When looking at different types of relays, it is first important to classify by the number of poles and throws. The most common types of relays are: SPST (single pole, single throw), SPDT (single pole, double throw), and DPDT (double pole, double throw). The number of switches inside a relay refer to the poles of the relay, while the number of circuits being controlled is known as the throws. We will briefly cover the difference between these types of relays, then take a closer look at common relay forms.
What are the Different Types of Relays?
- Single Pole Single Throw (SPST): SPST relays have one pole and one throw. The single pole can only control one circuit. While the single throw means that the single pole has only one position from which it can conduct. Therefore, it is either connected, closing the circuit, or disconnected, causing an open circuit.
- Single Pole Double Throw (SPDT): Similarly, SPDT relays have a single pole to control one circuit at a time. Whereas, the double throw refers to the relay having two positions where it can conduct. Based on the position of the pole, one throw is normally open (NO) while the other is normally closed (NC). A SPDT has two possible output connections, providing the opportunity to be connected to one output circuit, or the other.
- Double Pole Double Throw (DPDT): The DPDT is a specialty relay, also known as a motor reversing relay, that can control two isolated individual circuits. The double throw refers to the poles being able to conduct in two separate places. It is important to note that a DPDT can be used as a single throw relay when one side is not connected.
What are the Most Common Relay Forms?
Finally, it is important to understand the relay form. Relay forms are not only categorized by the number of poles and throws, but also by the default position of the relay. In the table below, we cover the standard forms for relays.To improve user convenience, our web store allows for filtering based on relay forms. You can find products available in the following forms: 1 Form A, 1 Form C, 2 Form A, and 2 Form C.