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Circuit Controlling Devices
Switch consists of two contacting surfaces which can be isolated from each other or brought together as required by a movable connecting link.

Connecting link is Pole and when it provides a single path for a flow of current the switch is designated as single-pole, single-throw switch.

Throw - the number of circuits each pole cam complete through the switch.

Switches are designated by the number of poles and number of throws and also by the number of positions they have.

Toggle switches also known as tumbler-type switches are used for general purpose switching functions and are used extensively in various circuits.

Push switches are primarily for operations of short duration. i.e. when a circuit is to be completed or interrupted momentarily or when an alternative path is to be made available for brief periods.

Rocker button switches combine the action of both toggle and push-button switches and are utilized for circuit control of systems and equipment.

Rotary switches are manually operated and for certain operating requirements they offer an advantage over toggle switches in that they are less prone to accidental operations. More adapatable to multi circuit selection than the toggle type switches.

Micro-Switches are special category of switch and are one of the most extensively applied electrical devices in aircraft, performing a wide range of operations to ensure safe control of a variety of systems and components. The term micro switch designated a switching device in which the differential travel between make and break of the operating mechanism is of the order of a few thousandths of an inch. Magnification and snap action of contact mechanism movements are derived from a pre-tensioned mechanically biased spring.

The method of actuating micro switches depends largely on the system to which it is applied but usually it is either by means of lever, roller, or cam, these in turn being operated either manually or electrically.

The operating cycle of a micro switch is defined in terms of movement of the plunger.

The contacts of micro switches operate within sealed evacuated chambers filled with an inert gas, e.g. nitrogen.

Rheostat is a controlling device containing a resistance, the magnitude of which can be varied, thereby adjusting the current in the circuit in which it is connected. Rheostats normally adjust circuit resistance without opening the circuit, also they are constructed to serve as a combined on-off switch and variable resistor.

Time switches. its principle varies but in general it is based on the one in which a contact assembly is actuated by a cam driven at constant speed by either a speed-controlled electric motor or a spring driven escapement mechanism.

Time switches also operated on thermal principle, where the contact assembly is operated by the distortion of a thermal element when the latter has been carrying a designated current for a pre-determined period.

Homing control circuit is to reset the time switch after use so that it will always re-commence at the beginning of an operating cycle.

Mercury switches are glass tubes into which stationary contacts or electrodes, and a pool of loose mercury are hermetically sealed. Tilting the tube causes the mercury to flow in a direction to close or open a gap between the electrodes to make or break the circuit in which it is connected.

The rapidity of make and break depends on the surface tension of the mercury rather than on externally applied forces.

Mercury switches are applied to systems in which the angular position of a component must be controlled within a narrow band of operation and in which the mechanical force required to tilt a switch is very low.

Pressure switches are used for measuring high and low pressure and connected to warning lights and for indication that an operating pressure has been attained for the period during which the system is in operation.

Pressure switches are also applied to systems requiring that warning or indication be given of changes in pressure with respect to a certain datum pressure.i.e. as a differential pressure warning device.

Thermal switches are applied to systems in which a visual warning of excessive temperature conditions, automatic temperature control and automatic operation of protection devices are required.

Common principle adopted for thermal switch operation is based on the effects of differences of expansion between two metals, usually invar and steel.

Proximity switches are used in several circuits required to give warning of whether or not passenger entrance doors, freight door, etc. are fully closed and locked.

Since they have no moving parts they offer certain advantages over micro-switches which are also applied to such warning circuits.

Relays are electromagnetic switching devices by means of which one electrical circuit can be indirectly controlled by a change in same or another electrical circuit.

In basic form a relay may be considered as being made up of two principal elements, one for sensing the electrical changes and for operating the relay mechanism, the other for controlling the changes.

The sensing and operating element is a solenoid and armature and the controlling element is one or more pairs of contacts.

Relays are designated by their pole and throw arrangements.

Relays are also classified by the order of making and breaking of contacts, whether normally open or normally closed in the de-energized position, rating of the contacts in amperes and voltage of the energizing supply.

Grouping of relays are also related to basic form of construction, e.g. attracted core, attracted armature, polarized armature, and "slugged", and the current carrying ratings of the controlling element contacts. i.e. whether heavy-duty or light-duty.

Attracted Core, Heavy Duty relay : heavy duty refers specifically to the amount of current to be carried by the contacts. carbon contacts are provided to absorb the initial heavy current and thereby reduce arcing to a minimum before positive connection with the main contacts is made.

Polarized armature relays are used in applications where the value of control circuit elements and voltages may be only a few milliamps and millivolts. The attraction and repulsion effects of magnetic forces are substituted for the conventional spring-control of the armature and contact assembly.

Slugged relays are used where it is required to slowly operate the contact assembly either at the stage when the armature is being attracted, or when it is being released.

Principle of Slugged relays: the build up or collapse of the main electromagnet flux is slowed down by a second and opposing magnetizing force.

The relay usually incorporates a ring of copper or other non-magnetic conducting material in the magnetic circuit of the relay, in such a way that changes in the operating flux which is linked with the slug originate the required opposing magnetic force. In some slugged relays, the effect is attained by fitting an additional winding over the relay core and making provision for short-circuiting the winding, as required, as by means of independent contacts provided in the main contact assemblies.