Olympic 50 - Power Supply

S1-4 is the date code switch. This switch causes the
display to show the last software revision date in the format
MM.DD.YY. Always return S1, 2, 3 and 4 to off before
returning the Scale to nursery service.
Display
Refer to Display Card schematic, Fig. 4-19.
Data is clocked serially into two display drivers with a
common clock. The format is one start bit followed by 35
data bits. On the 36th clock, all 35 data bits are automati-
cally latched to the outputs of the display drivers.
Each driver has a brightness control, Rl & R2. Current of
a group of lit segments can be measured at El. High
efficiency 7 segment displays are used. This allows
uniform segment brightness at only 4.8 mA per segment.
Ifa display segment burns out, replace all six displays to
retain uniform brightness. To avoid heat build-up in the
power supply, never set segment current above 4.8 mA/
segment.
Power Supply
Refer to Wiring Diagram, Fig. 4-15 and Main Board Sche-
matic, Fig. 4-16.
Alternating current power from the power cord comes into
the chassis. The chassis has F1, a 1/4 amp, normal-blow
fuse, a filter and a transformer (F1 is 1/8 amp for 220 Vac
units).
The secondary of the transformer T1 is connected to the
Main Board through Pi-J1. CR3 through CR6 forms a full
wave bridge rectifier. CR2 is a transient voltage suppres-
sor, and C43 and C44 provide filtering.
VR2 is a +12V voltage regulator. All Scale current comes
through VR2. All these circuits are live when the Scale is
plugged in, whether it is on or off.
When the Scale is off and the front panel On/Off switch (or
the Recall switch) is pressed, Q7 is biased on, supplying
SS50-N1-220V(B09)
power to U17. U17, a switch debouncer, provides some
delay and toggles flip-flop, U18, on. This turns on Q8
which energizes relay K1.
When the On/Off switch is released, Q7 turns off, but now
Kl’s-contacts supply power to keep itself and everything
else on.
To turn off the Scale, the On/Off switch is pressed again.
Q7 turns on and a signal goes through U17 which toggles
U18 off and turns off K1. The Scale is still on due to Q7,
but when the On/Off switch is released, Q7 turns off and
the Scale is off.
The Recall switch cannot tum the scale off. The pC can
toggle U18 off (independently of the On/Off switch) by -
applying a low directly to pin 3 of U18. The On/Off switch
(or Recall switch) must be held for at least 1/5 second for
the piC to recognize it.
If the switch is hit very quickly and let go, then the pC will
not know whether the On/Off switch or the Recall switch
was hit, and the pC will turn the Scale off. This is normal.
The pC will turn the Scale off after 10 seconds of recall or
when the very low battery threshhold is reached.
Voltage regulator, VR3, provides digital +5 volts. DSl, a
small LED situated on the Main Board, shows when the
Scale is on. U2, a switching regulator, provides a negative
voltage. VR1 provides a regulated +5 V for the analog
circuitry. The negative voltage from U2 is applied to U5
which has a gain of -1. This inverts the +5 V analog toa
tracking -5 V analog voltage.
Low battery voltage and very low battery voltage is detected
by U6 and U3 respectively. Threshold adjustments are set
by R31 and R27 respectively.
Front Panel The front panel contains five membrane
switches. Each switch, when activated, shorts the line to
ground. Membrane switches have some resistance (they
do not give a direct short). Several ground traces are used
to keep this resistance as low as possible (Typically 30Q
to 500).
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