spdt relay switch

Single Pole Double Throw SPDT Relay

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

spdt relay

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


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  • Vaibhav

    I have a project of “beam switching antenna for sensor network operated at 2.4 Ghz frequency”,in that there’s a switching circuit which works to select on of the antenna out of 6 different antenna.
    In the paper that I m referring they have used spdt switch for beam selection,I.e the o/p of one one of the selected decoder I/p is applied to the spdt relay and one of the spdt switch volatge control terminal is held at high voltage and other is needed with o/p of decoder order to switch antenna why they have used spdt switch ,why can’t spst switch be used…does reflection of signal back into the circut takes place if we use spst switch instead of spdt?

  • prachal jadeja

    I have to move my motor in forward as well as reverse direction can i use this spdt relay for it ?

  • Tony

    Yes they are definitely odd, guessing that’s why nobody has one and can’t get it till next week lol. Everyone has the SPST NO relay which is the cooling relay and of course that isn’t the one that I need lol. Thank you again for your help!

  • Tony

    Thank you very much for the reply. That is exactly what I planned on doing but wanted to confirm it would/should work. I received some bad advice from a “professional installer” telling me to use a sequencer, told me how to wire it and that he had done it “several” times and it set my blower motor on fire lol. So of course I had to buy a blower motor and capacitor too. My understanding is that as long as the relay is rated the same or lower that it should work correctly and in the event of failure that the relay would be damaged and not anything else, is this correct?

    • Jim Keith

      The relay contact is connected in series with the motor –this sometimes is called a “permissive” circuit because the motor limits the relay contact current. Relay failure is unlikely without first having a motor failure.

      Another cause of relay contact failure could be caused by rapid cycling of the relay –such overheats and blows out the contact material. If this is happening, you should be able to hear the malfunction.

      A relay that is used in this application is often called a contactor. Also, NC contacts are truly an oddity –form “C” or DT contacts are commonly available and can be connected to perform the same function.

  • Tony

    Quick question, working on my furnace which has two relays, one for heating and another for cooling. Both relays are SPST. The heating relay is NC and the cooling relay is NO. The heating relay is bad but can’t get a SPST NC relay until next week. I can’t seem to figure out why they didn’t use a SPDT relay to begin with but my question is can I use a SPDT relay to replace the heating relay? Thanks in advance for your advice!

    • Jim Keith

      Use the SPDT relay, but do not connect the normally open terminal. They saved perhaps $0.05 by using the SPST relays…