3DQDVRQLF
56
Chapter
4
EURO 4D Supplement
2.8. Secondary Supply Protection
On the secondary side of the power supply a number
of protection circuits are used which are discussed
below.
2.8.1.
3.3V Protection
In the case of the 3.3V supply failing, IC4854 which
(houses a latch circuit) is monitoring the supply via pin
2, would sink current via pin 1 of IC4854. This would
result in the capacitor C4873 discharging very rapidly,
causing transistor Q4858 to switch ON. When
transistor Q4858 conducts the base bias from
transistor Q4856 is removed, stopping the output of
the 5V supply.
2.8.2.
5V (Conditional Access) CA Protection
The 5V supply output from pin 3 of IC4852 is fed via
resistor R4873 used to monitor the current flow to the
Conditional Access (CA) module. Resistor R4873
develops a voltage drop that is fedback to IC4852 pin
4. and used to regulate the 5V output supply.
However if the current flow reaches to high a level, the
voltage drop developed across R4873 would cause
transistor Q4857 to conduct. When transistor Q4857
conducts a HIGH level is fed to the latch circuit made
up of transistors Q4854 / Q4855. The HIGH level fed
to the base of Q4854, biases the transistor into
conduction. Now with transistor Q4854 conducting,
the base of transistor Q4855 is pulled LOW, this
resulting in Q4855 also conducting. With transistor
Q4855 conducting pin 2 of IC4852 is then pulled LOW
switching OFF IC4852.
Cancellation of the latch circuit operation is achieved
by switching OFF the TV disconnecting the A.C. input
to the power supply circuit.
2.8.3.
12V Protection
Likewise the 12V supply line is also monitored, in this
case for over voltage conditions by transistor Q4859.
Transistor Q4859 is connected to the 12V supply line
via the potential divider resistors R4882 / R4883,
while the base of transistor Q4859 is connected to the
5V supply line via resistor R4879.
In normal operation the resistor configurations of the
emitter and base junctions ensures that the transistor
Q4859 is switched OFF. However if an over voltage
condition arises and the 12V supply rises by
approximately 1V the emitter of Q4859 would become
more positive with respect to its base, this would result
in the transistor Q4859 being biased into conduction.
To prevent any voltage spikes or ripples causing the
transistor Q4859 to conduct, capacitor C4874 is used.
When transistor Q4859 is switched ON then a HIGH
level is fed to the base of transistor Q4860. This HIGH
level would thus result in transistor Q4860 being
biased into conduction, pulling the base bias of
transistor Q4856 LOW, switching off the 5V supply.
Summary of Contents for EURO 4 Chassis
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