Seca 727, 728 Service Manual

Service Manual

Variants:

7271321004
7271321124
7271321334
7271321614
7271321624
7271321804
7271321844
7281321009
7281321129
7281321619
7281321834

Content:

for seca 727, 728

Service Manual Number

17-05-01-260-i

Valid as of: 01.12.2010

Description:
High resolution baby scales with integrated LED-display

Function diagram 25-01-02-481 a
Electronics 25-01-02-467 e
Block diagram electronics 08-09-01-267 c
Description of RS232 interface 30-34-00-605 a
Description of faults 30-34-00-437 g
Replacement 30-34-00-483 e
Calibration 30-34-00-654
spare parts 30-34-00-486 k

Manual number: 17-05-01-260-i

Circuit description

Model 717,727,728,737,748,757 Blatt 1(3)

Associated circuit diagrams:

1. A/D converter circuit diagram 08-01-21-321

2. Microprocessor circuit diagram 08-01-21-322

3. Display unit circuit diagram 08-01-21-323

4. Switched-mode power supply
circuit diagram 08-01-21-354

Measuring element

A platform load cell is used as the force
measuring element. 4 wire resistance strain
gauges are attached to its surface at suitable
points which are connected in a bridge circuit.
When a load is applied, the spring body is
deformed in such a way that the two resistors
forming a half bridge are extended and
compressed. This causes the resistance to be
increased or reduced so that the bridge is
detuned, causing a change in the output signal:

=⋅ ⋅

UkUF

aS

In order to ensure a higher signal yield for the
seca 717, the measuring element and the A/D
converter are supplied with 10V; usually, a 5V
power supply is provided.

Analog to digital converter

The A/C converter directly processes the small
output signal of the strain gauge sensor. It
functions according to the principle of signal­dependent pulse-width modulation. All digital
functions of the A/D converter are implemented
via software in a microcomputer. The reference
potential of the A/D converter is ≈ U/2, since the
positive input of integrator 402 is connected to a
bridge output via resistor 506. During a predefined
total time T, first the input voltage +Ue is
connected to integrator 402 via resistors 507 and
506 and then the reference voltage U/2 with FET
switch 600/1,2,13, via resistors 510, 517 (for
model 727 only), 602, 604, 511, 512, 513 and 514.
The components are selected so that the
integrator integrates up during this phase
whenever an input voltage is applied until
comparator 502 reacts. The response threshold of
the comparator is determined by resistors 410 and

411. Resistor 413 causes positive feedback and
prevents the comparator oscillating. The
microcomputer detects that comparator 502 is
triggered and switches FET switch 600/1,2,13 off.
The integrator now runs down until time T has
expired. The interval between T = 0 and the
moment the comparator reacts is a measure for
the input voltage Ue.

Trimmer 602 compensates for the local gravitation
(GAL value). Trimmer 604 is used to fine-adjust
the slope. As the adjustment range for 604 has
been deliberately kept small, the slope can be
adjusted using the combination 513, 514.

∝

UF

a

Resistors 508 or 509, which can be used
alternatively, are used for coarse adjustment of
the zero point. Resistor 412 helps the output stage
of amplifier 402 to increase the negative output
range.

Resistors 515 or 516 allow characteristic curves to
be corrected.

For incubator scale 748 only:
For reasons of electromagnetic compatibility, the
analog to digital converter circuit is provided on a
separate board. Reducing the analog signal path
by mounting the board in the base plate and using
interference-suppression capacitors 400, 401,
501, 603 considerably reduce electromagnetic
susceptibility.
The 5V supply voltage for the board is fused on
the main board with a 100mA fuse to limit the
energy on the board in the event of a fault.

Temperature compensation

For temperature compensation of the strain gauge
sensor, a fixed-value resistor is connected in
parallel with a temperature-dependent resistor. To
compensate for the sensitivity's t.c. value, the
combination 511,512 is used (NTC). The positive
t.c. value of the test value can be compensated for
by the combination 405, 406 (PTC).

Zero point, sensitivity and test value are partly
interdependent. To determine these values, the
modules (electronics + sensor) are measured at
10°C and 40°C (zero point, slope, test value) and
the results are entered into a computer. The
computer uses a complex computing routine to
calculate the optimum temperature compensation.

Microcontroller and display

The central computing and control element is the
microcontroller (µC) 520.

It fulfils the following functions:

• Digital A/D conversion

• Calculation of the zero point

• Binary BCD segment conversion

• Display control using multiplex operation

• A/D converter test

• Testing the CPU and the memories (RAM and

ROM)

• Overload detection

• Monitoring the supply voltage (digital)

• Automatic zero point monitoring

• Taring function

• Hold function

All functions are implemented via software in the
programmed memory (ROM) of the µC.

Sequence of operations

08.02.98 Law 25-01-02-467 Index E

Circuit description

Model 717,727,728,737,748,757 Blatt 2(3)

When the start button is pressed, the following
steps are executed:

Starting

The µC is started with the reset logics (see below)
and the program is executed.

Self-test

A test digit is written into all of the RAM cells in
succession, then read out and compared.

The main CPU commands are checked via
computer operations to see that they function
correctly.

The sum of the digits of those memory values
which are important for the weighing result is
continuously calculated and checked for
correctness.

The µC outputs the processed 7-segment
information to the segment port. Via Darlington
driver 419 (8 transistors) and resistor network 317
(8 resistors), the cathodes of the LED's are set to
0 V. The shared anodes are connected to + 5V via
the relevant digit transistor.

Overload detection

The current measured value Mi is checked with
respect to two limit values:

a) Overranging
If F = F
(d = graduation on the display)

b) Overshooting the limit
If F = F
displayed.

+ 9d, "STOP" is displayed.

max

lim

(ca. F

+ 20 %), "EEEEE" is

max

In the event of fault, "EEEEE" is written to the
display.

Zero point determination

After starting, "SECA" is displayed for approx. 1
second. During this period, the zero point is
determined. The measured value Mo obtained is
saved and subtracted from the relevant measured
values.

For incubator scale 748 only:

After "SECA", "-UP-" flashes in the display. The
scale must be relieved by at least 0.5 kg in order
that the zero point can be determined. Once the
zero point has been determined, a sound is
emitted for approx. 1 second which signals that
the scale is ready for weighing.

Zero point follow-up

If the current measured value Mi changes only
slightly in relation to the zero point value Mo within
a given time (C = 0.5 d/sec), the current measured
value is regarded as the zero point (Mi = Mo).

Weight calculation

The weight is calculated from the current
measured value Mi minus the zero point value Mo,
divided by ne ne is the internal step count per step
displayed.

MM

()

−

0

F

Here ne = 10.

Display

The current weight F is displayed on a 7-segment
LED display. The display is controlled via multiplex
operation. Consequently, actuation faults affect all
segments and are detected immediately.

i

=

n

e

C) A/D converter limit values
If the bottom or upper limit of the A/D converter is
overshot, "EEEEE" is displayed.

Range switch-over for model 717

By pressing the weighing range switch-over
button, where normally the tare button is fitted, the
scale can be switched over between weighing
range 1 and 2:
Weighing range 1: 15kg / 5g
Weighing range 2: 6kg / 2g

Hold and tare function for model 727

The Hold/Tare button has two functions.
Tare range: up to 0.4 kg
Hold range: from 0.4 kg

Taring function for models 737 and 757

The taring function is activated by pressing the
tare button.

Hold function for models 717,737,748 and 757

The hold function is activated by pressing the hold
button.

Taring

If the taring function is activated, this is detected
by the µC, tare indicator 27 on the display board is
switched on and the weight on the scale is tared
off.

The zero point Mo is subtracted from the
measured value Mi and the result is saved as Mt.
The weight now results from:

MMM

−−()

0

F

it

=

n

e

08.02.98 Law 25-01-02-467 Index E

Circuit description

Model 717,727,728,737,748,757 Blatt 3(3)

Zero follow-up and overload detection continue to
operate as usual, whereas the measuring range is
overshot if:

M

FF d

If measured value Mi is smaller than the zero point
Mo by the value Mt, the taring function is
cancelled again.

The value Mt 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 0­potential, 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

- 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

=+−

range from 6 – 15 V at a controlled output
voltage of 5V or 10V.

max

1

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.

08.02.98 Law 25-01-02-467 Index E

Circuit description

Model 717,727,728,737,748,757 Blatt 4(3)

Diode 102 is used as polarity reversal protection
and overvoltage protection. Diodes 110 and 111
protect against discharge during battery operation
and against polarity reversal.

Electrolytic and tantalum capacitors are distributed
evenly over the printed circuit board as backup
capacitors for the operating voltage.

08.02.98 Law 25-01-02-467 Index E