A
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CALEX
FaxFACTS:
315
1997
Models 166 and 167 Bridgesensors
http://www.calex.com
2401 Stanwell Drive
Concord, CA 94520-4841
(510) 687-4411 Fax (510) 687-3333
Note: Pin 31 should be grounded if the
V/F converter is not used.
gain of approximately 500, and a bridge supply voltage of 10
Volts. A 15 pin mating connector is included. A circuit diagram
of the MK 166/7 is shown in Figure 7.
Application Information
Figure 2 illustrates a typical weighing application. The load
cell signal is amplified and converted to frequency by Models
166 or 167. Scaling is set by changing the amplifier gain or by
the V/F scale factor adjustment. These two adjustments allow
the user to treat the amplifier and the V/F converter
independently if desired. In Figure 2, the amplifier gain would
be set so that -10 volts output would represent full scale, 1000
pounds for example. The V/F scale factor would be set for full
scale output of 10 kHz or 100 kHz. If a 0.1 second time base
were selected for the Model 166 and a 0.01 second for the
Model 167 then a load of 999 pounds would be displayed as
999. Stability of the system depends on the load cell stability,
the amplifier drift stability, the V/F drift stability and the
accuracy of the time base. For better resolution, the display
could be expanded to six digits by doubling the counter and
digit circuits and by increasing the time base to one second
and 0.1 second respectively. For the ultimate in time base
accuracy and stability, a crystal clock could be used to
generate the time base.
Overall accuracy depends on calibration and the amplifier
gain control can be used to provide a system calibration
adjustment. System error is the root sum square (RSS) of the
various error contributions. If the following errors with respect
to the full scale are assumed,
Load Cell
±0.005%/°C (3 mV/V cell)
Amplifier
±0.002%/°C
V/F Converter
±0.01%/°C
Time Base
±0.001%/°C (crystal clock)
Then the theoretical overall error is .011%FS/°C.
Gain
Amplifier gain is set with one external resistor. The MK166/7
mounting kit provides two potentiometers in series for a fine
and coarse gain adjustment. There is also a place on the PC
card to install a single fixed resistor in place of the
potentiometers. The gain equation is G = 10 + 200 k
/Rg
where Rg is the external gain resistor. To illustrate, a gain of
500 would require an Rg of 408 ohms. The accuracy of the
gain equation is ±2%. If it is necessary to set the gain very
accurately, the best procedure would be to calibrate the
amplifier against a known voltage standard.
Bridge Power Supply
The bridge power supply voltage is adjusted with a single 10k
potentiometer. To reduce internal heating which could cause
undesired amplifier drift, the load current should be kept
within the limits indicated in Figure 3. The MK166/7 includes
an adjustment potentiometer on the PC card. It is also
possible to remotely adjust the bridge supply voltage by
applying a positive reference voltage to pin 16 of the 166 or
167 modules. The output voltage will follow the reference
voltage, that is, +8 Volts applied to pin 16 will produce a +8
Volt bridge supply voltage.
The + and - sense lines can be used to provide load regulation
at the load. If it is necessary to drive more than one or two load
cells, the sense lines in combination with a separate power
supply and transistor will allow the same regulation and
stability but with more output. For example, consider a typical
case where four 120 ohm load cells were to be operated in
parallel. Current required is 334 mA at 10 Volts. Figure 4
shows how to connect the Models 166/167 to solve this
problem.
FIGURE 2. Typical Weighing Application