Buzz
Main Points to Remember
- Relays employ an electromagnet to switch circuits on and off mechanically, enabling a small current to control a much larger one.
- Widely used in household devices and vehicles, relays help electronic systems manage high-power components like motors and lights with ease.
- They can also be configured in sequences or used for Boolean logic operations, allowing for sophisticated control of complex electrical systems.
A relay is a basic electromechanical switch consisting of an electromagnet
and a series of contacts. Relays are embedded in a vast array of devices, and some of the earliest computers relied on them to execute Boolean gates.
This article will explore the functionality of relays and some of the practical uses they have.
Construction of a Relay
Relays are surprisingly straightforward devices. Every relay contains four key components:
- Electromagnet
- Armature that responds to the electromagnet's pull
- Spring
- Set of electrical contacts
The diagram below illustrates how these four components function together:
This diagram shows that a relay consists of two distinct and independent circuits. The first circuit, located at the bottom, powers the electromagnet. A switch controls the power to this electromagnet. When the switch is turned on, the electromagnet activates and pulls the armature (blue). The armature then functions as a switch in the second circuit. When the electromagnet is energized, the armature completes the second circuit, turning the light on. When the electromagnet is off, the spring pulls the armature away, breaking the circuit and turning the light off.
When purchasing relays, you usually have control over a few key specifications:
- The voltage and current required to activate the armature
- The maximum voltage and current that can flow through the armature and its contacts
- The number of armatures (usually one or two)
- The number of contacts for the armature (typically one or two — the relay shown here has two, with one unused)
- If the contact (in case there is only one) is normally open (NO) or normally closed (NC)
Uses of Relays
Essentially, the purpose of a relay is to use a small amount of power from the electromagnet — typically supplied by a low-power electronic circuit or a small dashboard switch — to activate an armature that can control a much larger power load. For example, the electromagnet may require just 5 volts and 50 milliamps (250 milliwatts), while the armature is capable of handling 120V AC at 2 amps (240 watts).
Relays are commonly found in household appliances, where an electronic control is used to activate devices like a motor or a light. They're also widespread in vehicles, as the 12V supply voltage demands large amounts of current for most systems. In modern cars, manufacturers have begun integrating relay panels into the fuse box to simplify maintenance. For instance, the six gray boxes in this image of a Ford Windstar fuse box are all relays:
In situations requiring the switching of substantial power, relays are often cascaded. This means a small relay activates the power needed to drive a larger relay, which then controls the power to drive the load.
Relays can also be utilized to implement Boolean logic. For more details, see How Boolean Logic Works.
