Re: [CAD_CAM_EDM_DRO] Re: E-stop relay welds contacts for load dump resistor

Dan,

The inrush blink may not be a problem right now, but if you happen to
have another load running on the circuit, then the combined current
drain of the load plus the inrush may trip your breaker with
unpredictable results.

Also, a big inrush current is hard on the diodes in your rectifier
bridge. Each current surge will eat a bit off their lifetime, leading
to premature failure.

If the condenser bridges the relay contacts, you will pass significant
AC current, and there will be some voltage applied to your transformer
even when the relay is open. A capacitor looks like a resistor to AC.
Worse than that, when the relay does close, the entire stored energy of
the cap will discharge across your contactor, so you will be back in
spot weld city again. Old point systems in auto ignitions had to be
replaced periodically, as the contacts burned away, but they were DC
systems, so there was no AC leakage through the cap.

Dave

On 1/30/2011 12:26 PM, kzf817 wrote:
> Very encouraging that all of your strategies are things that I had considered as possibilities, so at least I know that I am not totally lost here. I actually already purchased a second 7 ohm resistor, which I eventually blew, but also ordered a 10 ohm along with it just in case. I was also thinking of pairing a big contactor to my ES Relay to carry the Cap dump load. Just have to source one that is suitable. For right now, I am temporarily using a standard 100 watt light bulb, placed in a socket on top of my enclosure in plain view, as my dump resistor. It reads about 12 ohms, and also gives me a visual indication of what is happening during bleed time, and an instant warning if the contacts are welded. If contacts are welded, the light bulb stays on, and I know to cut the power right away. This prevents me from eating $13 resistors (with shipping) while I isolate the problem.
> If the shop light "blink" from inrush current is not indicating a larger problem, then I won't worry about it. It does not cause any other electrical problems, and I don't blow any fuses or breakers.
> What do you think about the idea of using a condenser (capacitor) immediately in front of the relay contacts, to soften the "hit" that they see from the current inrush, and reduce the initial arc jumpover on contact close? Kind of like what auto manufacturers used to always do with points style ignitions to keep from buring the contacts on breaker points. Do you think that this might be an effective means to preserve the contacts? Thanks!
>
> Dan.
>