A relay with five pins typically has two to operate the coil and three to incorporate an SPDT switch function with a common and a normally open contact (open when the relay is unenergized and connected to common when the relay is energized) and a normally closed contact with the opposite function. This is sometimes referred to as a Form C contact.
Complex switching, SPST NO, SPST NC, SPDT, 2PDT, and 4PDT are all common types of relays. The last would necessitate the use of 14 pins.
So, what exactly are SPDT relays used for?
The standard relay has four pins and is an SPST NO relay that closes when actuated. When actuated, an SPDT relay can perform the function of an SPST NC relay. This could be used to have a disabling circuit interrupt some operation.
