Instruction manual for devices of the MASI20 series
12
V 1.0
2.4.3 Voltage drop
System-related limit values regarding system power supply must be strictly observed if maximum
functional safety and fault-free operation are to be ensured.
Always ensure that the system voltage, measured at the slave furthest away from
the system power supply, does not drop below 26,5V DC.
A voltage drop, dependent on the load current, results in the system cable as the sensors and actuators
are supplied centrally from the AS-Interface power supply unit.
In critical cases, optimisation can be achieved by changing the location of the
system power supply unit within the system layout and by using power supply
cables of a greater conductor cross-section (only for round cables).
AS-Interface modules with digital inputs support the direct connection of commercially available sensors.
The voltage needed to operate the sensors is supplied by a slave internal power supply taken from the
bus.
With the lowest admissible supply voltage at slave level and with regard to voltage drops in the slave,
optimal power supply of sensors is always guaranteed.
The sensor power supplies of different AS-Interface modules supplied by the bus
cable are not grounded and should in no case be coupled !
2.4.4 Cable cross-sections
The chief determining factor in selecting a suitable transmission cable in regard to energy transfer is the
DC resistance.
Initial conditions :
Ÿ
the cable length is 100 m
Ÿ
31 slaves are connected to the cable at regular intervals, and
Ÿ
the current consumption of 65 mA is the same for all slaves (total current : 2 A).
The table below shows the resulting drop of voltage between the power supply inputs and the
connecting point of the last slave:
Cable cross-section in mm²
0,75
1,5
2,5
DC resistance in mOhm/m
52,0
26,6
16,0
Max. voltage drop in en V
5,4
2,7
1,6
Table 2.4–1 :
Influence of various cross-sections
