Tascam 122 Service Manual page 21

through resistors R540 and R541 are input
to the base of 0517, permitting a large
collector current @ to flow through 0517.
As a result, the output voltage rises rapidly.
d. As the output voltage rises, current flows
via paths @ and @. The emitter of 0520
is fixed at the zener voltage (VZO :;: 13 V)
by current flowing along path @ and the
zener diode.
e. If the output voltage rises for any reason
and the current flowing along path @
increases, the voltage drop across R544
increases and the base voltage of 0520
rises. When the base level exceeds the
emitter level, collector current @ begins to
flow to 0520.
f. The output voltage of the main regulator
has been rising during this time. However,
the collector current of 0520 works to
reduce the output voltage because the
current on path
CD
decreases.
g. When the voltage difference between the
base and emitter of 0520 reaches approxi ­
mately 0.7 V, the main regulator enters a
state of equilibrium . That is, drift of the
current on path
CD
tending to raise the
output voltage is cancelled by the tendency
to reduce the output voltage because a rise
of the output voltage results in an increase
in the current of 0520, an increase of the
current on path @ , and a decrease of the
cu rrent on path
CD
in this sequence.
POWER RECTIFYING SMOOTHING
TRANSFOR~ER
"' 1
CIRCUIT
...1
CIRCUIT
~I
Fig. 3-17
h. In the state of equilibrium, the output volt ­
age is determined by the following equa­
tion.
R544
R542 + RS44 x VOUT
=
VZD + Vee
R542 + R544
:. VOUT
=
X
(VZD + VeE)
R544
Substituting the actual values of the circuit
parameters, we have the output voltage as
follows.
4.7kn + 6.8kn
VOUT= . x(13+0.7):;:23.1V
6.8kn .
2) Regulating process
When the output voltage tends to rise (fall)
because of reduced (increased) load, the fol ­
lowing process stabilizes the voltage.
a. The output voltage starts to rise (fall).
b. The base level of 0520 rises (falls).
c. Since the emitter of 0502 is kept at a fix·
ed level, its base current increases (de ­
creases) and the collector current @
varies together with rising (falling) of the
base level.
d. As a result, the base current
CD
of 0517
decreases (increases) and its collector cur ­
rent varies similarly.
e. Thus the output voltage falls (rises) to the
initial level.
SMOOTHING
VOLTAGE REGULATOR
.. \ CIRCUIT
"I
REGULATED
OUTPUT
VOLTAGE
OV
Main regulator
21
-
0542
~
0543
"""­
0544
I
R540
R!>41
C520 R545
C518
,J,
". -
...
CD
,r;;(
0519
R542 R543
~OS20
~
"'~j
t'
0512
R~44
1
C514
@
®