Re: Powerdump relay (now with free arcwelding)

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Abby Katt
<cncgroupnospam@s...> wrote:

> Hi all,
>
> I have a RELAY configuration on my PSU whereby the power being shut off
> (E-stop / limits / etc) causes the transformer to be disconnected and
> the capacitor to be dumped through a power resistor. The problem I am
> getting is that the relay has been arc-welding itself in the

ON-Position

> (and lately in the off-position).
> The old RELAY was rated 20A, and the PSU was a 55V/500W linear one,

with

> the power resistor calculated to dissipate 10A.
>
> I'm about to install a relay rated at 30A N.O and 3A NC (The best I can
> seem to find for this voltage range) and was wondering if I could use
> some of the capacitors I have spare for preventing DC brush-motor
> spikes, to somehow limit the arcing?
>
> Any advice?
> Thanks!
> Abby

Abby: Several things to "cure" the relay arcing. 1.) Use one relay
set of contacts to turn on and off the AC on the primary of the
transformer. Lower current and less arcing. Use the other set of
contacts to drive a second relay to dump the load across the resistor.

OR use an electronic load dump (SCR, FET, etc) to dump the load
triggered by the contacts. There are lots of small relays rated at
15A 250VAC. Their DC rating goes way down. Relays are not good ways
to switch high current DC (but the lowest loss and easiset to
implement) so auto mfg's use them a lot.