If there's one thing that can add unnecessary wear and tear on your equipment, it’s forgetting to turn it off right after use. This is especially true with a compressor, since it's controlled not only from the main power switch, which turns it on, but also from a pressure switch—which turns it off when the air pressure is built up, and back on after the pressure has dropped.
So when wrapping things up after a day in the home workshop, if the compressor has a good pressure built up in the tank, causing the pressure switch to keep it from running at that moment, it’s easy enough to forget turning the power switch off. After a while, when the pressure drops from leaks along the air line, it will start again without anyone realizing it, and will keep running that off and on cycle until someone shuts the power off. There is, however, an easy fix for this, which is the addition of an automatic shutdown device set up with a timer along the power line going to the compressor (Fig. 1).
If it’s for a larger stationary compressor hooked up to 230 volts, it would probably be wired-in through a junction box. This would be the location where the control device should be installed. Before getting technical, here are a few terms and abbreviations that will be used in the following text:
Closed or closing means making contact completing a circuit.
NC means "normally closed", the current can flow through until the switch is opened.
Opened or opening is breaking continuity in the circuit.
NO means normally opened; the circuit is opened at that point until contact is established.
SPDT (single pole double throw) means a set of contactors with one common, one NC contact, and one NO contact.
Auto Shut Off Device Operation
Building an automatic shut off device will require a Time-Delayed relay (more commonly called a timer) (Fig. 1a), preferably rated at 220 VAC to eliminate the installation of a transformer.
This type of relay, at a reasonable price, doesn’t usually rate at more than 10 amps, thus requiring an alternate Power relay capable of handling up to 15 amps or more to run a stationary compressor. This power relay should have at least three contactors NO rated at a minimum 15 amps (or at the compressor’s maximum requirements). A simple “Push-Button Momentary” switch will also be needed to activate both the time delayed relay and the power relay.
Note: In order to simplify the descriptions and explanations, there will be references to “pin numbers” and “terminal numbers” that are relevant to figures 2 and 3. Those numbers, however, may not correspond or coincide with someone else’s relays.
A timer with “delayed activation” is one type where the coil’s countdown is activated but which runs the preset length of time before closing or opening the contactors. This is shown in figures 2 & 3 as option A (the blue wire), where pin #5 (NC) on the timer (for this example) is connected to terminal 13 of the power relay. It's easier to see in the close-up of figure 3.
A timer with “instant activation” is another type where the coil and the contactors are all activated at the push of the button, closing or opening the contactors instantaneously. It is shown in Figures 2 & 3 as option B (the mauve wire) where pin #13 of the Power relay is connected to pin #6 (NO) on the timed-delay relay (for this example) instead of pin #5.
The only difference in the wiring is the blue and the mauve wire in the diagrams. In both options, by closing the push-button switch, the voltage is sent to both coils and it remains “ON” by flowing through the contactors of the time-delay relay and the closed contactors 13 and 14 on the Power relay. It will remain as such until the time runs out on the timer. Re-activation will occur when the push-button is pressed again. The 220 volts get to the compressor motor through the power relay’s terminal L1 for one side and L2 for the other side of the 220 volts.
1. The timer must be a time delayed relay with an adjustable dial to adjust from half an hour to at least four hours or more, with a 220 VAC coil but with one set of contactors SPDT. Prices vary.
2. Plastic or metal electrical enclosure box with cover, large enough to accommodate the timer and its socket (Fig 1a) and the power relay. These pieces are about $20 (available at home improvement stores, but also check online, since prices can vary widely).
3. Momentary Push Button Switch SPST (single pole, single throw)
Wiring the Device
Safety Warning: before starting the project, THE BREAKER POWERING THE COMPRESSOR MUST BE SWITCHED OFF.
The new enclosure should be prepared to receive two cables by drilling holes in the side and making sure the wires will not conflict with the location of the relays. A couple of holes can also be drilled through the back to screw the box to the wall.
The junction box by the compressor can then be opened up to show all the wiring. In the case pertaining to the drawings, there is also a knife-switch involved in the circuit. If there isn’t one, simply disregard it from the drawing.
The wire coming from the main electric panel and going to the junction box should be disconnected from the knife-switch or any other wire in the junction box. Both the black and red wires can then be hooked up to a two-wires cable added to the circuit to feed into the new device’s enclosure box.
The black wire can be used as the main feed for activating both relays (even though it will also work with the red wire, it is a good practice to standardize the methods of wiring throughout all electrical projects, since it marginally decreases the chances of making mistakes). Inside the enclosure, both wires from the 220 V feed are connected on the power relay, the black wire to terminal T1 and the red to terminal T2. A jumper is used to tie T1 to pin #8 of the timer, and another from that point to one side of the push-button switch. A third jumper goes from T2 to A1 on the Power relay and a fourth is added from A1 to pin #7 on the timer.
To wire up the control section that will activate both relays, a jumper is added from the free terminal of the push-button switch to connect to the 2nd side of the power relay coil on terminal A2, a sixth jumper to go from there to terminal 14, with a seventh jumper linking 14 to pin #2 on the other side of the coil of the timer.
The Automatic Shut-Off device now being completed, the only thing left is to connect the compressor motor to terminals L1 and L2 and press the push-button switch, after adjusting the cycle duration on the timer. After this is done, the original power switch for the compressor is left in the “on” position.