# Current sensing relay

I have a 240v/30a table saw and want to automatically start my dust collector, 240v/20a by means of a current sensing switch and relay. I have a 30a double pole relay w/ 24v coil. A 24v transformer 40va, and a Dwyer series ccs current switch, amprage sensing range 0 -200a ac, switch rating 1a @ 240 vac. My problem, in my very limited understanding, is the switch voltage in the sensing unit. Can i use these items. If not what do i need?

• Why not wire the collector in parallel with the table saw, after the power button? Commented Aug 16, 2016 at 22:05
• Where are the links to the datasheets for the components? Put all the info in the question and not in the comments. Commented Aug 16, 2016 at 22:16
• @RickC Have you made a good attempt at a circuit design? Commented Aug 16, 2016 at 22:23
• Try an HVAC supply house. Such switches are commonly used to turn on a humidifier when the blower comes on. Or have you already exhausted that idea? If so, why?
– jonk
Commented Aug 16, 2016 at 22:40
• Just Google "current sensing relay" and you will find many options including items for sale on Amazon and Ebay. Commented Aug 16, 2016 at 23:17

You can probably use them, but DON"T DO IT! You clearly know nothing about wiring practices, and your project has plenty of room for you to kill yourself or burn down your house.

If you must, this is the general approach

simulate this circuit – Schematic created using CircuitLab

The motor current closes the switch output, which applies 24 VAC to the relay coil. The coil contacts switch 220 to the blower.

A few caveats. First, although you have a 24 volt relay, you do not specify that the coil is AC. If it's a DC input, you're out ouf luck (unless you rectify the transformer output). Second, you do not specify the coil current requirement. If you need 2 amps on the coil, the power needed from the transformer is 2 x 24, or 48 VA, and your transformer is too small. Third, I recommend you get a spark arrestor for your relay contacts. If you don't, sooner or later your relay will fail from the transients involved with switching the blower.

• Thanks for your quick response. I have been using a commercial unit that has accomplished the task for 11 years. It burned out, the business went under, and I can't find a unit to replace it. The circuit you drew is accurate, except the saw and the blower are on separate circuits. Can you help me with this dilemma, with or without my parts? Commented Aug 18, 2016 at 23:24
• Sorry, what dilemma? And all you need to do is put the blower switch on the appropriate load wire. The fact that this gives you trouble is another indication that you should not be messing around with this. Get an electrician. Commented Aug 19, 2016 at 0:08

There is an off-the-shelf solution for this problem for \$75 that will save you designing a circuit yourself.

... as @Barry already indirectly suggested.