Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.20 Burden with 2-wire transducers
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
59
Example 2: Connecting a transducer and other devices
If multiple measuring devices are connected in series, the sum of all connected resistances
may not exceed the value of the maximum burden.
If you have connected a 2-wire transducer to a supply voltage of 8.5 V, you will need to allow
for the resistance of the measuring device connected
②
.
Maximum transducer burden: 820 Ω
Transducer resistance upon a voltage drop of 8.5 V: 425 Ω
Maximum resistance of additional devices connected: 395 Ω
The resistance of the measuring instrument may therefore not exceed 395 Ω.
①
2-wire transducers
②
Digital multimeter
③
Zener diode
U
V
Terminal for feed voltage at the channel
I+ Terminal for current input
U
min
Voltage drop at 2-wire transducer
R
Mess
Resistance at measuring instrument
U
Mess
Voltage drop at the measuring instrument
Figure 2-28 Connecting a transducer and other devices to the circuit
Calculating admissible voltage drop
The maximum additional resistance in the current loop may not exceed 395 Ω at a maximum
current of 20 mA. You can calculate the voltage drop at the connected measuring instrument
(U
Mess
) as follows using Ohm's Law:
U
Mess
= R
Mess
* I = 395 Ω * 0.020 A = 7.9 V
The voltage drop at the measuring instrument should therefore not exceed 7.9 V.