Brochure/Catalogue

One particular challenge for the circuits in the load circuit
of optocouplers and solid-state relays is posed by the
different load types of the possible applications (resistive,
inductive, capacitive load). Depending on the application,
it is important to be aware of the effects that these loads
have on the module being used, and how an appropriate
protective device needs to be designed.
In general, it must be ensured that the power loss at the
amplier semiconductor does not exceed a permissible
limit value over a long period of time. This would lead to
overheating and nally to the destruction of the component.
Switching resistive loads
Because the current strength in the load circuit and the
voltage across the amplier semiconductor are inversely
proportional to each other with resistive loads, these types
of loads do not usually pose a problem. In this case, it is
sufcient to observe the maximum current and voltage
strengths of the modules.
One particular case is the switching of incandescent bulbs.
Due to the low cold resistance, overcurrents of 10 to 20
times the operating current can occur when switching
on. The components must be designed for these possible
overloads, which correspond to the effect with a capacitive
load.
Switching capacitive loads
Capacitive loads occur when there is a capacitor in the load
circuit. This acts as a short circuit at the moment of switch-
on, causing a high inrush current.
Compared to many other electromechanical relay modules,
an amplier semiconductor is more robust against very
short current peaks (< 10 ms) when switching on capacitive
loads, because it does not contain any mechanical parts that
can weld together. Inrush current peaks that are too high,
too steep or too long can still lead to the destruction of the
amplier semiconductor.
Solid-state relays – Switching loads
Switching inductive loads
With inductive loads, which are mainly present when
coils are used in the load circuit, the problem arises when
switching off. Due to the current ow in the coil, a magnetic
eld builds up which then suddenly collapses and generates
a high induction voltage. An amplier semiconductor needs
to be protected from these voltage peaks, otherwise it will
be destroyed. Many solid-state relays are equipped with
protective suppressor circuits at the output, but these often
only provide protection against very small inductive loads.
Therefore, when switching inductive loads with solid-state
relays, it is highly recommended to have an additional
external protective suppressor circuit parallel to the load. This
is especially important if inductive loads are to be switched
with a frequency faster than 0.5 Hz. When switching
inductive loads faster than 5 Hz, specially designed solid-
state relays should be used.
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Technical appendix/Glossary
W.10 2737920000
Solid-state relays – Switching loads