Relays

A relay is an electromagnetic switch operated by a relatively small electric current that can turn on or off a much larger electric current. The heart of a relay is an electromagnet (a coil of wire that becomes a temporary magnet when electricity flows through it). You can think of a relay as a kind of electric lever: switch it on with a tiny current and it switches on (“leverages”) another appliance using a much bigger current. Why is that useful? As the name suggests, many sensors are incredibly sensitive pieces of electronic equipment and produce only small electric currents. But often we need them to drive bigger pieces of apparatus that use bigger currents. Relays bridge the gap, making it possible for small currents to activate larger ones. That means relays can work either as switches (turning things on and off) or as amplifiers (converting small currents into larger ones).

How relays work

Here are two simple animations illustrating how relays use one circuit to switch on a second circuit.

A simple animation showing how a relay uses electromagnetism to link two circuits.

When power flows through the first circuit (1), it activates the electromagnet (brown), generating a magnetic field (blue) that attracts a contact (red) and activates the second circuit (2). When the power is switched off, a spring pulls the contact back up to its original position, switching the second circuit off again.

This is an example of a “normally open” (NO) relay: the contacts in the second circuit are not connected by default, and switch on only when a current flows through the magnet. Other relays are “normally closed” (NC; the contacts are connected so a current flows through them by default) and switch off only when the magnet is activated, pulling or pushing the contacts apart. Normally open relays are the most common. 

Here’s another animation showing how a relay links two circuits together. It’s essentially the same thing drawn in a slightly different way. On the left side, there’s an input circuit powered by a switch or a sensor of some kind. When this circuit is activated, it feeds current to an electromagnet that pulls a metal switch closed and activates the second, output circuit (on the right side). The relatively small current in the input circuit thus activates the larger current in the output circuit:

Animation showing how an electromagnetic relay works

  1. The input circuit (blue loop) is switched off and no current flows through it until something (either a sensor or a switch closing) turns it on. The output circuit (red loop) is also switched off.
  2. When a small current flows in the input circuit, it activates the electromagnet (shown here as a dark blue coil), which produces a magnetic field all around it.
  3. The energized electromagnet pulls the metal bar in the output circuit toward it, closing the switch and allowing a much bigger current to flow through the output circuit.
  4. The output circuit operates a high-current appliance such as a lamp or an electric motor.

Relays in practice

A typical relay with its plastic outer case removed, photographed both from in front and from directly above, looking down into the switch mechanism.

Photo: Another look at relays. Top: Looking straight down, you can see the spring contacts on the left, the switch mechanism in the middle, and the electromagnet coil on the right. Bottom: The same relay photographed from the front.

Relays don’t always turn things on; sometimes they very helpfully turn things off instead. In power plant equipment and electricity transmission lines, for example, you’ll find protective relays that trip when faults occur to prevent damage from things like current surges. Electromagnetic relays similar to the ones described above were once widely used for this purpose. These days, electronic relays based on integrated circuits do the same job instead; they measure the voltage or current in a circuit and take action automatically if it exceeds a preset limit.

 

Other types of relays

What we’ve looked at so far are very general switching relays—but there are quite a few variations on that basic theme, including (and this is by no means an exhaustive list):

  • High-voltage relays: These are specifically designed for switching high voltages and currents well beyond the capacity of normal relays (typically up to 10,000 volts and 30 amps).
  • Electronic and semiconductor relays (also called solid-state relays or SSRs): These switch currents entirely electronically, with no moving parts, so they’re faster, quieter, smaller, more reliable, and last longer than electromagnetic relays. Unfortunately, they’re typically more expensive, less efficient, and don’t always work as cleanly and predictably (due to issues like leakage currents).
  • Timer and time-delay relays: These trigger output currents for a limited period of time (usually from fractions of a second to about 100 hours, or four days).
  • Thermal relays: These switch on and off to stop things like electric motors from overheating, a bit like bimetallic strip thermostats.
  • Overcurrent and directional relays: Configured in various different ways, these stop excessive currents from flowing in the wrong direction around a circuit (typically in power-generation, distribution, or supply equipment).
  • Differential protection relays: These trigger when there are current or voltage imbalances in two different parts of a circuit.
  • Frequency protection relays (sometimes called underfrequency and overfrequency relays): These solid-state devices trigger when the frequency of an alternating current is too high, too low, or both.

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