GB936085A - Improvements in or relating to radiographic apparatus - Google Patents

Abstract

936,085. Protective systems for X-ray tubes. GENERAL ELECTRIC CO. Ltd. Jan. 6, 1961 [Jan. 11, 1960], No. 932/60. Class 38 (4). [Also in Group XX] Radiographic apparatus for use with X-ray tubes with different power ratings, having three independent controls for setting the voltage, anode current, and exposure time, has means provided for deriving a voltage signal dependent in strength on the power to be dissipated in a selected X-ray tube under the exposure conditions selected, and applying said voltage to the trigger electrode of a gas discharge tube, the arrangement being such that in the event of the signal exceeding a predetermined value (i.e. an unsafe combination of control settings has been selected), a discharge is initiated in the discharge tube which prevents the X-ray tube from being energized. Means may also be provided for deriving a further voltage signal dependent upon the voltage itself to be applied to the X-ray tube, and this signal also applied to the trigger electrode of the gas discharge tube, so that too high a value will prevent the X-ray tube from being operated. Both signals may be derived together, an exposure being prevented if either is too high. In the arrangement shown, potentiometer 5 is connected across conductors 7, 8, the voltage across which is proportional to the voltage to be applied to the particular X-ray tube selected, out of a number having different power ratings, for exposure. A push-button station 9 selects the tube, and shorts out a portion of the potentiometer 5, so that its effective value is proportional to the maximum safe voltage. A voltage signal is derived by a slider 14, which is connected via a contact 39a and rectifier 15 to the trigger electrode 2 of a neon tube 1. The other signal is supplied by a transformer 6, which is fed with a signal equal to the voltage drop across a resistor 16 and a potential divider 17; these are in series with a tapped resistor 18, and connected to the secondary of a transformer 19 across conductors 7 and 8. The secondary winding is tapped at three points, as shown, connected to contacts 20, 21, 22 on a pushbutton station 23. Four further contacts 24, 25, 26, 27, corresponding to the ratings of the X-ray tubes, are connected to the sliders of banks 28, 29, 30, 31 of an exposure time selector switch, these sliders being linked together. Thus the value of the voltage between the slider and the start of the secondary winding is proportional to the voltage across the X-ray tube, and also to the anode current, i.e. to the power to be dissipated in the tube. The rectifiers 15 and 32 constitute an " OR " gate through which a signal either from the slider 14 or the transformer 6 may pass, whichever is larger. The neon tube 1, fed by supply conductors 33, 7, has a series relay 38, which operates a contact 38b in series with relay 39. Relay 39 operates a contact 39a in the slider 14 line, and also a main contact (not shown) which controls the supply for the X-ray tube. When a voltage above the safety maximum is selected, a discharge in the neon tube is triggered off. Relay 38 is energized, which moves switch 38b to its second position, thus extinguishing the neon tube discharge, opens contact 39a to interrupt the signal from slider 14, and opens the main X-ray contacts; also, switch 38a is energized to operate a warning indicator lamp 40. If, on the other hand, the selected voltage is itself within the safe limit, but the power dissipated will be excessive, the transformer 6 supplies a signal which triggers off the neon tube, the subsequent sequence of operations being as before. Means are provided so that the behaviour of the indicator lamp 40 shows which of the two settings, voltage or power, is too high.