5
4. SYSTEM DESIGN FOR ZONE 2
4.1 Transmitter loops
A BA324ND indicator may be connected in series
with almost any 4/20mA current loop and
calibrated to display the measured variable or
control signal in engineering units.
Fig 2 illustrates a typical application in which a
BA324ND indicator is connected in series with a 2-
wire transmitter and controller.
There are two basic design requirements:
1.
In normal operation the voltage and
current applied to the terminals 1 and
3 of the BA324ND must not exceed:
Ui
=
4V dc
Ii
=
30mA
Providing the maximum input current
in normal operation is less than 30mA,
the maximum voltage between the
indicator terminals will automatically
be limited by the internal safety
components.
2.
The 4/20mA loop must be able to
tolerate the additional 1.1V required to
operate the indicator.
Fig 2 BA324ND in a transmitter loop
In practice it is only necessary to ensure that in
normal operation the maximum current flowing in
the loop is less than 30mA.. In the example
shown in Fig 2 this current is determined by the
maximum current from the transmitter.
Considering the example shown in Fig 2, the sum
of the maximum voltage drops of all the
components in the loop must be less than the
minimum power supply voltage.
Minimum operating voltage of 2-wire Tx
10.0
Maximum voltage drop caused by controller
5.0
Maximum voltage drop caused by BA324ND
1.1
Maximum voltage drop caused by cables
0.4
---------
16.5V
Therefore at 20mA the power supply in this
example must have an output greater than 16.5V
4.2 Remote indication
The BA324ND indicator may be driven from any
4/20mA signal to provide remote indication. Fig 3
shows a typical application in which the output
from a gas analyser drives a BA324ND. Again it is
necessary to ensure that the loop complies with
the two design requirements listed in
section 4.1
