Autotronics - Electrical Switches

Introduction


   - Electrical component which can break an electrical circuit.

   - Interrupting the current or diverting it from one conductor to another.

   - A manually operated electro mechanical device with one or more sets of electrical contacts.

   - Closed – contacts are touching and electricity can flow.

   - Open – contacts are separated and non conducting.


Contacts

   - The simplest -> 2 pieces of metal called contacts to make a circuit when they touch and separate to break the circuit.

   - Material of contacts are chosen base on its resistance to corrosion.

   - This is due to the problem of insulating oxides form at the metal which prevents the switch from working.

   - Materials of contacts are chosen base on the electrical conductivity, hardness (resistance to abrasive wear), mechanical strength, low cost and low toxicity.



Actuator

   - Moving part which applies the operating force to the contacts.

   - E.g. toggle , dolly, push-button.



Arcs and Quenching

   - When the wattage or voltage are large-> electron flow across opening switch contacts is sufficient to ionize the air molecules as the switch opened.

   - Gas plasma / electric arc is formed.

   - The disadvantage of gas plasma :

      o Low resistance

      o Able to sustain power flow.

      o Very hot and capable to erode the metal surfaces.

   - To minimize arc formation and preventing contact damage :

      o Use fast-moving switch mechanism. e.g. a spring operated tipping point mechanism.

      o Use a series of nonconductive blades spanning between switch contacts.

     o Use a puffer to blow a sudden high velocity burst of gas across the switch contacts.



Contact Arrangements

   - SPST -> Single Pole Single Throw

   - SPDT -> Single Pole Double Throw

   - SPCO -> Single Pole Change Over

   - DPST -> Double Pole Single Throw

   - DPCO -> Double Pole Change Over



Biased Switches

   - Switch that contain a spring to return the actuator to a certain point.

   - E.g. momentary push-button

     o Push-to-make (Normally Open)

     o Push-to-break (Normally Close)



Special Types

   - Designed to respond to any type of mechanical stimulus :

      o Vibration

      o Tilt

      o Air pressure

      o Fluid level

     o The turning of a key

     o Linear or rotary movement

     o Presence of a magnetic field

   - Mercury tilt switch

     o Consists of a drop of mercury inside a glass bulb with 2 or more contacts

     o When the bulb tilt contacts will be connected by mercury.

     o Better than ball tilt switch.-> unaffected by dirt, debris and oxidation.

     o Used when arcing is dangerous e.g. presence of explosive vapour.

   - Knife switch

     o Consist of flat metal plate hinged at one end and a fixed contact.

     o Usually it is not enclosed.

     o Used in electrical transmission and distribution, gang-operated switches which us used in circuits up to the highest voltages.

     o Disadvantages :- slow opening speed