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SPDT Relay: Quick Guide to its Working and More!

A relay is an electronic switch. Like all circuits, it has a control signal and input signals. Also, the controller has an electric circuit consisting of a coil wrapped around a soft iron core that operates it. The switch receives input signals from a live power source with control connections and then controls the output.  This article will help you understand how the spdt relay operates. You will also learn how to connect the relay in a circuit.

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What is Spdt Relay?


Spdt is short for single pole double throw. It is beneficial in modern applications because of its internal configurations. It consists of five control terminals. Two electromagnetic relay coils, one common terminal (CM), one normally closed terminal(N/C), and one normally open terminal(N/O). We can always refer to these terminals as contacts. From these features, we find that the spdt relay coil has two different configurations making it unique from other control relays. 

When we have continuous power applied on the coil of wire wrapped around the solenoid, the normally open and standard terminals get charged. When there is no coil current, the magnetic circuit is relaxed, the common terminal and the normally closed terminal get assigned. 


an electric switching relay

(an electric switching relay.)


How Does an Spdt Relay Work? 


The spdt relay has two electrical contacts in position.


(circuit explaining how the spdt relay works when there is no current flow within the coil.)  


Diagram (1) above represents an electrical circuit with no direct current voltage passing through the single-coil form. As a result, the T terminal connects to the first contact providing contact resistance. This connection makes the dc flow through fan one but not fan 2.  


(circuit explaining how the spdt relay operates when flowing within the coil.)  


On the other hand, diagram 2 explains current ratings in the electromagnetic coil. When the T terminal connects to get 2, the current initially flowing through fan 1 has a broken rating making the diversion open 2. Consequently, this causes the T terminal to connect to contact 2, providing contact resistance. Hence the current now flows through fan two and not fan 1. 

The accurate operating characteristic of the spdt can be the ability of the relay to switch between two magnetic circuits. However, this is through standard contact ratings and works as a control panel.


(a manual switch.)


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How to Connect Spdt Relay in the Circuit


This movable iron armature has five coil terminal points. These terminals include the two insulated coils, a common terminal, a normally open terminal, and a normally closed terminal.

One insulated coil represents one side of the circuit while the other is the opposite end. These terminals are where we apply the input power. When we apply the input power, polarity might be an issue. Importantly, using power on these relay coil terminals will make the switch close. 

However, in this case, polarity should not be an issue. It should work since one end becomes the positive voltage while the other becomes the negative one. It would help if you only worried about polarity when using a diode.


Understanding the spdt terminals(N/O, N/C, COM)


N/O is otherwise known as the normally open terminal. We connect the device that needs power on this switch. This device will remain off as long as the relay is out of the magnetic flux range. Once the truck is within the field at deactivation, the device turns on. 

N/C is otherwise known as the normally closed switch. Also, it works contrary to the normally open button. Its operating principle is to connect the already powered device on this switch when no coil current flows in the electromagnetic coil. The device connected to this switch will go off when the circuit track is within the magnetic field. Otherwise, the device will turn on.


an electronic printed circuit board

(an electronic printed circuit board.)


COM, the common terminal in full, is an essential relay feature. It is the terminal where we connect the first part of the circuit. However, if the relay has no first part, we can leave this terminal open. Moreover, something notable occurs when the relay is within the magnetic field and the switch is closed. Both the COM and the N/O will have a continuous rating. However, if the relay is outside the magnetic field and the control remains closed, the opposite happens. The COM and the N/C will now have the continuous rating.

After understanding how each terminal operates, we connect the relay to a circuit to light up an LED. Otherwise, the red LED goes off, and the green one lights up when we connect the relay to a power supply. Notably, the red LED lights up only when a current application is working the terminal's explanations.

Also, the relay only needs a low voltage application of 12v or less to power it. However, the 12v coil voltage is the maximum voltage rating the relay requires.  

an LED bulb

(an LED bulb)


SPDT Relay vs DPDT Relay


While spdt stands for single pole double throw, the dpdt stands for double pole double throw. Also, you should know that we can use a dpdt in place of an spdt relay. However, these two relays vary in features and operation. Below are some of the differences between an spdt and a dpdt.

  • Firstly, an spdt relay has three pins, but a dpdt runs on double that of the spdt.
  • Secondly, the spdt has two circuits, whereas a dpdt has double that of an spdt.
  • Also, unlike a dpdt that powers down or up an electrical device simultaneously, an spdt relay cannot. The spdt relay can only perform one action at a time.
  • Lastly, the dpdt has two circuit mechanisms, while the spdt has a single circuit mechanism.


industrial electric motors with relays.

(industrial electric motors with relays.)


Common Problems You Will Encounter


Can I use a dpdt in place of an spdt?


It is possible to use a dpdt in place of an spdt. However, we require one to be keen when doing this. It would help if you understood that a dpdt is somewhat like having two spots in one. So, the coil connections of an electric motor to a dpdt are made on one side of the relay.  

What is the role of an spdt?


The spdt has a low reluctance path and normal circuit conditions like all typical control relays. Also, it acts as a control panel by behaving like an on-off switch, as we discussed earlier. Additionally, this relay can switch one input and connect them to two outputs. Lastly, we primarily use this relay as an automotive relay, for example, in an industrial motor starter.  


We use spdt in most electric motors. It often works with a control signal that helps to control voltage.

We hope this article has taught you what you need to know about the spdt relay. If you liked this article, consider checking out more of our articles, and please reach out to us for help.    



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