High voltage protection circuit

Abstract

A horizontal fly-back transformer having a high voltage winding for generating high voltage pulses fed to the anode of a cathode ray tube is provided with a tertiary winding which in turn is connected to a rectifying-smoothing circuit, and a DC voltage in proportion to the anode high voltage is obtained from the rectifying-smoothing circuit. The DC voltage is divided by a resistor divider circuit and a division of the DC voltage is fed to the gate of a thyristor via a Zener diode which is turned on when the anode high voltage exceeds a critical voltage. A thryistor gate current which flow concurrent with the conduction of the Zener diode causes the thyristor to turn on so that the operating voltage o a horizontal oscillator circuit may be reduced until the horizontal oscillator circuit stops oscillating, thereby preventing generation of the anode high voltage. In addition to the aforementioned high voltage prevention circuit, a beam current detector circuit is connected in series with the high voltage winding and when the amount of beam current is below a predetermined value, detecting current dependent on the beam current is passed through a part of a resistor circuit. DC voltage for turning on the Zener diode and accordingly, the anode high voltage in proportion thereto depends on th beam current below the predetermined value so that an anode high voltage level at which the high voltage prevention circuit is operated is varied with the amount of the beam current.