Siemens SIMATIC 6DL1 136-6AA00-0PH1 - Calculation of the Remaining Supply Voltage at the Sensor

6.11 Calculation of the remaining supply voltage at the sensor
To ensure the function of the connected sensor, the remaining supply voltage U
min
must be
greater than/equal to the minimum supply voltage of the sensor.
To calculate the remaining supply voltage on the sensor, determine the following voltages
depending on the wiring scheme used:
• Determine the minimum supply voltage (UV
min
):
Power supply unit e.g. for SITOP 24 V ± 2%
UV
min
= UV – [Tolerance] = 24 V – 2% = 23.5 V
(Voltage drop on the supply line to the I/O module was neglected).
• Calculate the maximum voltage drop on the signal line (U
line
):
For example, for copper cable 500 m with ∅ = 0.5 mm²; at I
max
= 25 mA
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U
line
= 35.7 Ω × 25 mA = 0.9 V
• Calculate the voltage drop (U
Ri
) at the input resistance "Ri" of the I/O module:
– With Ri
max
= 150 Ω; at I
max
= 25 mA
U
Ri
= 150 Ω × 25 mA = 3.8 V
– In a high-availability application case, the voltage of the Z diodes used in the redundant
terminal block (U
Z
= 6.2 V) must also be considered with a tolerance of 5%.
U
Z_6V2
= U
Z
+ Tolerance = 6.2 V + 5% = 6.5 V
• Determine the voltage drop at the sensor supply U
sensor supply
:
U
sensor supply
= 1.2 V
Calculation examples
Example for the calculation of the remaining supply voltage (U
min
) on a 2-wire transmitter in an
application case with no high-availability:
• U
min
= UV
min
– U
line
– U
Ri
– U
sensor supply
• U
min
= 23.5 V – 0.9 V – 3.8 V – 1.2 V = 17.6 V
Example for the calculation of the remaining supply voltage (U
min
) on a 2-wire transmitter in an
application case with high availability:
• U
min
= UV
min
– U
line
– U
Z_6V2
– U
Ri
– U
sensor supply
• U
min
= 23.5 V – 0.9 V – 6.5 V – 3.8 V – 1.2 V = 11.1 V
Applications of the F-I/O module
6.11 Calculation of the remaining supply voltage at the sensor
F-AI 8xI 2-/4-wire HART HA
60 Equipment Manual, 09/2021, A5E50557845-AA