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A pressure switch on an air compressor automatically turns the compressor motor on and off according to the air pressure in the tank.
Example:
| Function | Typical value |
|---|---|
| Cut-in pressure | 90 psi |
| Cut-out pressure | 120 psi |
| Differential | 30 psi |
So, in that example, the compressor starts at 90 psi and stops at 120 psi.
A compressor pressure switch is basically a pressure-operated electrical switch. It senses the tank pressure mechanically and uses that pressure to open or close the motor power circuit.
Inside a typical pressure switch are several main parts:
The operation is based on force balance:
\[ F = P \times A \]
where:
As tank pressure increases, the force on the diaphragm increases. When that force becomes large enough to overcome the spring force, the switch trips.
When the air tank pressure is below the cut-in setting, the spring inside the pressure switch pushes the diaphragm mechanism into the “run” position.
The electrical contacts are closed, so power flows to the motor.
Result:
As the tank fills, pressure pushes harder on the diaphragm.
When the tank reaches the cut-out pressure, the diaphragm movement trips an internal snap-action mechanism. The switch contacts open quickly.
Result:
The snap-action is important because it prevents the contacts from opening slowly, which would cause arcing, overheating, and rapid contact wear.
On many compressors, when the pressure switch turns the motor off, it also opens a small unloader valve.
This valve releases the compressed air trapped between the compressor pump outlet and the tank check valve. This is why you often hear a short:
“psssht”
after the compressor shuts off.
This is normal.
The unloader valve does not empty the tank. It only relieves pressure from the pump head or discharge tube.
Its purpose is to make the next restart easier. Without unloading, the motor may have to restart against high pressure, causing:
When you use air tools, inflate tires, blow dust, or operate pneumatic equipment, air leaves the tank and the tank pressure drops.
When the pressure falls to the cut-in setting, the spring force again overcomes the diaphragm pressure force. The switch snaps back to the “run” position.
Result:
The cycle repeats automatically.
The two most important pressure switch settings are:
| Term | Meaning |
|---|---|
| Cut-in pressure | Pressure where the compressor starts |
| Cut-out pressure | Pressure where the compressor stops |
| Differential | Difference between cut-out and cut-in |
For example:
\[ 120 \text{ psi cut-out} - 90 \text{ psi cut-in} = 30 \text{ psi differential} \]
The differential prevents rapid cycling. If the compressor turned on and off at nearly the same pressure, the motor would start too frequently, causing overheating and premature failure.
Many small shop compressors have a differential of about 20 to 40 psi, but the exact values depend on the compressor design.
Electrically, the pressure switch is usually wired in series with the motor supply.
For a small single-phase compressor:
Many 120 V compressors use a switch that opens one line conductor. Many 240 V compressors use a double-pole switch that opens both supply legs.
The pressure switch must be rated for:
A common mistake is replacing a pressure switch with one that has the correct pressure rating but inadequate electrical rating. The switch contacts must be capable of handling motor starting current, not just running current.
Many pressure switches have one or two adjustment screws under the cover.
Common arrangement:
| Adjustment | Effect |
|---|---|
| Large main spring screw | Raises or lowers both cut-in and cut-out |
| Smaller differential screw | Changes the gap between cut-in and cut-out |
Usually:
However, designs vary. Some switches are factory-set and not intended to be adjusted.
Important safety note: do not raise the cut-out pressure above the compressor tank rating or safety valve rating. The tank, relief valve, regulator, hoses, fittings, and tools must all be rated for the pressure being used.
A useful analogy is a thermostat.
A thermostat turns heat on when temperature is too low and turns it off when temperature is high enough. A compressor pressure switch does the same thing, but with pressure instead of temperature.
The compressor pressure switch is not primarily a pressure regulator. It controls the motor. A regulator controls the output pressure delivered to tools. The tank may be at 120 psi while the regulator output is set to 80 psi.
Also, the pressure switch is not the final safety device. The compressor should also have a pressure relief valve, sometimes called a safety valve. If the pressure switch fails and the compressor keeps running, the relief valve should open before the tank pressure becomes dangerous.
| Symptom | Possible cause |
|---|---|
| Compressor will not start | Tank pressure still above cut-in, bad switch, no power, motor fault |
| Compressor will not stop | Bad pressure switch, blocked pressure port, welded contacts |
| Compressor starts and stops rapidly | Incorrect differential, air leak, undersized tank, faulty switch |
| Air leaks briefly after shutdown | Normal unloader valve operation |
| Air leaks continuously from unloader | Often a bad tank check valve, not necessarily a bad pressure switch |
| Motor hums but will not start | Failed unloader, bad start capacitor, low voltage, motor problem |
| Breaker trips on restart | Head pressure not unloading, weak motor capacitor, high current draw |
A brief hiss after shutdown is normally correct behavior. A continuous leak from the unloader valve usually points to the tank check valve leaking backward into the discharge line.
If you are checking or replacing a pressure switch:
Disconnect electrical power first.
The inside of the switch contains live terminals when powered.
Drain tank pressure before removal.
Do not unscrew the switch while the tank is pressurized.
Match the pressure range.
Use a switch with the correct cut-in and cut-out settings for the compressor.
Match the electrical rating.
Check voltage, current, horsepower, and pole configuration.
Check the unloader connection.
If the old switch has an unloader valve, the replacement usually needs one too.
Verify the safety relief valve.
It should be rated appropriately and should open below the tank’s maximum allowable working pressure.
Test operation after replacement or adjustment.
Watch the gauge and confirm:
Do not bypass a compressor pressure switch except for controlled diagnostic testing by a qualified person. A bypassed or failed switch can allow the compressor to over-pressurize the tank, which is dangerous.
Also, do not rely only on the tank gauge if adjusting pressure. Gauges can be inaccurate. If accuracy matters, verify with a known-good pressure gauge.
An air compressor pressure switch senses tank pressure using a diaphragm or piston. At low pressure, it closes electrical contacts and starts the motor. At high pressure, it opens the contacts and stops the motor. Many switches also activate an unloader valve to release trapped pump head pressure after shutdown, making the next restart easier. The key settings are cut-in, cut-out, and differential.