GB1255710A - Electronic switching system for switching an alternating current supply - Google Patents

Abstract

1,255,710. Control systems for X-ray apparatus. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 11 Feb., 1969 [13 Feb., 1968], No. 7312/69. Heading H2H. A switching system for use with X-ray apparatus comprises means whereby the firing of a thyristor included in the system is effected at the start of each cycle of an A.C. source from which the system is supplied. X-ray apparatus is supplied from an A.C. supply by way of a diode bridge G10-G13 and a thyristor B10, gating signals for the thyristor being derived from a transistor circuit which ensures that the thyristor is rendered conductive at the start of each half cycle of the A.C. supply. Capacitors C8, C9 are normally charged from a secondary winding T14 of a transformer having its primary winding T10 connected to the A.C. supply, the charging circuit including a rectifying bridge, filter circuit and a change-over contact 53A of a relay S3. A switching operation is initiated by energizing the relay S3 whereupon contact S3A changes to its lower position connecting capacitors C8, C9 to the base of a transistor B7 thus rendering the transistor conductive. Transistors B5, B6 form a bi-stable multivibrator circuit which changes state under the control of transistor B7. Thus, when transistor B7 is rendered conductive by the discharge of capacitors C8, C9, transistor B6 becomes conductive, transistor B5 cuts off and transistor B4 cuts off. The collector of transistor B4 then receives a constant potential, adjustable by a variable resistor R29 which voltage drives transistor B3 near to its operating point. To the base of transistor B3 is applied a further voltage obtained from a secondary winding T13 of transformer T10, this voltage being rectified by a bridge G7 and clipped by a Zener diode G6. Thus, at the collector of transistor B3 a series of pulse signals appear, corresponding to each zero point in the cycles of the A.C. supply. These pulses render transistor B1 conductive thereby producing firing signals for the switching thyristor B10 at the zero points of the cycles of the A.C. supply. Transistor B4 also renders a transistor B2 conductive to energize a relay S1. Closure of switch S1B starts a timing period set by R=f(t) and at the end of this period transistor B7 cuts off to release relay S1 to prevent further operation of thyristor B10. Manual operation may be effected by energization of a relay S2 by closing a push button switch. The diode bridge G10- G13 and thyristor B10 may be replaced by a pair of reverse-parallel connected thyristors in which case the output of transistor B1 is used to energize the primary winding of a transformer, the secondary windings of which supply the gating signals for the thyristors.