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Captflx 12-06-2009 09:19 AM

Relay, 24 volt to 110
I'm planning to wire my wood working dust collector so that I can control the on/off at each tool location. This will be done with a 24 volt transformer, 3 way switches and a 24 volt to 120 relay (contactor). I've done considerable wiring but never with a relay / contactor.

1. Because of the low voltage, it is not necessay to put the 24 volt wiring in conduit??

2. However, I believe that the relay should be in a junction box? If so, the box size requirements?

3. Will the relay / contactor have connection instructions?

4. My dust collector motor spec is 20 amps peak. I assume that is at start up. What are the particulars to look for in a transformer and relay / contactor?

J. V. 12-06-2009 10:21 AM

You need to post a drawing of your intentions. If you are asking why you need a drawing, you may just want to hire a professional to wire this up.

micromind 12-06-2009 11:33 AM

The 24 volt wiring will likely not need to be put in conduit, but depending on local codes, it might. Depending on the size of the transformer, there might not be enough energy available to start a fire. I would recommend fusing the system at the transformer.

The relay will need to be in a box. I usually use a 6X6X4 for one relay.

I doubt if the relay will have connection instructions, the terminals will be identified though. Connecting a relay like this is pretty easy.

Any device that controls a motor must be rated for the HP and voltage of the motor. The vast majority of relays are rated in volts, amps, and HP.

What voltage and HP is the dust collector? If it's small enough, you might be able to use 3 way and 4 way switches directly on the power line.


Billy_Bob 12-06-2009 11:41 AM

A relay is just an on/off switch!

The only thing is there are two additional connections for the 24 volts to power it.

Usually relays have wiring diagrams/specifications and these would be on the web site of the manufacturer. Like this...

The relay above looks like it could be attached to the side of an electrical box.

"Contactors" usually are designed to be screwed down to something like the back wall inside a large electrical box (which you would want to do to keep anyone from touching any parts which had 120 volts on them and to contain arcing like with regular electrical wiring).

And "relays" many times are designed to be used on a printed circuit board like in electronics.

But relay, contactor, solenoid - same thing - electrically operated switch.

As to conduit, this protects the wiring from being damaged. I would think that in a shop environment, low voltage wiring would need to be protected just as much as other wiring.

How a relay works...

Then there are different relays. Some are designed to be on only momentarily, others are designed to be on for hours. The latter would be called "continuous duty". Look for "Duty Cycle:" on the specification sheet. If it does not say, ask the manufacturer if the relay is designed to be on for long periods of time.

Also there are "latching relays". With these you push a button like a doorbell button for a moment and the relay turns on. Then push the button again and the relay turns off.

And then there are the relay "contacts". All about that here...

Captflx 12-06-2009 12:12 PM

Relay, 24 volt to 110
Thanks to all.
The dust collector is advertised to be 2 h.p., but I think the rating is suspect as many motors on tools from China are. They also have a peak amp rating as 20.

micromind 12-06-2009 01:40 PM

If it's 20 amps at 120 volts, it's pretty close to 2 HP.


Yoyizit 12-06-2009 03:44 PM


Originally Posted by Captflx (Post 362158)
4. My dust collector motor spec is 20 amps peak. I assume that is at start up. What are the particulars to look for in a transformer and relay / contactor?

So you should be able to find a relay that uses ~1A or less to drive the coil. At 24vac this is 24 ohms or more, coil impedance.
The relay coil in post #4 uses only 75 mA, for a current gain of 20/.075 = ~270.

A recent post showed one that worked on 10vdc max, 10 mAdc max. You can't get much safer than that. I'm sure it had electronics inside to amplify the signal and drive the relay.

Captflx 12-09-2009 08:45 PM

relay, 24 volts to 120
Thanks to all. Now I have enough info to visit an electrical house and ask the right questions.

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