Re: [CAD_CAM_EDM_DRO] Re: CNC power supply question

Bleeder Resistor

The supply voltage divided by the resistance in ohms gives you
the load (amps) that your supply will waste in heat. If your
supply is marginal to begin with, this may be too much.
For safety you probably want to discharge the capacitor down
to about 50 volts. Unless you have very large capacitorss
in the 1000s of mfd. and hi voltages (over 250 volts) and
insist on turning the switch off and immediately touching the
capacitor, bleeder resisors can be in the Kilo or Meg ohm
resitance. The way to help you decide, is after the power
supply is built, Take a hi impedance voltmeter and put
it across the capacitor. Turn the power supply off and watch
the voltage bleed off. If you use a low impedance analog (meter
with a needle) voltmeter, the internal resistance of the meter
itself will be discharging some of the charge.

This can be calculated, but I'm a little rusty. You multiply
the resistance in ohm times the capacitance in microfarads (this
is where I am rusty) and this gives the time constant in micro seconds
The voltage will decrease .707 times the original voltage in one
time constant . So if you have a 1,000 mfd capacitor and 1,000 (1k)
bleeder resisor and the capacitor is charged to 1,000 volts, Then
in one time constant (rc) the voltage will drop to 707. Or in one second
the voltage will drop to 707 volts. The next second it will drop to
.707 times 707 to about 500 volts.

IF I HAVE THIS WRONG, PLEASE CORRECT ME

bob