BECKHOFF DK9222-0310-0010 - Manual (Page 3)

I/O
Bus Terminals
Application Note DK9222-0310-0010
required for the load is approached ‘softly’; this accurate regulation protects the load. Trailing edge phase control dimmers are
provided with a current limiter function, which reduces the output voltage in the case of overload. Furthermore, trailing edge
phase control generates very little electromagnetic interference. Unfortunately, the types of switches used are very sensitive
and are destroyed even by brief overloading. As prevention therefore, complex protective circuits are integrated. 230 V general-
purpose lamps (ohmic loads) and lamps with EBs (capacitive loads) can be used as loads.
t
u
i
Trailing edge phase control
Voltage u,
Current i
Fig. 3 Change of switch-off point with trailing edge phase control
Universal dimmer
Trailing edge phase control cannot be used with capacitive loads, because a sudden increase in voltage would cause an
extremely high current flow. On the other hand, trailing edge phase control is not suitable for inductive loads, because a
voltage peak would occur when switching off the current. If a wire-wound transformer is operated in error with a trailing edge
phase control dimmer, then damage to the power transistor and the protective components of the dimmer is to be expected
due to the resulting inductive voltage peaks. Additionally, damage to the cable insulation and the transformer windings can
occur. If using wire-wound transformers, it is particularly important to ensure a high degree of stability and symmetry in the
leading edge phase control dimmer, in order to prevent the development of direct current components in the primary winding
of the transformers with risks of overheating and cable breakage. In order to avoid this selection problem, universal dimmers
have been developed in which both control principles are implemented. They are particularly advantageous and uncomplicated
to handle, because they automatically recognise the type of load when switching on the connected mains supply and therefore
use the appropriate control principle. However, the electrical behaviour of the connected load must also be uniform in this case.
A further advantage of universal dimmers is that a change of the lighting elements (i.e. a change of the type of load) does not
entail changing the dimmer, because it is suitable for every load (inductive, ohmic and capacitive).
Practical problem: uniform brightness regulation
In practice, the operator expects that a quarter of a turn of the rotary switch of the dimmer will also lead to a change in the
brightness by a quarter. In case of conventional phase controls without integrated characteristic, however, the expected effect
does not occur, since only the switch-on/off point is controlled and not the power. Since the brightness corresponds to the
power, a change of the switching point by 5 % does not result in an actual change in the brightness of 5 %. Depending upon
New Automation Technology Beckhoff 3
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