I think that I'm still gonna beat the same drum. Eliminate the relay that switches the PLC power line. Use a diode array of 3 diodes so that whichever voltage is highest feeds the PLC power input. Ensure that the AC power supply is higher than the float voltage present on the batteries when fully charged. No relay needed. Then try it and see how you make out. dwayne PS: the only problem with adding that large capacitor after the relay contacts is inrush current eventually damaging the relay contacts. But I still don't see the need for the relay and think that eliminating it will get rid of your problem. dwayne At 01:06 PM 4/10/2007, Carl Denk wrote: >Powered from the non-generator engine battery, all OK. If it's powered >from the engine's battery, upon starter energizing, voltage is pulled >down to 9 volts, and PLC resets, had that issue some time ago, and added >a second battery (from a homebuilt airplane we had dismantled) with the >diodes to select the higher voltage one as described earlier. No >problems when powered off either battery, except when starting. Issue >only when switching from the switching power supply to battery, when the >relay coil is deenrgized and spring pulls contacts open. No problem when >coil is energized and contacts are switching. -- Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 22 years of Engineering Innovation (1984 - 2006) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist