Micro and Reed Switches
Retired Lecturer, Leicester University
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 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
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.
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.
Contemporary reed switches tend to be encapsulated in opaque plastic, but an older model displaying the action is shown in Fig.6.