24-volt Relay Controlling 120 volts


I decided I wanted to make 120 volt AC things get switched on and off via a 24 volt DC signal — so I bought a nice 10amp, 240volt relay at the local Circuit Specialists electronic store and started wiring stuff up as best to code as I know how. (Every time I see this kind of project online, there’s nothing but comments about how the featured project is totally unsafe… with very little in the way of recommendations on what “acceptably safe” would look like. So please, if you comment, do so by providing reference to alternatives that you’d find acceptably safe.)
relay wiring

First, I figured out which contacts on the relay go to which terminals on the relay socket. A multimeter and 24vdc bench power supply helped a bunch with this.

The first major issue was that this wiring scheme required 120vac power line to be on the same side as the 24vdc signal line — I had hoped to keep them separated on opposite sides, but that’s not how the socket was wired.




Then I chopped up a cheap power strip and wired it into the box (using a metal box is a good idea here — they can’t crack or burn, and if you ground them properly they’ll trip the breaker rather than sit around creating a hazard.)

I also tinned the tips of the stranded wires at each terminal, because I didn’t want any fraying to eventually cause sparks. Use solder sparingly and wipe flux clean with a wet rag.

Finally, I bought two inline fuse holders for the low voltage wires — idea being that the last thing we want is for a bunch of 120vac to get out onto the 24v line and heat something up– hopefully the low-amperage fuses blow before much else would. I should’ve also put fuses on the 120vac, but didn’t think ahead and didn’t have spare room.



Here’s a closeup of the box. I added a bit of electrical tape over the AC and DC lines on the left to discourage any possibility of them touching even though they’re screwed in tight and not going anywhere. Note the screws for the grounding wires, grounding the chassis. I would’ve liked to solder them up with a bus wire or bus bar, but this seems like a good alternative and I like keeping the cables un-stripped as much as possible. Note the strain-relief plugs, the terminal block on the outside, and the screws holding everything down (had to drill thru the junction box with a black drill bit and a drill press — more solid than tape!)



24v wiring


Here’s a closeup of the terminal block. I got a bit happy with the soldering and the wire gauge is overkill, but it works. These things are lifesavers when trying to install stuff in the field or troubleshoot — way easier to deal with than crimp-on or solder-on connectors for semipermanent low voltage stuff. (For mobile stuff, a wiring harness with crimp/solder connectors is nice, it can just be impossible to repair/adjust/test in the field. Everyone has a screwdriver!)



the relay


Closeup of the relay I’m using in case you’re interested. An NTE R10-11D10-24. No reason other than that it was convenient and fit my specs.






boxed up and labeled


Tested and closed up and labeled! Documentation is the most important part of anything that’s gonna last for more than a couple months — what the hell is it and how do I fix it?

I probably should have put 10 amp fuses in the box to protect the relay, but I never anticipate using an entire 10 amps on this project. Wrote “10A MAX” everywhere just in case.






Here it is installed! That’s CAT5 wire which is spliced into a thicker 24V wire upstream. The most this thing should deal with is 50mA during coil activation, and Power Over Ethernet greatly exceeds that, so we should be good. Labeled the wire as to what is what for completeness’ sake.


And that’s it! Now to find something to plug into it…



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