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How Relays Work
by Marshall Brain

A relay is a simple electromechanical switch made up of an electromagnet and a set of contacts. Relays are found hidden in all sorts of devices. In fact, some of the first computers ever built used relays to implement Boolean gates.

In this edition of HowStuffWorks, we will look at how relays work and a few of their applications.

Relay Construction
Relays are amazingly simple devices. There are four parts in every relay:

  • Electromagnet
  • Armature that can be attracted by the electromagnet
  • Spring
  • Set of electrical contacts
The following figure shows these four parts in action:

In this figure, you can see that a relay consists of two separate and completely independent circuits. The first is at the bottom and drives the electromagnet. In this circuit, a switch is controlling power to the electromagnet. When the switch is on, the electromagnet is on, and it attracts the armature (blue). The armature is acting as a switch in the second circuit. When the electromagnet is energized, the armature completes the second circuit and the light is on. When the electromagnet is not energized, the spring pulls the armature away and the circuit is not complete. In that case, the light is dark.

When you purchase relays, you generally have control over several variables:

  • The voltage and current that is needed to activate the armature
  • The maximum voltage and current that can run through the armature and the armature contacts
  • The number of armatures (generally one or two)
  • The number of contacts for the armature (generally one or two -- the relay shown here has two, one of which is unused)
  • Whether the contact (if only one contact is provided) is normally open (NO) or normally closed (NC)

Relay Applications
In general, the point of a relay is to use a small amount of power in the electromagnet -- coming, say, from a small dashboard switch or a low-power electronic circuit -- to move an armature that is able to switch a much larger amount of power. For example, you might want the electromagnet to energize using 5 volts and 50 milliamps (250 milliwatts), while the armature can support 120V AC at 2 amps (240 watts).

Relays are quite common in home appliances where there is an electronic control turning on something like a motor or a light. They are also common in cars, where the 12V supply voltage means that just about everything needs a large amount of current. In later model cars, manufacturers have started combining relay panels into the fuse box to make maintenance easier. For example, the six gray boxes in this photo of a Ford Windstar fuse box are all relays:


In places where a large amount of power needs to be switched, relays are often cascaded. In this case, a small relay switches the power needed to drive a much larger relay, and that second relay switches the power to drive the load.

Relays can also be used to implement Boolean logic. See How Boolean Logic Works for more information.

For more on relays and related topics, check out the links on the next page.

Lots More Information!

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