Service manual Description of the electronics
05.08.03 /Jensen / Rei 34-34-00-645 b
2
Display (only DMS low-end module and SLC highend)
Whenever eight new measured values are available, i.e. approx. every 0.8 s, the weight is displayed.
The display is an LCD which is controlled by duplex operation. A maximum of five digits as well as a
few special characters can be displayed.
Explanation of the special characters:
•The battery symbol is used to alert of the fact that the battery is going flat.
•Hold indicates that the weight display has frozen.
•Tare or Net indicate that a weight on the scale has been tared off.
•The left arrow indicates if additional explanations are given next to the display.
•The weight units on the right show the unit in which the measured weight is displayed.
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.
To perform this measurement, a capacitor is charged every 2 seconds alternately via a reference
resistor and an NTC with a linearization resistor connected in parallel. The time until the capacitor has
reached a specific voltage is measured. The temperature is calculated from the ratio of the two times.
To reduce measured value fluctuations, a mean value of 16 temperature values is always formed. The
fluctuation then amounts to approx ±0.2°C.
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
calibrated. A linear and a square term are used for this purpose.
Power supply
Power supply is provided by AA size batteries (batteries or rechargeable batteries) and a plug-in power
supply unit. The batteries pass the voltage via reverse voltage protection diodes to a 3V in-phase
regulator. A coil which suppresses interference and a buffer capacitor come next. This 3 V supply
voltage is supplied to the digital part of the circuit.
Another in-phase regulator generates the voltage for the analog part. The µC can set this regulator to
3V, 5V or 10V.
In order to operate the module on rechargeable batteries a recharging circuit can be connected. This
circuit charges the rechargeable batteries at a constant current when a power supply unit is connected.
When the supply voltage is applied, a power-on reset is initiated at the µC via an RC element.
To monitor the condition of the batteries, their voltage is measured every 2s. For this purpose, a 2.2 V
reference voltage is integrated upwards and the time until the voltage has reached approx. 9% of the
supply voltage is measured. Afterwards, a downwards integration is made from 2.2V to the threshold.
From these two results, a measured value for the voltage is calculated. If the voltage drops below a
certain limit, the user is alerted by the battery symbol in the LCD. If the next threshold is undershot,
„bAtt“ is displayed and weighing is no longer possible.
Operation (only DMS low-end module and SLC highend, with the exception of
vibration-sensitive switch and platform switch)
Four buttons are provided; one is used to switch the module on and off, the others can be programmed
as required. These three buttons can be pressed just lightly or longer (longer than 1.5s), so that six
events are available. The actions tare, hold and unit switch-over can be assigned to them.
If you press another button when switching on the scale, the scale will switch to the calibration counter
and recalibration mode.
Instead of the normal start button a piezo diaphragm can be soldered to the PI connection. If the piezo
diaphragm registers a vibration, a transistor switches and generates an interrupt signal which causes
the µC to start.
