On Monday 02 May 2016 19:17:41 andy pugh wrote: > The PSU on my lathe is something I made myself. On the input side are > two SSRs, one for power on/off and the other switches-out a soft-start > resistor. > There is also s discharge capacitor that is switched in when the main > input SSR is switched off. > > Today things went a bit strange, blowing the breaker and then the > discharge resistor. This was with the PSU powered up, but not turned > on. > > It turns out that I have an unanticipated failure mode, if both input > SSRs fail closed-circuit. > > I have tested the SSRs on the bench, running a light bulb, and they > both light the bulb with nothing connected to the control terminals > (And with the control terminals shorted together). > > So, I am wondering if there is a problem running a rectifier input > with SSRs. I found something in an Omron document suggesting that > turn-off might be unreliable, but that doesn't seem to be the issue > here, the relays are now incapable of turning off a (filament) light > bulb. > > Did I get unlucky, or are SSRs a bad choice for this application?
You may have been tapped by the EMP of a nearby lightning strike, Andy. Considerations about that is why my lashup has a hard metallic switch for the final off in the form of a small rocker/thermal breaker in the end of the outlet strip that powers my rig. When thats on, I am doing similar to you in that I have 2 40 AMP SSR's, each pumped on by a charge pump signal, the first one in series with the switched primary power but with a 50 ohm 200 watt R in series, controlled by the 2nd machine on button. 5 seconds later, after the 50 ohm 200 watt resistor has had a chance to charge up the excessive microfarads, another SSR bypasses the resistor, so that charge pump gate also turns the SPPWR button green to tell me its safe to fire up the spindle motor. At powerdown, I unclick that second button to put the resistor back into the circuit so an odd half cycle power that might occur as the charge pump bucket fades, is at least controlled so it doesn't trip the wall breaker going off. My bleeder is quite a bit higher R than yours, but its only bleeding 3 or 4 watts full time, so discharge time is several minutes. It can and has gotten quite noisy here several times in the last 2-3 days, and I am thinking the hard switch has probably saved me from some of those bumps. Yet that hard switch is no more complex than a small rocker button on the 6 socket power strip that runs it all. I think its also a slow circuit breaker but if it is, its never tripped on my watch, the 30 amp in the service box has beat it every time. But with this setup now, I've reduced the service breaker to a 20 and have had no further tripages. While these SSR's are cheap enough I bought spares, I'd have 2nd thoughts about having them exposed to line power on a 24/7 basis. Could be just my paranoia, but... And IIRC your line voltage is 2x mine. Mine is around 127 RMS over on this side of the salt water pond. I also suspect yours have been in service a lot longer than mine. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Find and fix application performance issues faster with Applications Manager Applications Manager provides deep performance insights into multiple tiers of your business applications. It resolves application problems quickly and reduces your MTTR. Get your free trial! https://ad.doubleclick.net/ddm/clk/302982198;130105516;z _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
