Sony KDP-51WS550 Training Manual page 23

4. AX-1 Protection Circuits
FBT Over Current Protection (IK-Prot 1)
(Reference Figure 4-2)
The secondary current in FBT is monitor by Q8007 at FBT/pin 1 (ABL). The +135V is used as the reference
voltage through the resistor voltage divider consisting of R8040, R8039, R8038, R8037, R8043, R8036, and
R8035. The combined voltage drop across R8036 and R8035 is used to turn off Q8007 and trigger the protection
circuit. In normal operation 0.6V is applied to the base of Q8007 turning it ON and a Low is present at the
collector. A High at the collector is needed to activate the protection mode.
As the current in the FBT secondary increases the ABL line will pull more and more current through the resistor
network of R8035, R8041, and R8082. The voltage drop across R8035 will decrease causing the combined
voltage drop across R8036 and R8035 to decrease. At the same time the 0.6V at the base of Q8007 is decreasing
which turns OFF Q8007. A High will be developed at the collector through pull-up resistor R8022, and Zener
diode D8023 (5.1V) will conduct. D8005 is now forward biased and a High is applied to the Anode of D6803 and
D6122 causing them to conduct, applying a high to the LATCH circuit. Once the latch is activated, the voltage at
the base of Q6527 is reduced and it turns OFF, removing the ground path for the AC Relay and it turns OFF. The
TV shuts down and the Stby/Timer LED flashes a 7X sequence.
B+ OVP
(Reference Figure 4-2)
The voltage level of the +135V line is monitored using IC6801. If the +135V increases at IC6801/pin 1, the
voltage at IC6505/pin 2 will decrease. Once IC6505/pin 2 decreases 0.6V (more negative then the collector),
Q6802 will turn ON. D6121 is now forward biased and a High is applied to the LATCH circuit. Once the latch is
activated, the voltage at the base of Q6527 is reduced, turning it OFF and removing the ground path for the AC
Relay, turning it OFF. The TV shuts down and the Stby/Timer LED flashes a 3X sequence.
9V, 5V, 3.3V, and 3.3V DC Regulator OVP Protection
(Reference Figure 4-2)
The 9V, 5V and both 3.3V regulators are protected against any over voltage condition using zener diodes (D6207,
D6205, D6203 and D6211). If any of the regulate circuits increase above the zener voltage (over voltage condition),
the zener will conduct and forward bias the appropriate steering diode (D6208, D6206, D6204 or D6212), which
in-turn will forward bias the B+ OVP steering diode D6121. D6121 is now forward biased and a High is applied
to the LATCH circuit. Once the latch is activated, the voltage at the base of Q6527 is reduced. turning it OFF and
removing the ground path for the AC Relay, turning it OFF. The unit shuts down.
Note that the regulator protection circuits are connected in parallel with the B+ OVP protection circuit. Consequently,
when the regulator protection circuits are activated, they will produce the same Stby/Timer LED 3X flash sequence.
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