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Micro  and  Reed  Switches 

Allan Mills

Retired Lecturer, Leicester University

allanmills1@hotmail.co.uk

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Modern electrical technology requires millions of switches, many of them operating at mains voltages to control currents measured in amperes. It is important that the circuit should be broken rapidly, for otherwise there is a risk of an arc being produced between the contacts. For this reason many switches are bistable devices: they are either ‘off’ or ‘on’, and snap rapidly between these two states when a control lever is moved manually. This in turn acts upon a tumbler which, fitted with a helical spring, comes to rest either side of centre.

 

The microswitch

The problem with the traditional switch is that, where large numbers are involved, it is both bulky and expensive since helical springs have to be individually wound and fitted. Therefore, in 1932, Peter McGall of the Burgess Battery Company invented a much more compact form.  Soon known as a microswitch, it was operated by pressure upon a protruding plastic pin. When the force becomes greater than a certain low design value the pin moves down through something like a millimetre, and operates the switch.  An example is shown in Fig.1; selectable contacts enables it to be normally ‘off’, but when activated to be ‘on’ –  or vice versa.  Relaxation of the pressure on the pin enables the switch to return to its original

Early and modern microswitches
Fig.1  An early Burgess microswitch (left, 28x16x5 mm)
alongside a modern lever-arm type by RS Ltd.
state, the snap action occurring rapidly with a reassuring click.  The microswitch is typically capable of handling up to mains voltages at a few amperes for over a million cycles. The principle of the microswitch is shown in Fig.2, where it will be seen that the motion of a rocking arm is controlled by a curved spring.
Microswitch components
Fig. 2  Physical components and internal layout of a typical microswitch
This is so shaped and positioned that a dual-faced contact at the extremity of the rocking arm moves rapidly between two fixed contacts. There is no stable intermediate position: it moves rapidly from one contact to the other under the influence of the pin.  The actual components are exposed in Fig.3, and are easily mass produced by stamping or etching from springy phosphor-bronze or beryllium-copper sheet. (The white pin shown in Fig.3 is not original, but was added to show where the actuator would normally press.)
Typical microswitch

Fig. 3 Mechanism of a typical microswitch.

 

 

Applications of microswitches in military equipment increased greatly during WW II, and models were made by many manufacturers in a large number of designs, sizes, and current-handling capacities, with various types of actuator.  It is now widely used.

 

The reed switch

The microswitch operated in air, for the moving pin could not easily be sealed. However, there was a need for a compact switch with the moving (and potentially sparking) contacts hermetically sealed in vacuum or inert gas within a suitable glass envelope.  This would allow the switch to be situated in potentially inflammable or explosive environments, such as a petrol tank.

In 1936 W.B. Elwood invented a device where mechanical actuation was replaced by a magnetic field acting on susceptible electrodes.  Nickel-iron alloy is both ferromagnetic and compatible with sealing into glass.  Fig. 4 illustrates the simplest form of such a magnetically-operated switch, the normally-open electrodes being brought into contact by the approach of a small permanent magnet.  Actuation by a small current flowing in a coil wound around the exterior of the switch produces a sensitive relay.

Reed switch details
Fig.4. Diagram of an incapsulated reed switch.

 

 

The thin strip of metal acting as the moving ‘reed’ was not mechanically pivoted; instead it moved by utilising an ingenious ‘tape hinge’.  A thin flat strip used as a reed would tend to bend into unwanted contact if the device were inverted or moved, but this is avoided by making the strip slightly concave.  Consider the metal tape measure shown in Fig. 5 (left): the curvature renders it remarkably rigid when extended, resisting a small downward thrust from the finger.  However, above a certain pressure the tape deforms as shown in Fig. 5 (right), with the straight portion retaining its rigidity. The transition between ‘straight’ and ‘bent’ is quite sudden, being accompanied by a definite click.  I would not be surprised to learn that some subminiature microswitches employ the same principle now that its patent has expired.

Reed switch simulation
Fig. 5  Tape measure, extended ((left) . Tape measure, deflected downwards.

 

Encapsulated reed switch
Fig. 6  Early reed switch, not encapsulated within plastic
(42 mm long x 4 mm diameter).

 

 

Contemporary reed switches tend to be encapsulated in opaque plastic, but an older model displaying the action is shown in Fig.6.