Panasonic TC-2588 color TV as an example, this paper introduces the principle of the protection circuit of Panasonic C150 movement.

Machine has a switch power supply output B voltage over-voltage protection, + overload current protection, combined load short-circuit protection, +46V voltage load short-circuit protection and 16V voltage load short circuit protection and other protection circuit. The protection circuit is shown in the following diagram.

1 protection execution circuit

The implementation of the protection circuit is composed of Q827, Q826, the simulation of thyristor circuit. Q805 and opto coupler D811 to simulate the thyristor trigger control circuit. Q805 is also the voltage reversal circuit, Q805's base external multiple fault detection circuit.

When the TV is normal, the Q805 base is 0V, the Q805 is off, and the switch power supply is not affected. Forward bias voltage was obtained in voltage loop optoelectronic coupler D811, D836 normal conduction, D811 in photosensitive audion guide pass. The simulated thyristor pipe base voltage of the circuit Q827 to ground short circuit, the simulation of a thyristor circuit is cut off, Q826 collector output low level, Q804 is also off. To switch power thick film circuit IC802 internal power switch tube base voltage does not affect switch normal high voltage of power supply output.

When the detection circuit of Q805 base sent protection voltage, Q805 base into high level, Q805 from off state into saturated conduction state, the anode voltage of D825 low, D825 cut-off, photoelectric coupler D811 cutoff, through "optocoupler switching power transmission to the" primary, as of a phototransistor D811, the base simulation of thyristor circuits by Q827 positive bias voltage, simulation of thyristor circuit, Q826 collector provides positive bias voltage to Q804, Q804 saturation conduction, the switching power supply circuit IC802 (1C802 3 feet thick film internal power switch Guan Jiji) on the short circuit voltage, switching power supply to stop working to achieve the purpose of protection.

2 protection detection circuit

Q805 base external three circuit fault detection circuit, Q805, the collector of the three circuit fault detection circuit. The various protection detection circuit is as follows:

(1) +B voltage protection circuit

Voltage resistance R850, R851, D831, and voltage regulator diode to form a switching power supply output voltage +B voltage detection circuit. When the switch power supply output B voltage is normal, partial pressure of the midpoint voltage of the circuit is lower than 10V, lower than the voltage of Zener diode D831 value, zener diode D831 cut-off unprotected trigger output voltage; when the switch power supply output B voltage is too high, the midpoint voltage of the circuit pressure exceeds 10V over voltage of Zener diode D831 value, and D831 breakdown conduction, the high level added protection execution component q805 base, q805 conduction produced flipped voltage, optocoupler "

D811 closing, simulating the thyristor circuit, enter the protection state.

(2) +8 load over current protection circuit

By the sampling resistor Q831, R834//R835R835 and other peripheral components to detect the +B load over current protection circuit. Adeptness + normal load, the sampling resistor R834//R835 voltage drop is small, forward bias voltage below Q831, cut-off Q831, the collector without triggering voltage output; adeptness output short circuit occurred leakage fault, causing B voltage over current and voltage on the sampling resistor R834//R835 drop increases, when greater than 0.7 V, Q831 conduction, the collector output high level trigger voltage, through the R831 d832 protection execution circuit q805 conduction, "optocoupler" D811 deadline, simulation of thyristor tube circuit conduction into the protection state.

(3) field overcurrent protection circuit

R476 and the sampling resistor Q454 to form the field over current protection circuit. Field output circuit can work normally, generated on the sampling resistance R476 voltage drop is very small, Q454 deadline, do not trigger output voltage; on the spot output circuit failure caused by excessive current and voltage on the sampling resistor R476 drop increases, when greater than 0.7 V, Q454 conduction and the electrode for high level through r534 r533, partial pressure, the D502 enable protection execution circuit q805 conduction, Optocoupler D811 cutoff simulation of thyristor tube circuit conduction into protection state.

(4) +B voltage protection circuit

D830 for +B voltage protection circuit. When the combined load short-circuit, or + rectifier, filter circuit fault caused by B voltage output, d830 anode through a short circuit grounding, grounding resistance is very small, d830 conduction, will D825 of the cathode voltage pull, cut-off D825, Optocoupler D811 cut-off and Simulation of thyristor tube circuit conduction into protection state.

(5) +46V voltage load short circuit protection circuit

D837 for +46V voltage load short-circuit protection circuit, when the +46V voltage load short-circuit, or +46V rectifier, filter circuit fault. Cause no +46V voltage output, D837 anode through short circuit connected to, the grounding resistance is very small, D837 conduction, will D825 of the cathode voltage pull, cut-off D825, Optocoupler D811 cut-off and Simulation of thyristor tube circuit conduction, into protection state.

(6) short circuit protection of 16V voltage load

16V voltage load short-circuit protection principle is: when the 16V voltage load short-circuit, 16V voltage disappeared, circuit protection "optocoupler" 16V voltage D811 power supply of disappeared or very low, "optocoupler'D811 cut-off into protection state.