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POWER
SOURCE
CIRCUIT
The
voltage
VF2
(-22
V)
is resulted
from
the
followtng
A power of AC 240
V passes through
the filter
circuit
(Ll)
and
is applied
to the primary
winding
of transformer
(Tl ).
equation
:
The output
voltage
of the secondary
winding
of transformer
(Tl)
is then full-wave
rectified
by the diode bridge (Dl
-
D4)
and smoothed
by the electrolytic
capacitor
(C2)
to become
DC
voltage Vp (-32V).
vF2
(V)
= VZDl
+ VZD3
+ VZD4
-
VBE
ial)
Then the voltage Vp is delivered
to the constant
voltage ctrcuit
made up of transistor
01
and zener
diodes
ZDl,
ZD3
and
ZD4 where it is made into VF~
(-22V).
VDD
(-12V)
is a voltage to be applied
to the LSI and it de-
pends on the zener diode ZD3 and ZD4.
(VFl-
VF2)
refers to a voltage to be applied
to the heater of
the
display
tube
and
it
is obtained
from
the
followrng
equatron:
(VFl
-vF2)
= (il
+
12)
X Rf
In which,
Rf
is the heater's
resistor
and 11 and i2 are load
currents
caused
by the
resrstors
R9
and
R8
located
In the
heater,
Thus the heater's
voltage
(VFJ-VF~)
IS specified
at
3.5v.
the transistor
Ql
to have a part of the current
running
In Ql
be by-oassed:
this results
In a reduction
of the current
con-
sumptlon
of this transistor.
Ltkewlse,
resistor
R6
leads
to a
ruduction
of the current
consumption
of zener diode ZD3 and
Resistor
R7
IS provrded
between
the collector
and emitter
of
ZD4.
Cl and C5 are noise by-pass capacitors.
VRSl
240V
50Hz
11
D2 d
!hDl
it
D4
l-
D5
R'
r
T
T
R6
c5
+ c4
ZD4
R8
ZD3
Lmo
T
GIN D
VREF (-6.2V
I
I
._
/
T
VDD (--12V)
-
!2
I
t
-i1
,
----~--VW
(-18.5V)
R2
8
4
Rc
,
, DC, 3.5V
I
I
I
i
-----I
-vF2
(-22v)
Ql
tz c3
Rf:
HEATER
OF
+
R7
FLUORESCENT
DISPLAY TUBE
m
I
Vp (-32V)
Figure
10. Power Source
Circuit
46
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