Re: [CAD_CAM_EDM_DRO] resistor for power supply

cadcamcenter wrote:

> Hi all,
>
> So far all the suggestions for the power supply for drives shows the
> connection from the output of the bridge rectifier direct to the
> capacitor (some may have bleed resistor, LED etc in parallel)
>
> An power engineer (who may not have very much experience with CNC
> items) mentioned that there will a large current surge on initial
> power up (when V on cap is 0) and suggested putting a resistor. My
> interpretation of his suggestion
>
> rectifier R
> .. .-----/\/\/\-----
> / \ |
> / \ _____
> / \ _____ C
> \ / |
> \ / |
> .. \./---------------
>
> What does the list think? And if this is an improvement what kind of
> relationship R = function (C,I,etc) should there be between R, C etc.

I've never needed a surge limiting resistor, but if you did, the way to
hook one up would be to use the above diagram plus a normally open (NO)
relay. The relay's contacts would be connected across (in parallel with)
the resistor and the relay coil would be across (in parallel with) the
capacitor. If the voltage across the cap is higher than the rated coil
voltage of the relay, you'll need a resistor in series with the coil to
limit the coil current. What happens is this:

1. With the power off the cap voltage is 0V, the relay is in it's
normally open state, therefore the resistor is in series between the
bridge and the cap.

2. Power is turned on and the current into the cap is limited by the
resistor.

3. As the cap is charged, the voltage across it increases.

4. At some point the cap voltage gets high enough to cause the relay to
actuate and the contacts close across the resistor shorting it out of
the circuit.

5. When the resistor is shorted, it cools off and rests ;)

Matt