The pressure switch on an air compressor might be a small part – but it does an important job.
Think of it as the system’s automatic control point.
It watches the pressure in the receiver tank – turning the compressor on or off whenever necessary to keep things steady.
Without it, the compressor might just keep running. Or it might not start at all. Either way, you’re left wasting energy and wearing down parts – and, in some cases, creating a safety risk.
In simple terms, the pressure switch makes sure the compressor only runs when it should. It protects the motor and cuts down on energy use. It also helps keep the air supply steady right across the system.
In this guide, we’ll look at how pressure switches work – why they matter – and what to check when you’re maintaining or replacing one.
The Core Functions of a Pressure Switch
The primary purpose of an air compressor pressure switch is to automate the pressure cycle.
It monitors the air pressure within the air receiver and controls the electric motor. This automation provides four key functions.
- System Automation: The switch handles the starting and stopping on its own. When pressure falls to the cut-in point, the motor kicks in. Once it hits cut-out – the switch shuts it back down.
- Energy Efficiency: The motor only runs when needed. This means energy isn’t wasted. It’s worth noting that even a small pressure increase – just 1 bar above what’s required – can push energy use up by around 7%.
- System Safety: The pressure switch is also a built-in safeguard. It’s the first thing that stops the system from climbing past its safe working limit.
- Equipment Longevity: The switch controls the compressor’s duty cycle, which helps prevent thermal overload. It also cuts down on wear from quick, repeated starts – keeping the motor running smoothly for longer.
Understanding Mechanical Pressure Switch Operation
A mechanical pressure switch works by balancing the pressure of the air in the tank with the tension of a spring inside the switch.
This system automatically controls the motor. The contacts close to start the motor again when the system pressure drops to a certain level. The contacts open to stop it when the pressure gets too high.
What is the Cut-In and Cut-Out Cycle?
The cut-in and cut-out points together create what’s known as the differential – the range in which the compressor operates.
This differential is important to stop rapid cycling, which would quickly cause the compressor motor to overheat and break.
- Cut-In: As air is used, system pressure begins to drop. After some use, the spring starts to push harder than the air. It shifts, and the diaphragm moves down. That’s when the contacts meet and the motor kicks in.
- Cut-Out: Once the motor’s running, pressure builds again. The rising air pressure pushes the diaphragm back up, forcing the contacts apart and breaking the circuit – which stops the motor.
Why is a Snap-Action Mechanism Important?
The snap-action mechanism is a critical design feature.
It makes sure the electrical contacts open or close right away – not gradually. That quick movement stops arcing from happening, which would otherwise create heat and damage the contacts over time.
While the diaphragm and spring manage air pressure, another small but essential component works alongside it – the unloader valve.
The Role of the Unloader Valve
The unloader valve is a small valve that is usually part of the pressure switch assembly.
The unloader valve protects the compressor. When the motor shuts off, the valve opens. A burst of air escapes – helping to release pressure in the line.
This means that trapped pressure is safely released before the compressor starts up again.
This process ensures the motor restarts against zero head pressure, preventing the high “inrush current” that causes overheating and motor failure.
Key Types of Compressor Pressure Switches
Pressure switches are broadly categorised as mechanical or digital.
Mechanical switches are ideal for smaller or piston-type compressors, while digital switches suit larger industrial or rotary screw models requiring programmable precision and integration with controllers.
What is a Mechanical Pressure Switch?
Piston compressors often have mechanical switches.
They use a spring and diaphragm system to open and close high-voltage electrical contacts by moving them. They are strong and cheap, but not as accurate as digital models.
How Do Digital Pressure Switches Compare to Mechanical Ones?
Digital pressure switches, also called transducers, are used in the Atlas Copco GA series of modern rotary screw compressors. They use a solid-state electronic sensor to get very accurate pressure readings.
There are no moving parts to wear out. If needed, the unit can also be set to send a low-voltage signal back to the main controller – a simple, reliable setup.
| Feature | Mechanical Pressure Switch | Digital Pressure Switch (Transducer) |
| Technology | Spring & diaphragm | Solid-state electronic sensor |
| Best For | Piston compressors, smaller units | Rotary screw compressors, industrial systems |
| Accuracy | Good, sufficient for standard use | High precision, programmable |
| Integration | Standalone, switches motor directly | Integrates with controllers (e.g., Elektronikon®) |
| Wear | Contacts are a wear item | No moving parts |
Identifying Common Pressure Switch Issues
Some common problems with a pressure switch are that the compressor doesn’t start, runs all the time, or air leaks from the unloader valve.
But when these symptoms appear, the fault is often somewhere else in the system.
Take a compressor that won’t start, for example – the real issue could be a faulty tank check valve or a bad unloader valve. If air keeps leaking from the unloader once the compressor’s off, it points to the check valve being the problem, not the switch itself.
A compressor that runs without stopping is a critical air compressor safety hazard, often caused by welded contacts or a major system leak.
UK Safety Regulations and PSSR Compliance
UK law says that the pressure switch is a safety-critical part.
The Pressure Systems Safety Regulations 2000 (PSSR) require a formal inspection system. A written scheme of examination is required for any system that has a pressure in bar multiplied by the volume of the vessel in litres that is more than 250 bar-litres.
This legal document, signed by a “Competent Person,” lists how often and how thoroughly all safety devices, including the pressure switch, should be checked.
The Provision and Use of Work Equipment Regulations (PUWER) also say that this equipment must be kept in good working order. Also, pressure switches sold in the UK must have the UKCA marking on them.
Maintenance, Inspection, and Servicing
For safety and reliability, it’s worth keeping the system checked and up to date.
Pressure switches should be inspected about every three months – and always during planned compressor servicing.
That means checking for any visible damage and making sure the electrical connections are secure – no loose wires or corrosion. You should also test the unloader valve to be sure it’s working as it should.
As part of a scheduled UK compressor maintenance plan, J Ll Leach engineers verify the safe operation of the pressure switch. They ensure its cut-out pressure aligns with the system’s Written Scheme of Examination and PSSR requirements.
Atlas Copco Pressure Switches and Smart Control
Modern Atlas Copco pressure switch systems are often digital. They integrate with advanced Elektronikon® controllers. These controllers offer precise control over system pressure, which is vital for managing the relationship between CFM and PSI and reducing energy costs.
How SMARTLINK Monitors Pressure
The Elektronikon® controller connects to SMARTLINK, one of the most advanced air compressor monitoring systems.
With SMARTLINK, you can check system pressure in real time – even when you’re not on site.
If the pressure switch fails, it sends an alert so maintenance can be planned before anything stops working. That small detail prevents sudden downtime.
A good pressure switch might not look like much, but it does a lot. It keeps the whole compressed air system running safely, efficiently, and without interruption.
Get in touch with J Ll Leach for professional advice on how to stay compliant and keep your system safe.
We have certified engineers who can test, service, and replace pressure switches that meet PSSR standards for all major compressor brands, including Atlas Copco.
Get in touch with the nearest depot in Birmingham, Shrewsbury, or Stoke-on-Trent today.