Automatic Shop Compressor Control



Introduction: Automatic Shop Compressor Control

About: Retired Electronic Design Engineer. Member of The MakerBarn.

The air compressor is a vital piece of equipment in almost any shop. One thing that is important however, is shutting down the compressor while the shop is vacant. A blown line, or a thrown belt can cause the compressor to run continuously until the problem discovered. Leaving the shop and forgetting to shut down the compressor is an easy thing to do.

This is a simple project that greatly improve the safety of the compressor. In a nutshell, the power for the compressor is control by the shop lights. Turning off the lights, shuts down the compressor. Turning on the lights enable the compressor to run. No more leaving the compressor running all weekend.

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Step 1: What Is Needed

All that is needed is a low-voltage transformer, a contactor (relay), and a box to mount the contactor in.

The photo above shows a class 2 transformer from my junk box. You need is a transformer that has the same input votage as your lighting, and an output voltage of 24 Volts AC. Get a transformer rated for at least 40VA. A good example of a new transformer is the Functional Devices Inc model TR40VA015, it can be used for several input voltages. You can find them at or Grainger for just over $10.

The second photo shows a contactor. This was another item from my junk box, but an air conditioning compressor contactor works fine. Make sure the coil is rated for 24VAC and the contacts are rated for at least 40A at 240V. Use a 2 or 3 pole contactor, these allow you to break both sides of the 240V power. A good example is the Square-D model 8910DP42V14. has them for about $22. Check eBay, there are always plenty for sale.

The last major component is an enclosure for the contactor. These are inexpensive and available at home centers.

Step 2: Planning the Circuit

The circuit is shown in the schematic. If your transformer is designed for NPT Hub mounting, you can mount it directly to your lighting fixture. Wire the primary side to the lighting power. Read the instructions that come with the transformer to make sure it is correctly wired and set for the correct input voltage.

The connection from the 24VAC side of the transformer to the contactor can be done with 18ga doorbell or thermostat wire. Nothing fancy here.

The 240VAC output side of the contactor can be wire one of two ways. If your compressor has a plug, install an appropriate outlet box just below the contactor box. My compressor uses a 30A twist-lock plug, so it was fairly simple. If your compressor is direct wire (no plug), you need to have a disconnect switch either in front or after the contactor. In front (on the main panel side) would be best. If you don't already have a disconnect, an inexpensive disconnect for air conditioning compressors is a good choice. You can get them at the home centers.

Keep in mind. You must follow all local rules and codes when working on electrical devices. If you are unfamiliar with electricity, find someone who does know.

Step 3: Building the Contactor

The photos should help. I used a small terminal strip to connect the low voltage wires. It made installation easier. My compressor is on a 240VAC 30A circuit. The 10ga feed from the main panel comes into the top of the box and connects directly to the contactor. Below the contactor enclosure, is an outlet box with the outlet wired to the bottom contacts on the contactor. You may notice my contactor is a three pole device. I only needed two poles for my circuit. There is also a pilot lamp on the box. It is wired to the output of the contactor. My circuit is 240VAC, but my neon pilot lamp was only rated for 120VAC. So to overcome this problem, I wired a 330 Kohm 1/2 watt resistor in series with the lamp. The pilot lamp is not necessary, but I thought it would be a nice touch.

Step 4: Operating the System

Make sure you label which shop light switch controls the compressor. Turning on this bank of lights energizes the contactor and allows the compressor to run. Turning off the shop lights insures the compressor will not run.

A couple of mods:

If your compressor runs off 3-phase, no problem, use a three phase contactor and with all 3 phases through it.

If you compressor runs off 120VAC, again, no problem. Wire it as shown in the schemtic. Use the contactor to break both hot and neutral. Use a 120VAC pilot lamp (they are easy to find).

One thought, a neat addition to this would be a pressure switch set a little bit higher pressure than the compressor's switch. Use its contacts in series with the low voltage to the contactor. If the pressure switch in the compressor ever stuck closed, the emergency switch would shut down the compressor. A sonalert module with a diode in series, would sound whenever the contacts of the emergency pressure switch opened.

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