Brochure/Catalogue

Relay modules – Switching loads
Electromechanical relays are a varied and cost-effective
solution for a wide range of switching processes. They can
be used for level and power adaptation and form interfaces
between control, signalling and regulating equipment and
peripherals. In spite of rising raw material prices, they are
still very inexpensive and can be easily integrated into a wide
variety of circuit types.
Relay modules from Weidmüller are extremely reliable,
durable, and available in many different designs. The
diversity of their applications in the various industrial sectors
makes it necessary to select a suitable relay for each specic
application. The following applies: Due to their design, relay
modules are subject to mechanical and electrical wear, which
must be taken into account when relay circuits are set up.
Electrical consumers always form a mixed load with resistive,
capacitive and inductive components, although consumers
with a large inductive component are predominantly used in
practice. These include contactors, solenoid valves, motors
etc. We will take a closer look at these areas of application
over the following pages.
Switching of large AC loads
If large AC loads are switched, the relay can in principal be
operated until the specied maximum value of switching
voltage, current, or power is reached. However, when
switching AC loads, the switching voltage has a much
smaller inuence on the service life of the relay contact than
the switching current. The reason for this is that the arc that
occurs when the relay is switched off usually extinguish
automatically at the next zero crossing of the load current.
In applications with inductive loads, an effective protective
circuit should be provided, as otherwise a signicantly
reduced service life can be expected.
Switching of large DC loads
Relays can only switch off relatively small direct currents
because the zero crossing for extinguishing the arc is
missing here. The maximum direct current value is also
dependent on the switching voltage as well as on design
conditions such as contact gap and contact opening speed.
Corresponding current and voltage values are documented
in load limit curves.
With undamped inductive DC loads, these values are lower
because the energy stored in the inductance can ignite an
arc that carries the current through the open contacts. The
resulting arc signicantly reduces the service life compared
to an resistive load.
An effective contact protection circuit can increase the service
life of the contacts by 5 to 10 times compared to inductive
loads that are not or unfavorably protected. Type1N4007
freewheeling diodes are preferably suitable for this purpose.
Relay modules – Switching loads
Switching of very low power circuits
Low power circuits with values below 30 V/10 mA are
mainly used in applications where signals has to be
transmitted to control inputs, e.g. to a PLC. Such low loads
do not produce a sufcient arc at the contacts.
However, this arc has two important functions:
On the one hand, it ensures continuous cleaning of the
contacts; on the other hand, it can penetrate non-conductive
foreign layers at the contacts. Such foreign layers are usually
created by oxidation or suldation of common contact
materials such as silver (Ag), silver-nickel (AgNi), or silver-tin
oxide (AgSnO). The foreign layers can increase the contact
resistance after a short time to such an extent that reliable
switching of low loads is no longer possible.
For these reasons, gold (Au) is used as the contact material
for relays switching small loads. It has proven itself due to
its low and constant contact resistance and its resistance to
ambient air containing sulphur.
W
Technical appendix/Glossary
W.72737920000