Automotive Relays Uncovered: Understanding Various Types & Forms

When selecting a relay for automotive or industrial use, it's important to understand how relays are classified. One of the key distinctions lies in the number of poles and throws. These terms define how many circuits a relay can control and how many switching positions are available.

In this article, we’ll break down the most common relay types—SPST, SPDT, and DPDT—before exploring the concept of relay forms, which are equally critical when specifying a component.

Common Relay Configurations: Poles & Throws

The number of poles refers to how many separate circuits the relay can control, while the number of throws defines how many different output paths each pole can switch between.

Here are the most widely used relay types:

• SPST - Single Pole, Single Throw: 
  • Contains one pole controlling a single circuit
  • Features only one output path: the circuit is either closed (ON) or open (OFF)
  • Ideal for simple ON/OFF switching applications

• SPDT - Single Pole, Double Throw: 
  • Features one pole that can connect to two different outputs
  • One contact is normally open (NO), the other normally closed (NC)
  • The relay switches between the two output paths depending on its energised state
  • Useful for toggling between two functions or redirecting signals

• DPDT - Double Pole, Double Throw:
  • Controls two independent circuits simultaneously
  • Each pole has two possible switching positions
  • Also known as a motor reversing relay due to its ability to change current direction
  • Can also be used as a single-throw relay if only one side is wired

Relay Forms: Contact Arrangement & Default State

Beyond poles and throws, relays are also categorised by their contact form, which describes both the switching configuration and the default (resting) state of the contacts. These forms are typically denoted by codes such as 1 Form A, 1 Form C, and so on. Understanding these designations helps engineers select the correct relay for specific load or control requirements. You can find products available in the following forms: 1 Form A, 1 Form C, 2 Form A, and 2 Form C.