Power Supply Bleed Resistor

[pcstru]

Hi,

I am building a power supply for the xylotex board based on an 18V torrodial transformer. I've two 10000uf Capacitors on the DC side of the Bridge rectifier, which need to be discharged on power off. I was going to chuck in a 4.3K .25w resistor across the +/- lines to do this. Basic question - is this OK or some kind of lurking fire hazard?

Thanks

[kreutz]

It won´t overheat, but at the rate of approx. 6 mA it is going to take forever to discharge those capacitors. Your resistor will dissipate approx 150 mW.

[pastera]

The time constant is 86 seconds - it will take over 4-1/2 minutes to get down to 1 volt.
If you have extras, put a few across the output to shorten the bleed down time, just make sure to space them out so that they can stay cool.

Because the supply voltage is so low, you really don't have to worry about the bleeder too much.

[pcstru]

Thanks. It is slow and probably unneccessary since with the controller hooked up, it discharges more rapidly than with the bleed resistor. Think I'll put an ultrabright LED + appropriate resistor across it so I can see when it is powered up.

This is my first real forray into CNC and today I managed to get all three axis on the router driven by EMC2. The machine Lives!