Re: [CAD_CAM_EDM_DRO] Power supply and E-Stop question

washcomp wrote:

>I'm in the process of completely redoing the control wiring on my
>CNC Bridgeport to incorporporate a keyboard emulator. I figured I
>may as well re-examine everything I've done and tweak what I can.
>
>1) My power supplies have bleeding resistors across the
>capacitors. I was thinking of relocating the bleed resistor to a
>N.C. set of contacts on the "stepper enable" relay with the other
>side of contact going to ground. This would put the resistor between
>the positive terminal of the capacitor and ground (negative of
>capacitor already connected to ground) when the power supply is de-
>energized and resistor would be ignored when power supply is working.
>
>Is this a good or a bad idea? (if bad, why not)
>

The only possible downside is that the contacts might not work someday,
and leave the caps charged.

>
>2) On E-stop, I am currently cutting the A.C. power to my stepper
>drive power supply. Should I change this to cutting power at the DC
>sidee of the capaciter (I will need 3 relay contacts instead of 1)?
>
>

I have done this on my servo amps. But, it is not a trivial problem.
Almost all servo and stepper drivers have substantial capacitors in them
to absorb the current pulses they generate. When you connect the main
capacitor bank in the power supply to the capacitors in the driver, a HUGE
current will flow. It is possible you could do this with a DC solid state
relay, which might be able to handle the surge. I used a 2-relay setup
to first charge the servo amp capacitors through a resistor, then
connect the
supply directly through a large contactor, then enable the servo amps.
See the 2nd schematic at http://jelinux.pico-systems.com/EMC.html
for how this works.

Also, you don't want to disconnect the drivers from the supply while the
motors are still moving. The generated voltage from the moving motors
can, under some circumstances, cause high voltage to be produced at the
drivers. So, if you do this, you want a "braking" resistor to come into the
circuit. The above circuit accomplishes this, as well, using the same
resistor
that charged the caps on turn-on to drain energy from the drivers on
e-stop.

Jon