Secondary Circuit (Switch Side)...
There are 4 types of switches:
- SPST (Single Pole Single Throw)
- SPDT (Single Pole Double Throw)
- DPST (Double Pole Single Throw)
- DPDT (Double Pole Double Throw)
Recognizing the type of switch that the relay is using, helps understanding the pinout and how the relay works. It also helps selecting the right relay for your application.
For example, for the relay presented in the introduction section we are dealing with a SPDT relay.
Since the switch side is normally connected to a higher voltage, when selecting a relay make sure that this part of the relay can handle the required maximum voltage and current otherwise you will destroy the switch.
Mechanical relays are the oldest type of relays, but they are still being used.
Advantages...
- They are simple in construction
- They are reliable
- No specific programming device is required
- People can understand these relays easily
Disadvantages...
- This type of relays require to be maintained periodically. Over time, the springs and the linkages inside the relay grow weak which can result in maloperation and false trips.
- No directional feature
- The speed of operation is limited by the mechanical inertia of the moving components