At 07:06 PM 4/9/2007, Carl Denk wrote: >The DPDT relay (Square D 8501KP12P14V20) has a 120 volt coil going to >the house power with the PLC input going to the common contacts, the 120 >power supply to the N.O. contacts so if there is 120 volt power >available, the power comes from the 120 volt supply. Each battery goes >to one of the N.C. contacts so if the 120 isn't there, the batteries are >the supply. I take it that the PLC loses power during the time while the relay is switching between its contacts? If so, I expect that to be the problem. Can I make a possibly simple suggestion? Ensure that your DC supply while operating from AC power is higher than the float voltage on the batteries. Then use diodes to OR all 3 sources of power: one diode anode from each of the batteries and from the AC power supply. The cathodes of all the diodes feed the PLC power input. By picking the output voltage of the AC-powered DC supply to be higher than what is present on the batteries, you ensure that the batteries are not loaded. When the AC power disappears, the diodes smoothly transition to whichever battery has the highest voltage. Easy to test to see if this fixes your problem. I suspect that it will. dwayne -- 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