Monostable vs Bistable Valves: Which to Pick

Monostable valves return to a defined rest position when the signal drops, while bistable valves hold their position even without a signal. This basic property decides safety behaviour, wiring and air consumption. This guide shows when each type is the right choice.

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5/2

Typical directional valve function

Solenoid on a monostable valve

Solenoids on a bistable valve

2-8 bar

Common pilot pressure range

Inhalt

Basics
Safety
Selection
Frequently asked questions

What separates monostable and bistable?

A monostable valve has a defined rest position it returns to by a spring (or pilot air) as soon as the signal drops. A bistable valve (impulse valve) has two stable positions and stays in the last commanded position after a short switching pulse - even after a power or pressure loss.

The number before the slash gives the ports, the number after gives the switching positions. A 5/2 directional valve has five ports and two positions and is the most common form for double-acting cylinders.

Which type is safer on signal loss?

The choice depends on the wanted behaviour under fault. If a cylinder must retract in a defined way or release a clamp on power loss, a monostable valve is right: it takes the safe home position by itself. If a load must hold its position so a workpiece does not drop or a process does not start uncontrolled, a bistable valve makes sense.

Monostable: safe home position, ideal for emergency stop and defined retraction.
Bistable: holds the load in place, good for heavy or suspended loads.
Always assess the full safety chain, not the valve alone.
For suspended loads, also check load-holding valves or spring accumulators.

The fault behaviour must match the machine risk assessment. A bistable valve holds position on power loss - depending on the application that can be safe or hazardous. When in doubt, align the safety function with a qualified engineer.

How do I choose for my application?

Beyond safety behaviour, signal type, energy use and control effort matter. Bistable valves need only a short pulse and so save coil heat and energy during long hold times. Monostable valves are simple to wire and need only one output from the controller.

Signal & control

Match a held signal (monostable) or a pulse pair (bistable) to your PLC.

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Energy & heat

For long hold times the impulse valve saves coil heat and power.

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Sizing

Set nominal size, function and fault behaviour together.

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Rule of thumb: short cycle times and a defined home position favour monostable, long hold phases and position retention favour bistable.

Frequently asked questions

Does a bistable valve lose its position on power loss?

No. A bistable impulse valve holds the last commanded position even without power or a control signal. Only a fresh pulse on the opposite side switches it over. That is the key difference from a monostable valve, which spring-returns to the home position.

Why does a monostable valve need a held signal?

Because the spring constantly pushes the valve toward the home position. The valve only holds the switched state while the solenoid is energised. When the signal drops, it returns at once. That means it draws some energy continuously while switched.

Can I replace a monostable with a bistable valve?

Technically often yes, if size and ports match. But you change the fault behaviour and the actuation: instead of one held signal you need two pulse outputs. Check the machine safety function before you swap.

Find the right pneumatic valve

Our pneumatics specialists help with function, fault behaviour and nominal size for your control.

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Safe fault behaviour

Home position and position retention clearly compared.

Right function

5/2, 5/3 and impulse valve matched to the application.

Efficient operation

Air and energy use explained per valve type.

Expert advice

Pneumatics specialists support your selection.