Fema I4 Series, User Manual

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SERIES I4 · Model I4L
Section INDUSTRIAL . ISOLATED SIGNAL CONVERTERS
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A ‘ junction box ’ is a connections box where several load cells can be
connected. The ‘ junction box ’ then offers a single set of output terminals,
that will be connected to the instrument.
The ‘
junction box ’ provides 4 or 6 terminals, like a normal load cell: two
terminals for millivolt signal, two terminals for excitation voltage, and
eventually two additional terminals for ‘ sense ’ wires. If ‘ sense ’ terminals
are not present, you can connect the ‘ sense ’ wires to the excitation
voltage terminals of the ‘ junction box ’ or directly to the ‘ electrical middle
point ’ of the power wires of the load cells. Last option is to short circuit
the ‘ sense ’ terminals to the excitation voltage terminals as indicated at
section 7.2.
If the ‘
junction box ’ provides an output signal that is the addition of
each of the millivolt load cell signals, configure the instrument for the
appropriate input signal range.
Example : four load cell signals of 2 mV/V, powered at 5 Vdc, each load cell
provides a maximum of 10 mV signal. The output of the ‘junction box‘ will
be 40 mV maximum, so select the 0/40 mV input signal range.
If the ‘ junction box ’ provides the mean value of the four load cell signals,
then the input signal range must be selected to 0/10 mV.
7.6 Connections with a junction box
Table 5 | Connections with a junction box
I4L terminals
Junction box
(connections box)
signal - Vexc +
sense -
Vexc -
sense +
1 2 3 4 5 6
Using 3 load cells is the optimal way to distribute the weight on a plane,
although it is common to work with 4 load cells in applications with
tanks, hoppers and similar.
When working with multiple load cells, the optimal connection is the
one that makes the wires of the load cell converge in the same central
area, so that all the cells are at the same ‘ electrical distance ’ from the
instrument.
Use the same type load cell and connect the wires to the central area as
indicated below. Configure the instrument as indicated in this manual,
assuming that :
• the nominal weight of the system is the addition of the nominal
weight of each cell (3 x 100 Kg = 300 Kg for 3 cells, or 4 x 100 Kg = 400 Kg
for 4 cells)
• the ‘ sense ’ wires are carried to the central area together with the Vexc
wires, but are not propagated to each individual cell. If you do not want
to use the ‘ sense ’ wires, see section 7.2.
Table 6 | Direct connection to 3 load cells
Vexc+
Signal-
Sense-
Vexc-
Sense+
1 2 3 4 5 6
I4L terminals
Table 7 | Direct connection to 4 load cells
Vexc+
Signal-
Sense-
Vexc-
Sense+
1 2 3 4 5 6
I4L terminals
7. Practical information (cont.)
7.7 How to calculate the input signal range
The input signal range selected at the instrument must be able
to accept the whole range of signal that the load cell can provide.
This value is obtained by multiplying the sensitivity of the load cell
(expressed in mV/V) with the excitation voltage value, which is 5 Vdc
for this instrument.
• Load cell sensitivity = 2 mV/V
• Excitation voltage = 5 Vdc
• Maximum signal = 2 mV/V x 5 Vdc = 10 mV
• Select ‘ Input signal range ’ = 0/10 mV
• Code 011 for 4/20 mA output or code 110 for 0/10 Vdc output
7.8 Connections with 3 or 4 load cells