1. ## Bleeder resistor

Hi all,

I want to determine the capacitor discharge time for my power supply :
Here is some spec :
- 42,43 V
- 11,78 A
- Filter capacitor : 56000uF

My question : what is the best discharging time ? It
Can I choose the time I want or should I respect some rules about this time ?

For example, I've calculated this :
C = 56000 uF R = 330 ohm
330 x 0,0560 = 18,48 s
18,48 x 5 = 92,4 s or 1mn 32s

Is this too small ?

Thanks for all your explanations !

2. Picking bleeder resistors is a tradeoff. Faster bleedoff costs more power dissipation. For example, your 330 ohm resistor will dissipate about 5.4 Watts as long as it's hooked up (P = V^2 / R). Power dissipation is heat in your cabinet and lost power supply capacity.

There are some people that have mentioned power controllers that switch the bleeder resistor in when the AC power is removed. You could make such a thing yourself with a relay. The AC to the power supply would also go to the coil on the relay. Hook one side of the power supply output to the wiper on the relay. The bleeder resistor would go between the Normally Closed relay contact and the other side of the supply output. Size the relay to deal with the current flow for the resistor you chose. You could be pretty aggressive with the resistor value and bleed down time because the resistor would not be driven continuously. One consideration is that I am not sure this would be an OSHA legal thing, because if the relay fails, the bleeder does not work.

Good Luck,
BobH

3. If the resistor fails it won't work either, solid state relays may be more reliable and unless you are planning to poke around in the case you can take your time discharging the caps. I tend to short them out manually just to be on the safe side.
Amplexus Ender

#### Posting Permissions

We are the largest and most active discussion forum from DIY CNC Machines to the Cad/Cam software to run them. The site is 100% free to join and use, so join today!