The Single Pole Double Throw SPDT relay is quite useful in certain applications because of its internal configuration. It has one common terminal and 2 contacts in 2 different configurations: one can be Normally Closed and the other one is opened or it can be Normally Open and the other one closed. So basically you can see the SPDT relay as a way of switching between 2 circuits: when there is no voltage applied to the coil one circuit “receives” current, the other one doesn’t and when the coil gets energised the opposite is happening.
Video presentation of how the SPDT relay works
How does the Single Pole Double Throw relay works?
In figure 1 no DC voltage is applied to the coil so the terminal T is connected to contact 1 therefore the current can flow through fan 1 and it cannot flow through fan 2.
In figure 2 when DC voltage is applied to the coil and terminal T is now connected to contact 2 threfore the current doesn’t flow anymore through fan 1 but now it flows through fan 2.
Related product: Fan Cooler