I am trying to build 2 wire pressure switch with indicator. basic block diagram is, From AC power supply, positive (Live or +) is connected to the relay and the other end of the relay is connected to the pressure instrument. So most of the voltages will be drop on the relay when relay is switched on and small voltage drop on the instrument. The other end of the instrument is connected to neutral line of the power supply.
If the power is directly connected to pressure instrument, instrument will get fired. So i am using the relay in series. the motor is connected to this relay (C and NC). I have the circuit which measures the pressure and displays in LCD with micro controller. I am using a set point and have a NPN/PNP output. When measured value is greater than set point, the output goes on/off(i.e, relay goes ON/OFF). Even when the relay is ON/ off (i.e, the relay is open/close), pressure instrument must get power from the power supply and all other components in instrument must work. how to attain this?
How do i switch the motor ON/OFF when process value crosses set point? how to design this feedback circuit?
please help.