The following description applies to the schematic shown below. This circuit was designed to extend the time that an air handler blower would run after the compressor has stopped, in an air conditioner system. This type of a circuit could also be used to control the blowers on a broadcast transmitter as well, allowing the transmitter to be stopped but fully cool down the transmitting tubes. The primary design goal was to have a circuit that could be added on top of an existing system and also would not require any external power source for operation.
This circuit was designed to accept 24-28 VAC as a control input and switch a 120 VAC blower motor. The motor that I needed to control draws 6 amps when running so a 10 A solid state relay module was chosen for the back half of the circuit. The front half of the circuit takes the 26 VAC and first rectifies it via FWB1. R1 (approx. 750 ohms) is a series limiting resistor that is adjusted to keep the current thru D1, D2 and the optical input to SSR1 at an acceptable current. Resistor R2 in combination with C1 provide the time delay for the circuit. R1 is adjusted for the desired delay. For a 2.5 minute delay the resistor value is 6.8K. The zener diodes D1 & D2 (15 V and a 9.1 V zener diodes in series) are used to limit the input voltage to the solid state relay module (SSR) to 24 volts.
Return to Home page