Stabilizing Circuit - Toshiba e-STUDIO170F Service Manual

7.6.2

Stabilizing Circuit

LVPS
~
DB1
- +
R7,R8,
~
R10,R14
Q3
PC4
R9
Q2
The stabilizing circuit monitors the +24V voltage in the secondary circuit of the transformer (T1) through
the shunt regulator (IC1) and feeds back its status to the primary circuit through the photo-coupler
(PC4) to stabilize the power output.
IC1 compares the voltage produced by dividing the +24V power output through R23 and R25 with the
reference voltage. When the +24V voltage rises, the cathode current of IC1 changes to allow current to
flow through the LED of PC4. This causes the light detector of PC4 to conduct to turn on Q2 so that Q3
will turn off due to a drop in its gate voltage and cut off the current in the primary circuit of the trans-
former. When the current in the primary circuit is cut off, the emission power of the secondary circuit will
drop to decrease the output voltage for stabilization.
When this machine shifts to the super power save mode, the PSC signal output from the super power
save circuit on the Main PBA goes LOW to turn on the photo-coupler (PC5), thereby increasing the volt-
age to be applied to IC1. This allows IC1 to turn on PC4 at a voltage lower than that in regular opera-
tion. For this reason, the emission power of the secondary circuit drops so that the voltages of +24V
and ±12V will no longer be output.
And if the PSC signal goes LOW, Q9 is turned ON. This turns Q8 OFF to cut off the +5V voltage output.
e-STUDIO170F Circuit Description
D12
T1
PC4
R28
C26,
IC1
C29
R19
ZD1
Fig.7-53
7 - 72
R23 R24
Power supply
PC5
voltage
R36
generation
circuit
PC5
R25
+24V
14
+12V
2
-12V
3
+5VPS
6
+5V
7
Q8
R42
Main PBA
Q9
R43
SG
5
PSC
1
January 2005 © TOSHIBA TEC