Switching Of Inductive Loads - ProMinent DULCOMETER diaLog DACb Assembly And Operating Instructions Manual

Assembly and installation

9.4.6 Switching of inductive loads

54
If you connect an inductive load, i.e. a consumer
which uses a coil (e.g. an alpha motorised pump),
then you must protect your controller with a protec‐
tive circuit. If in doubt, consult an electrical techni‐
cian for advice.
The RC member protective circuit is a simple, but nevertheless
very effective, circuit. This circuit is also referred to as a snubber or
Boucherot member. It is primarily used to protect switching con‐
tacts.
When switching off, the connection in series of a resistor and
capacitor means that the current can be dissipated in a damped
oscillation.
Also when switching on, the resistor acts as a current limiter for the
capacitor charging process. The RC member protective circuit is
highly suitable for AC voltage supplies.
The magnitude of the resistance R of the RC member is deter‐
mined according to the following equation:
R=U/I
L
(Where U= Voltage across the load and I
load)
The magnitude of the capacitor is determined using the following
equation:
C=k * I
L
k=0,1...2 (dependent on the application).
Only use capacitors of class X2.
Units: R = Ohm; U = Volt; I
If consumers are connected which have a high
starting current (e.g. plug-in, switched mains power
supplies), then a means of limiting the starting cur‐
rent must be provided.
The switching-off process can be investigated and documented
using an oscilloscope. The voltage peak at the switch contact
depends on the selected RC combination.
Fig. 29: Switching-off process shown on the oscillogram.
= current through the
L
= Ampere; C = µF
L
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