.

Mariss

--- In CAD_CAM_EDM_DRO@egroups.com, "Wally K" <cnc4me@u...> wrote:
> How to calculate a bleeder resistor on a power supply. Assuming
> 80VDC and 10,000 UF. This resistor is a safety device to slowly
> discharge the capacitors in a power sypply to a non lethal leval
when
> the power supply is turned off
>
> The explanation and formulas for time constants are at this URL.
>
> http://www.sweethaven.com/acee/forms/frm1002.htm
>
>
>
> Also need to know I = E/R and P = I X E. Where
>
> E = Volts
> I = Current in amps
> R = Resistance in ohms
> P = Power in watts
>
>
>
> 1) Pick a time you want the dishcharge to happen in. 5 minutes
> sounds reasonable to me.
>
> 2) Pick a voltage you want to discharge to in 5 minuits. lets say
> about 5 volts.
>
> 3) Pick a RC time constant that will give you aproximitly that
> voltage. A time constant of 3 will give you 4 volts. Formula .05
X
> Vmax = discharge voltage. Because .05 X 80VDC = 4VDC (formula from
> URL above).
>
> 4) Calculate the resistance. were R = T/C.
>
> T = Time in seconds
> R = Resistance in ohms
> C = Capasitance in FARADS
>
> For time we will take the desired 5 minute time and convert it to
> seconds. Or 5 minuits = 300 seconds. Next we know we will wait 3
> time constants for the discharge we want, so divide 300 seconds by
3
> for 100 seconds.
>
> R = T (100 seconds) / C (10,000UF = .01 Farads). So R = 10,000
ohms.
>
> 5) Next calculate the power in watts the resistor will disapate. P
=
> I X E so we need to find the current to use the power formula. I =
> E/R
> I = E (80VDC) / R (10,000 ohms). I = .008 amps
>
> P = I (.008 amps) X E (80 Volts). P = .8 watts
>
> When picking a resistor wattage rating as a general rule never run
a
> resistor more than 50% of its rating. So .8 watts X 2 = 1.6 watt
> rating. 1.6 watt resistors are not made so go to 2 watt.
>
> 6) You are done, your bleeder resistor is 10K ohm (10,000 = 10K)
and
> 2 watt.
>
>
> Wally K.