Protection Circuit; Over Current Protection - Toshiba MM20E45 Technical Training Manual

This model is equipped with Over Current Protection circuit and X-ray Protection circuit which are both the same as those of
ordinary TV, and besides another system of X-ray Protection circuit is equipped.
The reason of having two systems of X-ray Protecion circuit, is to comply with the DHHS regulation.
DHHS require that X-ray will not be emitted under the worst condition with all controls adjusted, in situation that one component
is failed. Since this model has the circuit which controls high voltage, if X-ray Protection circuit is intentionally failed and High
Voltage Adj. is rotated to maximum, the high voltage exceeds the limit curve of CPT and so it can not satisfy the DHHS
regulation. To solve the problem, the most economical means is to provide another X-ray Protection circuit to stop operation,
and then protection circuits are provided with two systems.
First, Over Current Protection circuit and X-ray Protection circuit-1 the same as TV are explained, and next X-ray Protection
circuit-2 peculiar to this model is described.

1. OVER CURRENT PROTECTION

CIRCUIT
IF current of the main power supply for the TV set increases
abnormally due to failure of parts, etc. secondary breakdown
due to damage of associated parts, etc. or hazard such as
excessive heat, etc. may occur.
This model has a protection circuit whitch cuts off the relay
under abnormal conditions by detecting a current of the
180V line.
Fig. 1 shows the overcurrent protection circuit. If a load of
the 180V line is short-circuited and the current increases
excessively, a voltage drop will occur across R876.
SECTION X PROTECTION CIRCUIT
Fig. 1 Overcurrent protection circuit
When base-emitter voltage exceeds V
drop increased, Q870 turns on and a voltage obtained by
dividing it with R870 and R871 is applied to point
When the voltage at
voltage of D870 zener diode, the diode conducts and a gate
voltage is applied to a thyrister D862. The thyrister turns on,
a base bias is applied to Q863 and Q863 turns on. Then, the
base bias of Q862, which drives the relay, is dropped to zero
and the relay is cut off. Since the 5V power line connected to
the anode of the thyrister D862 through R874 is the standby
power line, D862 continues to work until the main power is
turned off. A series of operations shown above is carried out
for an instant time period, and threshold current is set to 2.0
~ 2.4 times the normal current, so the circuit will not operate
under normal conditions.
10-1
with the voltage
BE
increases by more than the zener
.