If there is one thing that can add unnecessary wear to your equipment, forget to turn it off immediately after use. This is especially true with a compressor, since it is controlled not only by the main switch, which turns it on, but also by a pressure switch – which turns it off when the air pressure is built up and on again after the pressure has dropped.
So when you pack things in after a day in the home yard, if the compressor has a good pressure built up in the tank, which causes the pressure switch to keep it from going at that moment, it's easy to forget to turn the power switch off. After a while, as the pressure drops from leaks along the overhead line, it will start again without anyone realizing it, and will continue to run off and on the bike until someone turns off the power. However, there is a simple fix for this, which is the addition of an automatic shut-off device set with a timer along the power line going to the compressor (Fig. 1
If it is a larger stationary compressor connected to 230 volts, it would probably be switched on through a junction box. This is the place where the controller should be installed. Before it becomes technical, here are some terms and abbreviations that will be used in the following text:
Closed or Closed means contacting to complete a circuit.
NC means "normally closed", power can flow until the switch is opened.
Opening or opening breaks the continuity of the circuit.
NO means normally opened; the circuit opens at that point until the contact is established.
SPDT (single-pole double cast) means a set of contactors with a common, an NC contact and an 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 valued at 220 VAC to eliminate the installation of a transformer.
This type of relay, at a reasonable price, usually does not value more than 10 amps, which requires an alternate power relay can handle up to 15 amps or more to run a stationary compressor. This power relay should have at least three contactors NOT rated with at least 15 amps (or at the highest requirements of the compressor). A simple "Push-Button Momentary" switch will also be needed to activate both the time-delayed relay and the power relay.
Note: To simplify the descriptions and explanations, there will be references to "Pin Numbers" and "Terminal Numbers" that are relevant to Figures 2 and 3. However, these numbers may not correspond or coincide with anyone else's relays.
A "delayed activation" timer is a type in which the coil countdown is activated but which runs the preset time before the contacts are closed or opened. This is shown in Figures 2 and 3 as option A (the blue wire), where pin # 5 (NC) of the timer (for this example) is connected to terminal 13 of the power relay. It is easier to see the close-up image in Figure 3.
A "instant activation" timer is another type where the coil and contactors are all activated by pressing the button, closing or opening the contactors directly. It is shown in Figures 2 and 3 as option B (the small wire) where pin # 13 in the power relay is connected to pin # 6 (NO) of the time-delayed relay (for this example) instead of pin # 5.
The only the difference in the cables is the Blue and the Mauve thread in the diagrams. In both options, by closing the pushbutton, the voltage is sent to both coils and it remains "ON" by flowing through the contact delay for the time delay relay and the closed contactors 13 and 14 of the Power relay. It will remain as such until the time runs out on the timer. Reactivation occurs when you press the button again. 220 volts come to the compressor motor through the power relay terminal L1 for one side and L2 for the other side of 220 volts.
1. The timer must be a time-delayed relay with an adjustable knob to adjust from half an hour to at least four hours or more, with a 220 VAC coil but with a set of contactors SPDT. Varied prices (Amazon).
2nd Electric or metal enclosure box with cover, large enough to accommodate the timer and its sockets (Fig. 1a) and the power relay. About. $ 20 (Home Depot, but also check online, price margin is ridiculously large).
3rd Momentary Push Button Switch SPST (single pole, single throw)
Wiring the Device
Safety Warning: before from the beginning of the project the switch that drives the compressor must be switched off.
The new enclosure should be prepared to receive two cables by drilling holes in the side and ensuring that the wires do not conflict with the location of the relays. A couple of holes can also be drilled through the back to screw the box against the wall.
The junction box at the compressor can then be opened to display all wiring. In the case relating to the drawings, there is also a knife breaker involved in the circuit. If there is none, simply ignore it from the drawing.
The wire coming from the main elements and going to the junction box should be disconnected from the knife switch or any other thread in the junction box. Both the black and red wires can then be connected to a cable with two wires added to the circuit to be fed into the new unit's enclosure box.
The black wire can be used as the main power supply to activate both relays (although it will also work with the red wire, it is a good practice to standardize the methods of wiring in all electrical projects, as it marginally reduces the risk of make mistakes). Inside the housing, both wires from the 220 V supply are connected to the Power relay, the black line to terminal T1 and the red to terminal T2. One jumper is used to tie T1 to pin # 8 in the timer, and another from that point to one side of the push button. 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 connect the control section which activates both relays, a jumper from the free terminal is placed on the pushbutton to connect to the 2 and side of the Power relay coil on terminal A2, a sixth jumper going from there to terminal 14, with a seventh jumper connecting 14 to pin # 2 on the other side of the timer coil.
The automatic shut off unit is now complete, the only thing left is to connect the compressor motor to terminals L1 and L2 and press the push button after adjusting the cycle duration of the timer. When this is done, the original compressor switch is left in the "On" position.