The things you need to know about micro switches and hysteresis

The invention of the microswitch dates back to over 80 years ago in 1932 and is attributed to Peter McGall of Freeport, Illinois, USA Who knows how many billions of these small and practical components have been made since then, but there can hardly be a house or a company where you can’t find at least one and probably many more. They are used in doors to detect an “open” or “closed” state, in vending machines to detect falling coins, in printers and copiers, in franking machines and in pressure switches for process applications.

Fast acting microswitches, the most common type, often use an actuation lever to produce a quick change with minimal physical pressure on the actuator. With industrial versions capable of operating for 10 million cycles or more, and also low-cost consumer types capable of at least one million operations, they are found in a wide range of applications, including lamps, solenoids, motors and flow switches . pressure. The actuator acts like a lever, with a small force applied to its free end which results in more force as it moves towards the end of the pin, where it rests on a plunger or pin. Even when force is applied to the actuator very slowly, its amplifying effect causes the movement of the switch contacts to always be very fast, a desirable feature in any switch.

During operation, the microswitches show hysteresis. Wikipedia defines hysteresis as “the dependence of a system’s output not only on its current input, but also on its history of past inputs. Addiction arises because history affects the value of an internal state ”.

In microswitches it is easier to think of it this way. When the actuator is pressed, there is a point where the switch is activated, connecting the common contact to the normally open (NO) contact. As the pressure on the actuator decreases, the point at which the switch returns to the non-activated state, with the common contact returning to the normally closed contact (NC), is not the same as the activation point, is subsequent. The distance between the trigger point and the release point is called differential motion or hysteresis.

In many cases, this short delay is a good thing. In fact, hysteresis is often deliberately introduced into electronic circuits to prevent “vibration” from switches as they oscillate around a defined set point. However, in some applications, excess hysteresis can be a disadvantage. This is especially true for mechanical pressure switches and thermostats, such as the one shown here.

The build-up of temperature or pressure is slow in most process applications. Hysteresis, sometimes referred to as “differential movement”, means that the point at which the microswitch activates as pressure increases will be different from the point at which activation reverses under decreasing pressure conditions. Hysteresis is 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 pressure changes. Some microswitches exhibit less hysteresis than others, and the figure is not always shown on the datasheet, so it may be worth wondering if this performance aspect is critical to your application.