GB1351844A - High voltage limiter circuit for television receivers - Google Patents

Abstract

1351844 EHT supplies RCA CORPORATION 7 May 1971 [11 May 1970 4 Jan 1971 29 March 1971] 13635/71 Heading H2F [Also in Divisions H3-H5] In a combined deflection circuit and EHT supply for a TV receiver, a protection circuit is included to reduce the line frequency in the event of excessive EHT voltage or in the presence of both positive and negative excursions of flyback pulses. The invention thus protects the phosphor on the cathode-ray tube from high energy electrons and reduces possible X-ray hazard. Fig. 3 shows a circuit for supplying line deflection currents to coils 661 and EHT voltage from a multiplier 901 and is of the type described in Specification 1,260,375. Failure of damping diode 621 results in a voltage across capacitor 671 having both positive and negative excursions. The negative excursion will cause diode 931 to conduct, reducing the voltage at the junction of resistors 261 and 281 and reducing the frequency of the line oscillator 161 so as to make the picture un-viewable and to reduce the excessive EHT voltage from multiplier 901. An EHT voltage regulator is included comprising a saturable reactor 108. Failure of the regulator will result in excessive positive voltage across capacitor 671 which turns on transistor 150, normally biased off by Zener diode 152, so as to reduce the frequency of line oscillator 161. In Fig. 1 (not shown) the voltage regulator comprising saturable reactor 108 is omitted and the protection circuit merely comprises a diode (35) connected between the S-shaping capacitor 641 and the gate of thyristor 681 and operates by becoming conductive if the voltage across capacitor 641 exhibits a negative excursion so as to cut off the gate drive of thyristor 681 so that the line deflection frequency and EHT voltages are reduced. Alternatively as in Fig. 2, an auxiliary winding 50s on the EHT transformer is coupled to a peak detector arranged so that in the event of failure of the damping diode 621 capacitor 61 will charge excessively positively, breaking down Zener diode 60 and so providing a continuous gate drive to thyristor 681. This action opens a breaker in the B + supply.