Isolated 24V Relay Controller

Posted on Feb 7, 2014

This is a common control voltage in industrial and telecommunications equipment, so the relays are common in surplus equipment. Many will actuate equally well on AC or DC, which makes them an easy solution when the control voltage is switched mechancially, as in the case of limit switches or magnetic reed switch sensors. Unfortunately, they`re not so simple to interface directly to microcontrollers as they

Isolated 24V Relay Controller
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require a separate supply and level shifting circuitry. Even so, one can easily control them with a separate supply and optoisolator. An optoisolator uses the illumination of a (mostly infared) LED to trigger a photosenitive resistor or transistor. This provides galvanic isolation between the control circuit and the device being switched, which also means that the power supplies don`t have to be the same voltage, or even share a common ground. Additionally, transistor-type optoisolators in DIP packages are extremely common and cheaply available if you don`t have a few in the parts bin. They also provide an easy method to control devices using a current loop. Using a current loop and a control current rather than a control voltage makes interfacing with a wide range of control voltages easy. I came across a few things that made this project possible and/or necessary. First, I had a large supply of 24V relays sitting around. Second, I had a few trays of Prem board-mount transformers with dual primaries (120/240 VAC operation) and dual secondaries that could be series-wired for 24 VAC output. Finally, I came across a bunch of 24V coil relays that could handle 5 Amps @ 125 VAC and came with sockets that could be board or panel mounted. I used one of them and a point-to-point controller in this relay-controlled receptacle project: The controller worked fine and could be driven by 3. 3/5/12 Volt control circuits without...

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