GB1251453A - - Google Patents

Abstract

1,251,453. Cathode-ray tube circuits. MATSUSHITA ELECTRIC INDUSTRIAL CO. Ltd. 29 May, 1969 [17 June, 1968], No. 27244/69. Heading H4T. [Also in Divisions C1 and H1] A degaussing arrangement for the picture tube of a colour television receiver comprises a resistor having a positive temperature coefficient of resistance (PTC) connected in series with the parallel arrangement of a resistor having a negative temperature coefficient of resistance (NTC) and a degaussing coil wound around the picture tube the two resistors being in thermal contact. In operation, when an A.C. voltage is supplied to terminals 4, 5 (Fig. 1) a heavy current flows initially through the PTC resistor 1 and the degaussing coil 3 due to the low resistance of the PTC resistor and the high resistance of the NTC resistor 2 when cold. As the temperature of resistor 1 increases due to the current flow its resistance also increases whilst that of 2, due to the heat communicated from resistor 1, decreases until the small current finally passed by the, now high, resistance of the PTC resistor 1 is shunted by the, now low, resistance of the NTC resistor 2 which is made lower than the resistance of the degaussing coil 3. Thus, the A.C. current flow through the degaussing coil 3 is reduced to substantially zero to effect a degaussing cycle, the parameters being selected so that this cycle is completed within about five seconds. The PTC resistor consists of a semi-conductive barium titanate ceramic comprising e.g. (by weight) 65% to 70% BaCO 3 , 28% to 30% TiO 2 , 0À1% to 3% SiO 2 and less than 0À5% of La 2 O 3 ; Nb 2 O 5 , Sb 2 O 5 or Bi 2 O 3 . These are mixed by wet milling and the mixture calcined at 1200‹C. for two hours, pressed into pellets and fired at about 1350‹ C. in air for one to two hours. The fired pellets are then provided with electrodes comprising e.g. silver, made by painting a silver paste on opposite surfaces of the pellets and firing in a nitrogen atmosphere, or, nickelphosphor alloy made by electroless plating. The Curie temperature may be raised by substituting lead for some of the barium and may be lowered by substituting strontium for some of the barium and tin or zirconium for some of the titanium the low Curie temperature resistor being employed when a fast degaussing cycle is required and the high Curie temperature resistor when a relatively large degaussing current is employed. The NTC resistor may comprise e.g. (by weight) 30% to 38% M n O 2 , 50% to 60% Co 2 O and 2% to 3% CuO, mixed, calcined at 900‹ C. for one half to one hour and fired in air at 1300‹ C. for one to two hours. The composition may be modified by incorporating Cr 2 O 3 , NiO and TiO 2 . Electrodes may be added by similar methods to those employed with the PTC resistors. Fig. 4 shows a disc-type PTC and NTC resistor 15 and 16 respectively the inner electrodes 17<SP>1</SP> and 18<SP>1</SP> of which are soldered at 19 to maintain good thermal contact. A thin metal sheet between these electrodes provides a common terminal and lead wires are soldered to the other electrodes 17 and 18.