Page 169 - Advanced Course

KNX ADVANCED COURSE

of the actuator and dimming step width and send interval of the controller should match
precisely:
If e.g. 1/64 dimming telegrams are sent every 2 seconds, the dimmer must be set so that
it makes a full cycle of 0 – 100% in 128 sec and no faster.
If it should also be possible to operate the dimmer manually, a shorter dimming period is
of course required. Differences can therefore be found in this regard between the
available devices as not all of them offer this possibility (see the note in the next section).

4.5 Parameterisation Notes, Flags, Possible Errors, Bus Load etc.

The previously named condition (dimming period of 128 sec) is optimised for the control
system but not for manual operation. A compromise must often be made: the actuator
dims at a slightly slower rate e.g. in 8 sec and the controller transmits at a slightly faster
rate to avoid stepwise brightness processes. This increases the bus load however.
Alternatively 1/32 is used as a step width instead of 1/64 to reduce the bus load again.
This produces a further problem in the case of very bright luminaires (over-dimensioned or
still new). The changes in brightness can already be greater than the set hysteresis after a
single step of 1/32. This results in continuous fluctuations as the luminaire continually
exceeds or falls below the target value.

500 lx Hysteresis
Interior
lighting

External brightness

Figure 3: Mismatch of dimming step width and hysteresis

The extension of the hysteresis or reduction of the dimming step width can provide some
help:

500 lx Hysteresis

Interior
lighting

External brightness

Figure 4: Correct ratio of dimming step width and hysteresis

Home and Building Management Systems KNX Association
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