Siemens S7-300 - Applications and Wiring Schemes; Calculation of the Residual Supply Voltage at the Transducer

Analog modules
9.3 SM 336; F-AI 6 x 0/4 ... 20 mA HART
Fail-safe signal modules
Installation and Operating Manual, 01/2010, A5E00085586-10
239
9.3.4 Applications and wiring schemes
Applications
The following applications are associated with the following wiring schemes:
Wiring schemes Application
A B C D E F G H
3: 1oo1 evaluation X X X X
4: 1oo1 evaluation, transducer not redundant, high availability X X X X
1oo2 evaluation X X X X 5:
1oo2 evaluation, transducer via 1-channel X X X X
6: 1oo2 evaluation, transducer via 1-channel, high-availability,
transducer not redundant
X X X X
7: 2oo3 evaluation, high-availability X X X X
8: 2oo3 evaluation, high-availability, transducer not redundant X X X X
9: 2oo3 evaluation with 3 modules X X X X
9.3.4.1 Calculation of the residual supply voltage at the transducer
C
alculation of residual supply voltage
Determine the following voltages, according to your wiring scheme, for the purpose of
calculating the residual supply voltage:
● Determine the minimum supply voltage (U
V_min
):
Power supply, e.g., for SITOP 24 V ±2%
U
V_min
= U
V
– |Tolerance| = 24 V – 2% = 23.5 V
(neglecting the voltage drop at the supply line to the module)
● Calculate the maximum voltage drop at the signal line (U
line
):
e.g. for copper line 500 m with ∅ = 0.5 mm²; with I
max
= 25 mA
5
/LQH

P
 ˖
PP

PPP

˖
U
Line
= 35.7 Ω × 25 mA = 0.9 V
● Calculate the voltage drop at the input resistance (U
Ri
) of the module:
– With R
i_max
= 175 Ω; with I
max
= 25 mA
U
Ri
= 175 Ω × 25 mA = 4.4 V
– When Zener diodes are used in a high-availability application, this higher voltage shall
be used, taking into account the tolerances.
e.g., when using Zener diodes with 5.6 V and 6.2 V:
U
Z_5V6
= U
Z
+ Tolerance = 5.6 V + 5% = 5.9 V
U
Z_6V2
= U
Z
+ Tolerance = 6.2 V + 5% = 6.5 V