3UG4622-1AW30 Datasheet, PDF (112/114 Pages) List of Unclassifed Manufacturers – Monitoring and Control Devices

Interface Converters
3RS17 interface converters
■ Overview
Interface converters perform the coupling function for analog
signals on both the input side and the output side. They are
indispensable when processing analog values with electronic
controls. Under harsh industrial conditions in particular, it is often
necessary to transmit analog signals over long distances. This
means that electrical isolation is essential due to the different
supply systems. The resistance of the wiring causes potential
differences and losses which must be prevented.
■ Function
Active interface converters
Active interface converters provide maximum flexibility for the
application by the use of an external supply voltage.
Configuration with active interface converters is extremely easy
because input and output resistances and voltage drops are
compensated by the auxiliary supply. They support complete
voltage isolation as well as conversion from one signal type to
another or reinforcement. The load of the measured value
transmitter is negligible.
Passive interface converters
Passive interface converters do not require an external supply
voltage. This advantage can only be used by current signals that
are converted 1:1. Reinforcement or conversion is not possible.
The converters are used for complete electrical isolation of
current signals and to protect the inputs and outputs. Passive
isolators do not operate reaction-free, any load on the output
produces an equal load on the input. When the passive
converter is to be used, the output performance of the sensor
and the input resistance of the analog input must be analyzed.
This technique is being increasingly implemented in the case of
pure current signals.
Calculation guide for passive converters
Note: Please note the following when using passive isolators:
The current-driving voltage of the measuring transducer U_E must
be sufficient to drive the maximum current of 20 mA over the
passive isolator with a voltage loss of U_V = 2.7 V and the load R_B.
This means that:
U_B ˜ U_E = 2.7 V + 20 mA × R_B
Distribution of the voltages in the case of passive isolators
7 _B
1
7 _V = 2 ,7 V
1
7_E
4_B
1
1
Electromagnetic disturbance and overvoltages can affect the
signals on the input side in particular or even destroy the analog
modules. All terminals of the 3RS17 interface converters are safe
up to a voltage of 30 V DC and protected against switching
poles. Short-circuit protection is an especially important function
for the outputs.
The devices are EMC-tested according to
• EN 50081 (basic technical standard for emitted interference)
• EN 61000-6-2 (basic technical standard for immunity to
interference)
The analog signals comply with
• IEC 60381-1/2
Input voltage depending on the load at I_a = 20 mA
The following diagram shows the input voltage U_E as a function
of the load R_B taking into account the voltage loss U_V. If the load
is known, the y-axis shows the minimum voltage that has to be
supplied by the current source in order to drive the maximum
current of 20 mA over the passive isolator and load.
7 _E /V
2 2 ,7
20
15
10
2 ,7 ⁵
0
0
100
200
300
400
600
2
4
6
8
12
Current carrying capacity of the outputs
N S B 01313a
800 4 B /O 1000
16 7 _A/V 20
A maximum output resistance is specified for current signals.
This resistance value specifies how large the input resistance of
the next device connected in series can be as a result of the
power of the converter.
For voltage signals, the maximum current that can be drawn
from the output is the decisive factor.
2-way isolation
In the case of 2-way isolation, the input is galvanically isolated
from the output. The "null potential" of the supply voltage is the
same as the reference potential for the analog output signal.
N S B 0_01314a
IN
OUT
V cc
2 - w a y is o la tio n
3-way isolation
For the 3-path isolation, each circuit is electrically isolated from
the other circuits, i.e. input, output, and supply voltage do not
have a potential connection.
N S B 0_01343
N S B 01312a
IN
OUT
V cc
3 - w a y is o la tio n
7/112 ^{Siemens LV 1 T · 2006}