Understanding micro switches and hysteresis

Who knows how many billion of these handy little components have been manufactured since, but there can hardly be a household or business premises where you won’t find at least one, and probably many more. They’re used in doors to sense an ‘open’ or ‘closed’ state, in vending machines to detect the dropping of coins, in printers and photocopiers, franking machines, and pressure switches for process applications.

Snap action micro switches, the most common type, often use an actuating lever to produce fast switching with very little physical pressure upon the actuator. With industrial versions capable of operating for 10 million cycles or more, and even low-cost consumer types capable of at least one million operations, they’re found in a huge range of applications, including in lamps, solenoids, motors, and flow and pressure switches. The actuator acts as a lever, with a small force applied to its free end being translated into a larger force as you move towards the pivot end, where it rests upon a plunger, or pin. Even when force is applied to the actuator very slowly, its amplifying effect means that the movement of the switch contacts is always very fast, a desirable characteristic in any switch.

In operation, micro switches exhibit hysteresis. Wikipedia defines this characteristic as 'the dependence of the output of a system not only on its current input, but also on its history of past inputs. The dependence arises because the history affects the value of an internal state'.

In micro switches, it’s easiest to think of it like this; when the actuator is depressed, there is a point at which the switch activates, connecting the common contact to the normally open contact. As pressure on the actuator decreases, the point at which the switch reverts to its non-activated state, with the common contact falling back onto the normally closed contact, is not the same as the activation point, it’s later. The distance between the actuating point and the release point is called differential movement, or hysteresis.

Cutaway showing the internal construction of a micro switch (Attricution: Benjamin D. Esham / Wikimedia Commons)

In many instances this short time delay is a good thing. In fact, hysteresis is often deliberately introduced into electronic circuits to prevent 'chattering' of switches as they oscillate around a defined set point. However, in a few applications, excess hysteresis can be a disadvantage. This is particularly true for mechanical pressure and temperature switches, like the one shown here.

As pressure increases in the chamber the membrane deforms, bulging outwards to depress the level on the micro switch (not to scale)

The temperature or pressure build-up is slow in most process applications. Hysteresis means that the point at which the micro switch actuates as the pressure increases will be different from the point at which actuation is reversed under conditions of decreasing pressure. This creates a disadvantage here because it limits the resolution of the pressure switch. In other words, it limits the system’s ability to detect very small changes in pressure. Some micro switches exhibit less than others and the figure is not always shown in the data sheet so it may be worth asking if this aspect of performance is critical to your application.

Actuators come in a variety of forms, as shown on these Omron switches

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