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Zero follow-up and overload detection continue to
operate as usual, whereas the measuring range is
overshot if:
M
=
+
−
F
F
d
1
max
If measured value M i is smaller than the zero point
M o by the value M t , the taring function is
cancelled again.
The value M t is added again to the zero point and
tare indicator 27 is switched off.
Hold operation
If the hold function is activated, the weight is
retained on display as soon as the value has
stabilized, until the hold function is activated the
next time.
Monitoring the supply voltage
A monitoring circuit for the supply voltage is
connected to pin 14 of the µC, which detects if the
operating voltage is too low. If this pin is at 0-
potential, the µC interrupts the normal measuring
cycle and writes "bAtt" into the display. If the scale
is operated on rechargeable batteries, the
electronics are switched off after a few seconds
via pin 8 of the µC in order to prevent exhaustive
discharge.
Switch-on time
The switch-on time is determined by the software
as standard. Continuous operation can be
achieved by soldering in jumper 618.
Power supply unit detection
If a power supply unit is connected to the scale,
transistor 109 is switched through via the battery
charging circuit (see below). This sets pin 33 of
the micro-controller to 0V and the switch-on time
is extended in accordance with the software
setting.
Power supply
The circuit has the following special features:
- Reliable function over a large input voltage
range from 6 – 15 V at a controlled output
voltage of 5V or 10V.
- Low power loss
Reset circuit
By connecting a power supply unit, a positive
pulse is issued via high-pass filters 116 and 117
and via diode 118. The same happens when the
start button is pressed (via 113, 114 and 115).
Resistor 112 ensures that capacitor 113 can be
discharged when the start button is open. (The
alternative connection for the start button at pin 3
08.02.98 Law
Circuit description
Model 717,727,728,737,748,757
t
n
e
of IC 126 prevents a reset when the scale is
switched on and allows a start button to be used
which is connected with the platform surface.)
The pulse from diode 115 or 118 is transferred to
the set input of flip-flop 126/B. Low-pass filter 141,
142 prevents a reset being triggered by conducted
interference. Resistor 119 is used as a pull-down
resistor for the set input.
As soon as flip-flop 126/B is set, capacitor 124 is
charged via resistor 125. The flip-flop is reset
shortly afterwards.
The pulse generated at the output of 126/B
switches on the 5V power supply via flip-flop
126/A and transfers a 5V pulse to the controller
via resistor 127 and transistor 128.
Low-pass filter 619, 620 ensures that no reset is
triggered by conducted interference.
Voltage stabilization
The reset sets flip-flop 126/A whose output sets
the shut-down input of switching controller 131 to
high. The latter generates a controlled voltage of
5V by means of diode 132, coil 133 and
capacitors 129, 130, 134 and 135. By means of
the LC combination 136, 137, this voltage is
smoothed for use in the A/D converter region.
Using resistors 139 and 140, the control threshold
is set above which the switching controller sends a
low-batt signal to the controller. Resistor 138 is
used as a pull-up resistor here.
If the voltage is to be switched off again, the
controller switches the stop signal from high to
low. Resistors 121 and 122 as well as transistor
123 perform a level conversion and inversion of
the signal, so that a positive edge is given to the
clock input of flip-flop 126/A, resetting it and
switching off the 5V voltage.
For the seca 717, the 10V in-phase regulator 451
is used which supplies the A/D converter and the
force measuring element using buffer capacitors
452, 453.
Battery charging circuit
The rechargeable batteries are charged via
stabilized power supply 103, 104, 106, 107, 108
and via diode 105. The charging current
corresponds to the float charge current of the
batteries. This current is relatively low and
increases the charging time, but the service life of
the batteries is extended considerably.
Voltage monitoring for the batteries is not
necessary. Diode 110 protects the batteries from
the unlimited current of the power supply unit.
Blatt 3(3)
25-01-02-467 Index E
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