Micro Switch

Overview of the micro switch

Micro switches are commonly known by a number of names, including “miniature Switch,hinge lever,0.39N,slv con,sldr tmsnap-action” switches. In essence they are electrical switches that use “over-centre” or tipping-point mechanisms to provide easy actuation using minimal pressure. Due to low manufacturing costs and long service life, they are now an exceedingly common choice with engineers and electricians – particularly as a single switch can provide up to 1 million cycles, with industrial varieties withstanding up to
10 million cycles.

Relative to switches that provide simple binary on/off states, micro switches can provide a degree of extra flexibility by offering the capacity to switch flexibly and reliably between specific positions of the actuator.

At its core, the micro switch's key feature is the amplification yielded by the actuator button: Switch,sim
                     rol lev,0.64N,slv con,wire tma tiny depression of the button produces a relatively large (and exceptionally fast) movement at the electrical contacts. Even if the actuation is relatively slow, the movement of the contacts is always very fast.

Micro switches are also defined by their ability to remain in the 'current state' until a significant reversal of the actuator has occurred. This innovation, termed 'hysteresis' means that small reversals will produce no action. This provides switched circuits that are broken sharply and reliably when the actuator is in receipt of the requisite reversal.

 

Technical aspects

A very widely used design amongst micro switches involves the internal use of two conductive springs. The first spring, a straight metallic strip, is fixed by a hinge at one corner of the switch at the other corner (horizontally), are a set of electrical contacts. A smaller, arched spring, usually made of Beryllium Copper (BeCu), is added under compression during the assembly process so that within the switch casing, it is always attempting to unbend. It is connected to the long, flat spring at a point close to the electrical contacts. Near to the centre of the flat spring, a fulcrum is positioned, while a depressible actuator nub touches the flat spring close to the hinge. The curved spring endeavours to pull the flat spring away from its hinged anchor point sideways, but the anchoring prevents this motion from occurring. It also exerts force to pull the flat switch upwards from the anchor point, although the geometry ensures that this force is proportional to the flat spring’s decreasing displacement when it is pulled downwards.

When the actuator is depressed by a force external to the switch, it causes the flat spring to flex while the contacts are kept closed by the curved spring. But eventually a point is reached where the flat spring’s flexion forces the curved spring to compress, whereupon the electrical contacts begin to move apart.

Even if there is no further movement of the actuator nub, the upward force of the curved spring diminishes in proportion to the downward movement of the flat spring, a development which accelerates the motion of the flat spring, affecting the normally open contact. The flat spring loses tension as it moves downward, but the switch is designed in such a way that acceleration is still the net effect of the operation. This is the tipping point or over-centre action mentioned earlier, and it results in a distinctly crisp feel and an audible clicking sound.

Once the actuator is released, the flat spring immediately begins to move upwards. As it does so, it causes the force exerted by the curved spring to rise. Again, the result is acceleration that ceases as soon as the normally-closed contacts are reached. The curved spring is designed to be powerful enough to move the contacts, even if it causes flexion in the flat spring, because during the transition from the downward to the upward direction the actuator does not actually move.

Despite the durability of the micro switch, the arched “return” spring is usually the first to fail toward the end of the switch’s life.

 

Where the micro switch is used in manufacturing

Micro switches have a wide range of applications in industrial and domestic settings. Microwave ovens are a case in point – here the microswitch provides an on/off state on the door interlock that means that the microwave will cease to operate when the door is open.  Elsewhere they’re also widely used as elevator levelling and safety switches. In office settings microswitches are part of photocopiers paper jam detection mechanisms. Microswitches have also proven invaluable in fire safety and disaster management technologies as they are commonly incorporated into gate valves on fire sprinkler systems (functioning as tamper switches, in effect), as indeed they are in many water pipe systems, where it is often exceedingly important to determine whether a valve has been shut or opened. Micro switches are an ideal solution for this purpose.

As an example of typical function, microswitches are often used to control electrical circuits in items as diverse as vending machines and industrial controls. They are also rated to convey current in control circuits, although some may be employed directly to control solenoids, lamps, small motors and other devices.

From an engineering perspective, the practical applications of the technology mean that, for example, action in devices built to vend food and drink can be precipitated by specially designed low-force varieties of micro switch capable of sensing coins; they may also be used to sense air flow in vending machines when a vane is attached to them. 

While micro switches can by operated without mediation by a mechanism, their uses also extend to being incorporated as a component of temperature, flow and pressure switches. These are commonly operated by a sensing mechanism like a Bourdon tube (the tube is governed by the principle that says that a tube that has been flattened will revert to its circular form under pressure). But in these applications, long-term accuracy depends fundamentally on the repeatability of the actuator during switching.

Timer mechanisms frequently employ micro switches, too, although these are usually designed with a comparatively slow-speed motor-driven cam to function properly in this application.

In addition to other uses, another common application for micro switches is to function as limit switches controlling electrically-driven machinery or machine tools: a metal housing encloses the snap-switch mechanism when incorporated into such equipment, and the switch comes with actuating plungers, rollers or levers to perform its designated functions.

 

How the micro switch differs from other switches

The key feature of the micro switch which differentiates it from other electrical switches is the large amplification of tiny movements of the actuator, which causes the springs configured within the switch to accelerate the opening and closing of electoral contacts, which part or join exceptionally rapidly as a result.. They are also capable of withstanding extraordinarily high levels of actuator repetition, making them considerably more durable than many other switches.