Want to feed a 24vac relay coil from 2 separate ac condensor 24 v thermostat wires, Contact side of the relay will control 12 V DC , 3 AMP load to a small water pump. I want to cover the case where one ac unit is on and powering the relay , then the 2nd unit comes on while first is on. I think I need to keep the voltage from one feeding back to the other if they are both on at the same time. Can I do this with diodes ? Would it be safer to current sense the the power going through the hot wires, this way the 24v lines never see each other. If I sense the current and convert this to a small voltage that I can measure using a microcontroller it could then make the decision to close the 12 v contact to turn on the pump.