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Solid-state relays - functionality
Solid-state relays are electronic components (switching
elements) that are used to switch a load circuit via a control
circuit. First of all, they allow applications with varying power
ratings to be switched with relatively minimal switching
currents. Secondly, they provide galvanic isolation for the
switching and load levels in order to protect components in
the event of malfunctions.
In contrast to electromechanical relays, solid-state relays
do not have any mechanical parts prone to wear. The
core element of a solid-state relay is an optocoupler. In an
optocoupler, a light signal is triggered in the control circuit
for the switching process via an LED, which causes a light-
sensitive semiconductor receiver to close a connected load
circuit to switch on the downstream switching amplier.
The transmitter (LED) and receiver (e.g. a phototransistor) of
the optocoupler are embedded in a light-conducting plastic
material and surrounded by a light-proof casing that protects
against outside inuences.
Solid-state relays – Overview
Switching amplication is required because optocoupler
can only switch comparatively small voltages and currents.
The combination of an optocoupler with the switching
amplication at the output means that this is now called a
solid-state relay.
The output of a solid-state relay
A voltage range is usually specied for the nominal
switching voltage of solid-state relays (e.g. 5...48 V DC),
which must neither be exceeded nor fallen short of.
The same applies to the continuous current. Frequently
exceeding this value can lead to premature wear and to
destruction of the semiconductor. Short surge voltages
(voltage peaks) are eliminated by appropriate protective
components such as diodes or varistors.
Depending on the output circuit with the appropriate
amplier semiconductor, either AC or DC loads can be
connected
Control circuit Load circuit
Transmitter
(LED)
Receiver
(Transistor)
Control circuit Load circuit
Optocoupler
Switching amplication
Solid-state relay
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Technical appendix/Glossary
W.8 2737920000
Solid-state relays – Overview