Dust collector relay box

I built this dust collector for my table saw. On my homemade machines I just add an extra socket to plug the dust collector into, but the table saw runs on 240 volts and the dust collector runs on 120 volts. I had previously hooked a mini dust collector between one of the phases and ground to get 120 volts, but that is risky because, if the ground connection breaks, this sends 120 volts into the chassis of my table saw.

So I built a relay box to turn on the 120 volts when it gets 240 volts from the table saw.

I used a solid state relay (Imitation Fotek, $3 from china). The relay needs 3-24 volts DC to activate and can switch up to 25 amperes AC. A 9 volt battery works fine to activate it, as does an old 5-volt cell phone charger AC adapter. The cell phone charger adapter can also run off 240 volts so it's perfect for the job. I tested it on the workbench using a motor that came from this machine.

The relay has a 1.5 volt voltage drop when under load. If the motor draws four amperes, that works out to 6 watts. So it needs a heat sink. I cut a piece of aluminium from some scrap for that.

The relay screws to the aluminium. I'm adding a dab of axle grease before screwing it on to get better heat transfer. Heat sink grease would be much better, but I didn't have any handy.

I'm using a metal 3-gang electrical box (the type used for house wiring) for the project box, and the aluminium plate covers two thirds of it.

I added one of these electrical connectors to supply the 240 volts for the cell phone adapter (the cell phone adapter can take 120 or 240 volts)

I soldered some wires to the prongs of the cell phone charger, then cut them shorter.

I mounted it in the electrical box by tying it in using some pieces of solid copper wire, and connected it to the power connector that goes in the side of the box.

Then wiring it up. Conveniently, the electrical box has a strain relief already built in, which I'm using for the main cable. This cable powers one of the sockets directly (always on), while the "hot" side for the other socket goes through the solid state relay. So one socket activates when the saw is switched on, the other acts more like an extension cord.

I don't have any convenient power outlets near the table saw, so I put a long power cord on the relay box. The extension cord function will come in handy for plugging in other tools.

Connecting it up under my table saw.

I then put it on top of the dust collector to keep it out of harm's way.

The combination of cell phone charger and solid state relay was a good way to handle this. Mechanical relays that run on 240 volts are harder to find and more expensive to buy.
An added bonus is that when I turn off the table saw, the filter capacitors in the cell phone adapter still provide power for a few more seconds, so the dust collector keeps running for a few more seconds to help clear out dust.

Update 2021
Still using this switch, though it now turns this dust collector on and off. The imitation fotek relay failed after about two years. I had another one of the same kind and sapped that one in instead.
There are also relays of this sort that can use 120 volts for their signal input, which would have eliminated the need for the DC adapter in this box.