Service Manual
Description of the LC2 electronics
23.07.07 Kr/Rei
Page 4(6)
30-34-00-745
Brief description of the LC weight module
Introduction
The LC weight module is used to determine weights using one or several load cell(s) with strain
gauges. The LC module bus supplies the processed measured values to the LC display module.
The following block diagram shows the main function blocks of the LC weight module:
Voltage
regulation
uC
Voltage
measurement
A/D converter
Programming
interface
LC module bus
Sw itch
on/off
logics
Load cell(s)
Temperature
measurement
Resonator
Mains
connection
/battery
Weight measurement
The weight module uses load cells with strain gauges in a double bridge circuit as sensors. The entire
analog part and the load cells, as well as the other peripherals, are supplied with a 5V supply voltage.
At full load, the load cells supply a bridge output voltage of 0.8mV/V. This voltage is measured by a
dual slope A/D converter; the different voltages supplied by the four load cells are multiplexed
accordingly. Every 500 ms, 12 measurements are carried out per load cell and the mean value is then
calculated. Before the mean values for each load cell are merged and converted into the weight value,
an individual corner correction factor is applied to them. A linear function is used to convert the results
into a weight value. The first stable value within the switch-on zero setting range after switching on the
scale is used as zero point. The zero follow-up range is monitoring during the operation of the scale.
Temperature measurement
In order to compensate for the load cell’s temperature drift, the temperature is measured regularly and
the measured value is adjusted accordingly. For this measurement, the voltage is cyclically measured
via an NTC, which forms a voltage divider together with a linearization resistor connected in series. To
reduce measured value fluctuations, a mean value of 8 temperature values is always formed.
Fluctuations are thus reduced to a third compared with individual measurements. The difference
between the current temperature and the temperature measured when the last zero point was
obtained is used to determine a projected zero point. To do so, one assumes that the weight change
per °C is constant. The weight value is then corrected in accordance with the zero point calculated and
in accordance with the difference to the temperature measured when the scale was adjusted.
Power supply
Power supply can be provided by AA size batteries or a power supply unit. The circuit cannot be
operated on rechargeable batteries. The unregulated voltage is connected to a 5V in-phase regulator
via reverse voltage protection diodes. The regulated 5V supply voltage is then available to the entire
circuit. The supply voltage for the analog part can be switched separately from the rest of the PCB via
a switch on/off logic. The digital part is permanently supplied with 5V.
Voltage measurement
To enable power supply unit and battery operation to be distinguished and in order to measure the
current battery voltage, the unregulated voltage is monitored by the microcontroller. Voltage
measurement takes place via a voltage divider and by means of the internal A/D converter.
