Relay Image


RELAY

1. Relay is an automatic switch that detects conditions in circuits and causes the contacts to change their positions to close or open the circuit as required.

2. Relays are used in industrial applications.

3. When the coil of relay is energised or de-energised, its contacts are On or Off. Contacts are isolated from the coil. They can be used independently to switch other circuits.

4. Relays are available in various shapes and sizes.

5. Relays are cheap, reliable and they have a long life.


TYPES OF RELAY

1. General purpose relays 

(i) AC relays

(ii) telephone relays

(iii) polarized relays

(iv) latch-in relays 

(v) interlocking relays 

(vi) stepping relays 

(vii) differential relays 

(viii) low capacitance relays.


2. Reed relays 

(i) Dry reed relays

(ii) ferred relays

3. Time delay relays

4. Thermal type relays

5. Rotary relays

6. Moving coil relays

7. Solid state relays (using transistors, diodes etc.)


Specifications of relays 

Rated voltage, rated current, rated frequency of operation, operate and release times, pick up and drop out voltage, contact bounce, overload capacity etc.


1. Rated voltage

This is the value of voltage upon which the performance of the relay is based. 


2. Rated current 

This is the value of current upon which the performance of the relay is based.


3. Rated frequency of operation  

This is the number of cycles of operation which a relay can perform per hour under specified conditions to make and break.


4. Operate and release times 

The time required for the relay to get energised or de energised after applying or removing the energising voltage It is few micro sec or m sec.


5. Pick up voltage  

Rated coil voltage is applied to the coil for 1 period of 1 to 3 seconds. The voltage is gradually reduced to zero. The voltage is then gradually increased until the relay operates and the pick up voltage is measured.


6. Drop out voltage  

Rated coil voltage is applied to the coil. The voltage is gradually reduced until the contacts return to the de-energised position and the dropout voltage is measured.