BECKHOFF DK9222-0310-0010 User Manual

I/O

Application Note DK9222-0310-0010

Bus Terminals

Keywords

Lighting control
Universal dimmer
Low-voltage halogen dimmer
Ethernet dimmer
Leading edge phase control
Trailing edge phase control
Short-circuit-proof
Network-capable
KL2751
KL2761

Basic principles of the dimmer function and network
capability

This application example describes the basic principles of the dimming of light, the important aspects

concerning the individual types of dimmer (leading edge phase control, trailing edge phase control,

universal) and the advantages of the use of a network-capable universal dimmer.

Basic principles

In order to dim the intesity of a light bulb, the flow of current is reduced, which corresponds to a regulation of the brightness.

Three principles can be employed for this: voltage divider, leading edge phase control and trailing edge phase control. The

voltage divider is not used because of its energetic inefficiency: the voltage for the light generation is divided by an adjustable

pre-resistor; the proportion of the unused power to generate light is dissipated by the resistor and converted into heat energy.

Phase control dimmers work considerably more efficiently, since in this case the current is switched on and off by means

of electronic circuits. The light bulb is switched at a frequency that is not discernable to the eye. Since the flow of current is

interrupted during the dead time, the power dissipation is considerably reduced in comparison with the voltage divider. The

ratio of the switch-on time to the switch-off time determines in both principles the flow of current or the quantity of emitted

light. The basis of both principles is the sine wave of the mains voltage. At a frequency of 50 Hz the voltage changes its polarity

100 times per second; it therefore also reaches the zero crossing point, at which there is freedom from both current and

voltage for a brief moment, 100 times per second.

Dimmer types

Electronic phase control dimmers control the effect produced by the light source by only letting the current flow over a certain

section of the alternating voltage half-waves. In the case of 230 V general-purpose lamps (‘incandescent lamps’), the operating

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Beckhoff

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Application Note DK9222-0310-0010

R, L, C R, L, C R L C

t

u

i

Leading edge phase control

Voltage u,

Current i

Bus Terminals

mode of the dimmer is unimportant because of the purely ohmic load. For all illuminants that are connected via electronic

ballasts (EBs), such as low-voltage halogen spots, the construction of the transformer is decisive for the applied control

principle. It should also be noted that mixed loads are not controlled: an ohmic load cannot be controlled together with an

inductive load in a the same circuit, even if the dimmer is suitable for both load types. The dimmer type and the type of the

connectable loads are identifiable by pictograms.

Fig. 1 Meaning of the pictograms: R = ohmic, L = inductive, C = capacitive, descending triangle = leading edge phase control,
ascending triangle = trailing edge phase control

I/O

Inductive loads – leading edge phase control

Low-voltage halogen lamps with conventional, inductive (= wire-wound) transformers are controlled by thyristor dimmers

based on leading edge phase control. In the leading edge phase control technique, the switch-on point of the switch is changed

in relation to the mains voltage half-wave. The thyristor thereby becomes conductive at a controllable point within the voltage

half-waves; the flow of current is automatically interrupted at the next zero crossing of the sine half-wave. This ensures that no

inductive voltage peak occurs when switching off.

Fig. 2 Change of switch-on point with leading edge phase control

Capacitive and ohmic loads – trailing edge phase control

Trailing edge phase control is used with electronic low-voltage transformers. In the trailing edge phase control technique, the

switch-off point of the switch is changed in relation to the mains voltage half-wave. The current begins to flow exactly at the

zero crossing of the voltage wave; the transistor dimmer terminates the flow of current at a controllable point within the half-

wave. Advantages of this circuit: the flow of current can be interrupted at any time; very accurate control is possible and the

flow of current is also interrupted immediately in the case of an overload or a short-circuit. The generation of current peaks on

input capacitors of the EBs is avoided, since the flat rise of the sine wave is used in order to charge the capacitor. The voltage

For application notes see disclaimer on the last page

Beckhoff

New Automation Technology

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