Control Current Influence - ABB i-bus KNX Product Manual

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ABB i-bus KNX
4.1.2

Control current influence

© 2008 ABB STOTZ-KONTAKT GmbH
Planning and application
The 1-10 V interface operates according to the current source principle.
Hereby, the ballast is a current source which generates a constant current.
The Light Controller (current sink) represents an electrical end stage, so that
the voltage value corresponds to the required dimming setting. The output
stage of the Light Controller output has been designed so that it can
counteract a maximum control current of 100 mA and generate a control
voltage of 1 V on the ballast. At a 1 V value the minimum brightness which
the ballast can set results.
If the number of ballasts produces a current greater than 100 mA, the Light
Controller output is no longer in a position to reduce the voltage on the
ballast to 1 V. In this case the minimum dimming value of the ballast can
not be set.
The same problems can occur if the control line is too long or if its cross-
section is too small. In these cases the resistance and accordingly the
voltage drop on the line is too large, so that the actual control voltage on
the ballast does not correspond to the required voltage value.
The control current generated by the ballast depends on the type and
manufacturer of the ballast. Typical currents are between 0.4 and 4 mA.
If an average control current of 2 mA is assumed, 50 ballasts can be
connected to a Light Controller output.
With a cable cross-section of 0.8 mm
is possible from the Light Controller to the ballast, or at a cross-section
2
1.5 mm 100 m is possible from the ballast to the Light Controller.
An open control line causes a voltage drop of 10 V on the ballast and affects
the maximum brightness of 100 %.
A short-circuited control line allows the voltage to collapse and the minimum
dimming value of the control device is set.
The polarity must be observed when the control line is connected. If the
polarity is reversed, the ballast can not be dimmed and the minimum
dimming value is set.
2
a simple control line length of 70 m
103