Autotronics - Relays

Introductions


- Electrically operated switch

- Use electromagnet to operate switching mechanism.

- Necessary to control a circuit by a low-power signal.

- Help to control several circuits using one signal.


Basic Design and Operation

- It works using the principal of electromagnet.

- Consist of :-

          o A coil of wire surrounding a soft iron core

          o Iron yoke provides a low reluctance for magnetic flux

          o A movable iron armature.

          o Sets of contacts

- When the electric current passed through the coil, magnetic field attracts the armature.

- Movement of movable contacts makes or breaks a connection with a fixed contact.

- When current is off the armature will return back to its original position by a force.

- The force is normally provided by spring or gravity.

- In low voltage application -> relays operate quickly to reduce noise.

- In high voltage application -> relays operate quickly to reduce arcing.



Types of Relays

- Latching relay

          o Has 2 relaxed states

          o Also called as impulse, keep, stay

          o When the current is switched off, the relay remains in its last state.

           o It consumes power only for an instant while is being switched, and it retains its last setting across a power outage.

           o A current pulse in opposite polarity will change the state.


- Reed relay

          o Set of contacts inside a vacuum or inert gas filled glass tube.

          o It protects against atmospheric corrosion.

          o Capable of faster switching speeds than larger types of relays.

          o Have low switch current and voltage ratings.


- Mercury-wetted relay

          o Contacts are wetted with mercury.

          o Used to switch low voltage signals due to low contact resistance.

          o Used for high-speed counting and timing applications.

          o Position sensitive and must be mounted vertically.


- Polarized relay

        o Placed the armature between the poles of a permanent magnet to increase sensitivity.

         o Used to detect faint pulses and correct telegraphic distortion.



Applications

 to switch on many circuits using only one control circuit at the same time.

 To switch on a high voltage circuit using a low voltage circuit.

 To separate 2 conductors with different characteristics in a circuit.



Advantages

 high switching load.

 don't need any coolant

 can be shorted without losing its function.

 It can switch from small signals until high frequency power.



Disadvantages

 high delay time

 sensitive to vibration.

 Life span is depend on the mechanical components.

 It can create noise during switching.