A
2401 Stanwell Drive Concord, California 94520 Ph: 925/687-4411 or 800/542-3355 Fax: 925/687-3333 www.calex.com Email: sales@calex.com
1
4/2001
Model 465 Bridgesensor
Features
!
Rugged, Compact and Fully Encapsulated
!
Complete System - Just Add AC Power
!
Ready to Use with Screwdriver Wiring
!
Stable and Accurate
Applications
!
Weighing with Load Cells
!
Long Term Structural Monitoring
!
Process Control Pressure Transducers
!
Low Frequency Strain Measurements
0
20
40
60
80
100
120
140
160
LOAD CURRENT (mA)
4
6
8
10
12
14
16
B+ OUTPUT VOLYAGE (VOLTS)
B+ VOLTAGE vs LOAD CURRENT
SAFE OPERATING AREA
FIGURE 1. Bridge Supply - Safe Operation
Transducer Excitation
Transducer bridge excitation is provided by an AC line powered,
adjustable, well regulated, low noise power supply. The
excitation voltage is adjusted by means of a molded-in
potentiometer, which allows the output voltage to be varied
from 4 to 15 Volts. The output ripple is extremely low and the
line and load regulation are 0.05%.
The sense lines minimize variations in output voltage with
changes in load current or lead resistance. It should be noted
that if the sense feature is not being used, terminal 1 must be
connected to terminal 2. Terminals 3 and 4 must be connected
also. The supply will provide up to 150mA of output current.
These features make the unit ideal for use with most common
strain gage bridge circuits of 120, 350, and 500 Ohms.
The supply has short circuit protection to protect it against
short term faults. The output recovers automatically from
short circuit conditions once the short is removed.
Safe Operation in the Unsafe Area
The curve (Figure 1) describes the region of safe operating
output current at each voltage setting level. If excitation is
desired at the 4 Volt level with 150mA, this can be accomplished
by using a resistor of at least 2 Watts in size in series with the
bridge. The right value resistor would drop 11 Volts allowing
4 Volts at 150mA across the bridge while minimizing internal
heating of the Model 465. The + sense line would regulate the
supply to the required 4 Volts. Normal operation of single
bridges at 10 Volts and under 100mA (all 120, 350, and 500
Ohm bridges) can be accomplished within the safe operating
area.
Instrumentation Amplifier
The built-in amplifier is a true differential input, low noise, low
drift, instrumentation amplifier. It has a high common mode
rejection ratio (CMRR) and is provided with an output offset
that is potentiometer adjustable. The minimum gain setting of
the amplifier is 40 and the maximum gain using the built-in
potentiometer is 250. The gain may be adjusted up to a
maximum of 1,000 by means of an external resistor connected
across terminals 12 and 13. The size of the external resistor
can be calculated using the gain formula in the specifications.
When doing this the coarse gain potentiometer should be
turned fully clockwise. The fine gain pot can then be used for
final gain adjustment. The output offset adjustment range is
0.5V.
The amplifier can withstand input voltages up to 15 Volts
without damage. The output of the amplifier is filtered to be
3 dB down at 3 Hz using a double pole Butterworth response
filter to minimize the effects of high frequency electrical and
physical noise on the system. The output of the amplifier is
10 Volts at 5mA making it compatible with modern data
acquisition techniques and systems.
Description
The Model 465 is a self contained, AC powered, signal
conditioning module for bridge type instrumentation. It contains
a precision differential instrumentation amplifier with filtered
output and a highly regulated, low noise, adjustable output
bridge excitation source. The unit is completely encapsulated
for use in rugged environments.
A
2401 Stanwell Drive Concord, California 94520 Ph: 925/687-4411 or 800/542-3355 Fax: 925/687-3333 www.calex.com Email: sales@calex.com
2
4/2001
Model 465 Bridgesensor
Specifications
(Typical @ 25C unless noted)
Getting Started with the Model 465
I.
Hook Up Procedure
A. Connect the + out of your load cell to the + INPUT,
pin 10.
B. Connect the - out of your load cell to the - INPUT,
pin 11.
Note:
If the SENSE are not used in your load cell application,
the connections in step C & D need to be followed. If
the SENSE are going to be used, do not jumper
them as described in steps C & D.
C. Connect B +, pin 4, to the + excitation of your load cell
and jumper the + SENSE, pin 3, to B +, pin 4.
D. Connect B -, pin 2, to the - excitation of your load cell
and jumper the - SENSE, pin 1, to B -, pin 2.
E. Connect the VAC power supply to the AC input lines,
pins 6 and 7.
II.
Turn On Procedure
A. Verify that the hook up procedure is complete.
B. Verify the correct AC voltage is applied to the 465; i.e.
100, 115, 220, 230.
C. Turn on the AC source supply to the 465.
D. Set the required EXCITATION supply voltage to the
load cell by adjusting B + ADJUST, Pot B
III. Calibration Procedure for Zero Adjustment
A. Jumper the + and - input terminals, pins 10 and 11,
together.
B. Connect a volt meter across the output, pins 8 and 9.
C. Adjust the OUTPUT OFFSET, Pot A, potentiometer
for zero.
IV. Full Scale Voltage Adjustment
A. Remove the jumper between the + and - input terminals
and apply a known load to your load cell, in most
cases it would be 100% of full scale.
B. Adjust the COARSE GAIN, Pot D, and FINE GAIN,
Pot C, potentiometers for the desired FULL SCALE
output.
C. Calibration is now complete. However, the user should
recheck the ZERO & FULL SCALE output before
continuing.
Rg = G - 1 - 1
40 k
150
1
Typical Bridge Application
Figure 2 shows a typical load cell application using a standard
350 Ohm bridge. Typical bridge output is 2 or 3mV/Volt of
excitation. With the power supply excitation voltage at 10
Volts an output of 20 or 30mV from the bridge can be
obtained. The common mode voltage of the bridge (the
instrumentation amplifier input signal level) is 5 Volts. This
is well within the 6.5 Volt common mode voltage range of the
amplifier.
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A
2401 Stanwell Drive Concord, California 94520 Ph: 925/687-4411 or 800/542-3355 Fax: 925/687-3333 www.calex.com Email: sales@calex.com
3
4/2001
Model 465 Bridgesensor
FIGURE 3
The gain must be set between 300 and 600 depending on the
output of the bridge. The built-in potentiometer set to a gain
of 200 would achieve an output voltage of 4 to 6 Volts. For
a higher level output an external resistor must be used. The
value of that resistor can be calculated using the gain
formula. It can also be arrived at empirically using a calibrated
input signal equal to the maximum input signal expected and
a resistance substitution box to adjust the gain until the
desired full scale output voltage is achieved.
Assuming a standard 350 Ohm bridge is used, the current
required from the excitation supply (set at 10 Volts) would be
28.6mA. If the leads were long enough to have 10 Ohms of
internal resistance there would be a drop of over 0.25 Volts
in both the plus and common side of the bridge. To eliminate
this potential error (especially where the current might vary
during the course of a measurement) the sense lines are
connected to measure and regulate the voltage right at the
bridge rather than at the output of the supply thereby
eliminating this potential source of error. The decision of
whether to use the sense leads or not depends entirely on
the lead length, its resistance and the effect of that error on
the measurement.
Application Suggestions
The Model 465 is designed to eliminate many of the ordinary
problems associated with bridge type measurements. Since
the whole system is in one case the common problem of
ground loops or circulating currents caused by poor wiring
practices is eliminated. It is recommended that lead lengths
be kept to a minimum. The use of shielded twisted pairs for
the input leads is recommended for most applications.
To minimize self heating errors the use of minimum excitation
power is suggested as is sufficient heatsinking of the transducer
wherever possible. For optimum stability a one hour warm-up
is recommended. Avoid large temperature changes or stray
magnetic fields.
The output leads should be kept short to minimize capacitive
loading on the output of the amplifier. The Model 465
Bridgesensor is ready to wire into your system, have power
applied, and to start making measurements for you.
FIGURE 2. Typical load Cell Application
Mechanical Specifications
Specifically designed for rugged field use the Model 465 is
completely encapsulated in epoxy using a vacuum potting
system to insure a complete seal against corrosive
environments. It is similarly protected against shock and
vibration and will provide years of reliable and accurate
operation.
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