A relay is a simple
electromechanical switch made up of an electromagnet
and a set of contacts. As simple as relays are however, they are
found hidden in all sorts of devices. In fact, some of the first
computers ever built used relays to implement Boolean gates. 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:
- An electromagnet
- An armature that can be attracted by the electromagnet
- A spring
- A set of electrical contacts
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
different 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 1 or 2).
- The number of contacts for the armature (generally 1 or 2 -
the relay shown here has 2, one of which is unused).
- Whether the contact (if only 1 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, for example from a small dashboard switch or a low-power
electronic circuit - to move an armature that is able to switch a
much large 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 6 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. So 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 the
article on Boolean logic
for more information.
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