Relay Selection

This wiki page describes the selection process for the relays used to control the Orbit water valves, and Wayne water pump.

Relays were chosen based on the requirements of the loads that needed to be switched. Both the Orbit water valves and the Wayne water pump are inductive loads which require AC, so it is necessary to have relays that can either tolerate or manage the inductive kickback produced when these loads are switched off. There is a semiconductor device sometimes called an alternistor which is a particular type of TRIAC designed to handle this type of load. A solid state AC relay is a nice IC package that accepts a DC control signal which will switch an AC supply, and makes use of one or more TRIACs to do so. Solid state relays can also include a zero-cross circuit which will turn on the AC signal just before or after the AC signal reaches 0V. This limits surges and spikes when the device is turned on. The current and voltage requirements of the Orbit water values are fairly low (0.19A @ 24 VAC), so the Panasonic AQH2213 zero-cross solid state relay which can handle 0.9 A at 600V was chosen for use with the valves. It is still protected with 42.3V varistors.

A solid state relay was also desired for use with the water pump, but its voltage and current requirements are significantly higher (120 VAC, 9.0 A). Solid state relays exist which would meet these requirements, but they are prohibitively expensive for the purpose of this project. Additionally, solid state relays have higher resistance than comparable ordinary mechanical relays, and would generate sufficient heat to impose thermal constraints on the design. That is, a heatsink and fan would be required given the footprint of the board. So, a beefy mechanical relay was chosen instead; namely the Panasonic ALF1T05. Since it would be subject to inductive kickback, the 120 VAC and 9.0 A rating requirements were doubled, and voltage-clamping varistors were installed on the switched hot and neutral lines to protect the contacts inside the mechanical relay and other components on the board.