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Digital lighting control: DALI or KNX?

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The digital lighting control common in Europe, but also worldwide, is DALI (Digital Addressable Lighting Interface).

DALI consists of a number of technical standards in IEC 62386 and compliance with these standards ensures the interoperability of equipment from different manufacturers.

Another system with which it is possible to digitally manage and control lighting, as well as manage building automation, is the KNX standard, the first open building automation standard.

In this case, devices from different manufacturers can interoperate without problems because all products are tested by a neutral third-party test laboratory to ensure interoperability.

Minimum attenuation level: DALI vs. KNX

The human eye does not perceive changes in light levels in a linear fashion and slight variations in perception often occur between individuals. This response causes a difference between measured and perceived light levels.

Three control curves are selectable: Linear, DALI and log, as found in the ETS application database for the KNX PWM driver.

The linear curve providesdriver output equal to the command sent by the button.

The DALI curve, on the other hand, is based on the formula specified in the DALI standard which offers the user exceptional dimming, this is because the light difference between the steps is designed to be constant and the minimum attenuation level is 0.1% (equivalent to 3% of perceived light) according to the DALI standard.

The log curve provides the logarithmic response. It is worth mentioning that the minimum output of the log curve provides only 0.01% (equivalent to 1% of perceived light) which opens up the possibility for some applications where low luminescence is required.

However, the user is free to select one of these three curves to obtain the best result.

The depth attenuation performance test performed using a 24 Vdc LED strip shows that it is possible to achieve a better low light intensity using the Log curve of the KNX PWM driver compared to the driver based on the DALI standard.

PWM attenuation frequency

The light change of the constant voltage LED lamp is generally realised using PWM (Pulse Width Modulation) by adjusting the current duty cycle, which results in changes to the typical current.

PWM is effective for precise light dimming requirements handling high dimming ratios at higher frequencies to avoid light flickering and stroboscopic effect, while maintaining minimal effects at the LED's colour temperature.

The generic DALI LED driver has a fixed output frequency of several hundred Hertz depending on the manufacturers design. Although a higher frequency design is preferred, it is a compromise to maintain product reliability due to the higher switching loss in the component within the driver.

The KNX PWM driver has a unique design according to which the output frequency can be changed by the user between 200 Hz and 4000 Hz and avoids the frequency below 8 0Hz, for which a flicker visible to the human eye is detected.

The 4000 Hz PWM frequency meets not only IEEE 1789-2015, but also the SVM (Stroboscopic Visibility Measure) for the new eco-design, which will come into force in the European market from 1 September 2021.

The SVM formula provides the summarised weighted spectrum for all frequency components up to 2000 Hz, which means that PWM at 4000 Hz is far beyond concern.

 

Equation 1: Ecodesign strobe visibility measurement requirement formula in from 1 September 2021.

In this case Ci is the relative amplitude of the i-th Fourier component; Ti is the visibility threshold for the strobe effect of a sine wave at the frequency of the i-th Fourier component.

KNX constant voltage LED driver applications

There are four types of application.

First of all, the LED strip as a typical application.

Secondly, a traditional 12 Vac lamp, but using LED technology (also available as a 24Vdc LED spot).

Finally, an additional DC/DC converter can be used to drive a constant current LED lamp.

LED strip applications
After ETS programming, the driver is positioned at the end point and the LED strip is connected to the driver output.

The KNX cable is then connected for telegram commissioning followed by connection to the 230 Vac mains voltage.

When the driver receives the command from the push button or any KNX input device, the light is controlled to switch on, off or dim.

12 Vac LED lamp control application for spot lighting
The advantage of this application is that even the end user could replace the lamp without safety problems.

The common base of the lamp pins is GU4, GU5.3, G4 and G53 and the conventional way to drive these lamps in the KNX system is to use a phase cut dimmer combined with a transformer.

Connection is very simple and several lamps can be connected in parallel. It is only necessary to program the driver on/off function if non-dimmable lamps are used.

For the dimmable lamp, the driver can be programmed to perform on/off and dimming functions.

The dimming range also depends on the type of lamp and the user can adjust the driver to suit, achieving the best performance using the ETS program.

24 Vdc LED lamp control application for spot lighting
The advantages are similar to the 12 Vac described above, in addition to the fact that the driving voltage is 24 Vdc.

It is possible to use the 24 Vdc LED driver to implement on/off and dimming functions and, again, several lamps can be added in parallel.

The video below shows a 60 W driver for several 24 Vdc LED lamps in parallel.

Theminimum output of the driver is set to 0.01% for some applications where low luminescence is required.

DC/DC converter to drive a constant current LED lamp
The last case concerns the possibility of being able to drive an LED module or a constant current lamp with a DC/DC converter between constant voltage driver and LED.

MeanWell provides the LDD, LDB and LDH series for use depending on whether step-up or step-down mode is required and it is also possible to add several lamps in parallel.

Conclusions

In conclusion, the KN PWM series constant voltage drivers are an optimal solution to avoid not only the incompatibility between DALI gateway and driver, but also offer many more functions to the benefit of the lighting system. In addition, the system is simplified without using the transformer as traditionally used, which means that the cost is reduced.

For information on Mean Well click here

For further information click here

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