SPDT Relay and DPDT Relay

In SPDT spdt relays (Single Pole Double Throw SPDT Relay) are very useful in some applications due to its internal configuration. It has one common terminal and 2 contacts in 2 different versions can be normally closed and the other opened, or it may be normally open and the other closed. So basically you can see the spdt relay as a way to toggle between 2 loops: when there is no voltage at the coil is one circuit receives current, the other is not and when the coil gets voltage is reversed.

SPDT relay circuit schematic

How SPDT relay work?

In Fig. 1 the DC voltage is supplied to the coil so the terminal T is connected to contact 1 therefore the current can flow through the fan 1 and it cannot flow through the fan 2.

In Fig.2, When the DC voltage is supplied to the coil and the terminal T is now connected to pin-2, so that current does not flow through the fan 1, but now it flows through the fan-2.

DPDT relay

In dpdt relay (double-pole two-position) is quite interesting and can be used in various ideas, including changing the direction of movement of the engine, as you can see in the picture below. It has 2 terminals and connectors 4 and you can look at the dpdt relay as the equivalent of 2 SPDT relays.

To switch between the 2 different loads with dpdt switch

we can use a dpdt switch to select between the 2 different loads, and also have a visual indication that the load is connected. In our case (Fig. 1) when there is no voltage at the coil the fan and green led turn on . When we apply voltage to the coil (Fig. 2) the relay switches its contacts and now the light buld and the red led does not turn on.

To change the direction of rotation of the motor with a dpdt relay

As you can see in the schematic diagram with 12V battery (or use other voltages) connected to the plus terminal T1 and minus terminal T2. Contacts 1 and 4 connected to 2 and 3 too.

Without voltage applied to the coil to positive battery connected to pin-1 (and-4) and minus 3 (and-2) so that the motor rotates in one direction (say clockwise). When voltage is applied to the coil of the relay switches and now T1 (+plus) is connected to pin 2 (and-3) and T2 (-minus) connected to 4 (and-1), so the motor changes direction of rotation.

What is this SPDT DPDT SPST DPST relays

SPST Single Pole Single Throw. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total. It is ambiguous whether the pole is normally open or normally closed. The terminology “SPNO” and “SPNC” is sometimes used to resolve the ambiguity.

SPDT Single Pole Double Throw. A common terminal connects to either of two others. Including two for the coil, such a relay has five terminals in total.

DPST Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. Including two for the coil, such a relay has six terminals in total. The poles may be Form A or Form B (or one of each).

DPDT Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil.

source: https://en.wikipedia.org/wiki/Relay