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A relay
is an electrical switch that opens and closes under
the control of another electrical circuit. In the
original form, the switch is operated by an
electromagnet to open or close one or many sets of
contacts. It was invented by Joseph Henry in 1835.
Because a relay is able to control an output circuit
of higher power than the input circuit, it can be
considered to be, in a broad sense, a form of an
electrical amplifier.
Operation
When a current flows through the coil, the resulting
magnetic field attracts an armature that is
mechanically linked to a moving contact. The
movement either makes or breaks a connection with a
fixed contact. When the current to the coil is
switched off, the armature is returned by a force
approximately half as strong as the magnetic force
to its relaxed position. Usually this is a spring,
but gravity is also used commonly in industrial
motor starters. Most relays are manufactured to
operate quickly. In a low voltage application, this
is to reduce noise. In a high voltage or high
current application, this is to reduce arcing.
If the coil is energized with DC, a diode is
frequently installed across the coil, to dissipate
the energy from the collapsing magnetic field at
deactivation, which would otherwise generate a spike
of voltage and might cause damage to circuit
components. Some automotive relays already include
that diode inside the relay case. Alternatively a
contact protection network, consisting of a
capacitor and resistor in series, may absorb the
surge. If the coil is designed to be energized with
AC, a small copper ring can be crimped to the end of
the solenoid. This "shading ring" creates a small
out-of-phase current, which increases the minimum
pull on the armature during the AC cycle.[1]
By analogy with the functions of the original
electromagnetic device, a solid-state relay is made
with a thyristor or other solid-state switching
device. To achieve electrical isolation an
optocoupler can be used which is a light-emitting
diode (LED) coupled with a photo transistor. |
