Carl, On Tue, 10 Apr 2007 10:03:45 -0400, Carl Denk wrote: > Just to summarize symptom: Relay drops out, switching PLC from 120VAC > > 12 VDC switching supply to 2 lead acid batteries connected with diodes > to relay. Switches OK when 120 is from utility, PLC resets when 120 is > from generator. Switch over to 120 supply is OK with either utility or > generator source. It could be that there is a voltage-step when you change from generated to battery supply - there will be at least a dip as the contacts change over (I assume they're not a make-before-break contacts). > Did disconnect one battery at a time, no change. Did add house > electrical load, as opposed to just exercising generator with no > transfer, no change. > > ?? > Replace relay - with what? Something solid-state that will switch without a gap? Perhaps just a set of diodes, making sure that the mains/generated voltage is above that of the batteries, so the latter are only used when needed? > Add capacitor to bridge during changeover - how big, where? This can't hurt, and may solve the problem. Start with a couple of hundred uF immediately after the relay, and see if it helps. But make sure it has a high enough voltage rating - it may see some spikes that would damage it. I'd go for 50V rating at least. > While only with generator source? Different voltages? Although they are all nominally 12V, the actual voltages - especially under load - will be different. You are introducing a sudden change of voltage, which may be enough to trigger the PLC to reset. I have two "proper" UPSs (made by APC) in series, which they don't recommend, and when the upstream one switches from mains supply to battery supply, the downstream one *sometimes* switches itself to battery supply, reporting something like: "High rate of change of mains voltage" - it reverts a few seconds later. Good luck! Cheers, Howard Winter St.Albans, England -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist