DE649523C - Device for operating discharge tubes with glow cathodes, in particular X-ray tubes - Google Patents

Description

Device for operating discharge tubes with hot cathodes, in particular X-ray tubes Devices for operating X-ray tubes are known in which for the sake of simplifying and making the manufacture of extra-high voltage transformers cheaper and the heating current transformer for the hot cathode X-ray tube has a common one Have iron core, so that when the high voltage is switched on at the same time the heating is switched on.

In these devices, the primary voltage is determined by the High-voltage transformer both the level of the tube voltage and the level the heating voltage and thus the temperature of the filament is determined. About the tube current to increase, you have to increase the primary voltage of the high voltage transformer, whereby at the same time the secondary voltage of the high voltage transformer, d. H. the tube voltage increases. However, the secondary voltage increases because of the voltage drop in the transformer less than the primary voltage.

A numerical example shows the relationships most clearly. With a secondary voltage of the high-voltage transformer of 6o kV, the secondary heating voltage on the filament should be 6 volts and the tube current 5 mA. In order to achieve a tube current of 50 mA, the heating voltage must be increased from 6 volts to g volts, ie by 50%. The primary voltage of the high-voltage transformer must therefore also be increased by 50%. This increases on the one hand the secondary heating voltage from 6 volts to g volts and on the other hand the secondary high voltage from 6o kV to go kV. Since the voltage drop in the transformer when the tube current is increased from 5 to 50 mA can be set at approximately 1 5 % of the initial voltage (6o kV), the secondary voltage of the high-voltage transformer at 50 mA tube current is go-g = 81 kV.

In general, it is desirable that the tube voltage, i. H. the Secondary voltage of the high voltage transformer, does not change when the tube amperage is increased. It has therefore already been proposed to reduce the voltage drop in Intentionally increase the transformer so that it compensates for the increase in voltage when it is increased the primary voltage and thus also the tube current strength. Such intentional deterioration of the transformer still brings the essentials Disadvantage with it that as a result of the strong voltage drop in the transformer difference between the loaded and the unloaded half-wave is greatly enlarged in an undesirable manner.

The tube voltage remains approximately constant when the Tube current intensity can be achieved without disadvantage according to the invention by that at least one of the two heating current supply lines has a variable resistance is switched on of such a nature that its resistance value only increases Current consumption decreases. Such a resistor, for example a highly saturated one Choke coil, has the property that it is always approximately with increasing current strength picks up the same voltage, since its resistance value decreases with increasing current strength.

The mode of operation of the resistor is illustrated using the numerical example mentioned and the figure clearly.

The high-voltage transformer a with the primary winding b, the secondary winding c and the heating current winding cl feeds the x-ray tubes e. In one heating current supply line to the filament, the highly saturated, iron-enclosed choke coil g flies, which is supposed to absorb the almost constant voltage of i.1 volts at every value of the tube current strength between 5 and 50 mA. Since the heating voltage of the filament is 6 volts at 5 mA tube strength and 9 volts at 50 inA, the heating current winding cd must have a voltage of 1.1-f- 6 = 20 volts at 5 mA and 1.1-f- 9 = 23 volts at 50 mA deliver. The increase in the heating voltage and thus also the primary voltage of the high-voltage transformer when the tube current strength rises from 5 to 50 mA is therefore 15%. Since the voltage drop of the high voltage transformer in this control range of the tube current is also 15 0/0, it follows that with the selected dimensions of the elements of the high voltage generator, the tube voltage remains practically constant when the tube current is increased. If so desired, the choke coil g can also be dimensioned in such a way that the oil voltage increases or decreases slightly when the tube current is increased.

Since in general the tube voltage in the loaded half-wave itself remains approximately constant is the increase in the primary voltage of the high-voltage transformer rd is directly proportional to the increase in voltage on the heating current winding. A voltmeter installed on the primary side can thus be calibrated directly in mA will. Because, as can be seen from the numerical example given, the voltage change is only about 15 0/0, the use of a measuring instrument is recommended suppressed zero point.

With constant tube voltage and a certain tube current strength the load duration of the tube may be limited by the load capacity of the tube do not exceed a certain maximum value. This maximum permissible load duration becomes smaller as the tube current increases. Since the establishment according to the Invention the tube voltage is kept approximately constant, then corresponds to each Value of the tube current strength an associated maximum permissible load duration. It It is therefore advisable to build the device in such a way that at every value of the tube current the maximum permissible load duration for this value is set on the time relay will. This can be achieved in a simple manner in that the device for Mechanical control of the primary voltage of the high-voltage transformer with the timing relay or electrically coupled in a corresponding manner.

In the case of a device with several voltage levels, it is advisable to switch part of the heating current winding when increasing or decreasing the voltage or the choke coil off or on; at the same time the scale of the measuring instrument calibrated in niA, preferably inevitably to be replaced.

Claims (3)

PATENT CLAIMS: i. Device for operating discharge tubes with Hot cathode, in particular X-ray tubes, in which the high-voltage transformer and the heating current transformer have a common primary winding, thereby characterized in that at least one of the two heating current leads to the hot cathode a variable resistor of such a nature is switched on that its resistance value decreases with increasing current consumption. 2. Device according to claim i, characterized in that a highly saturated, Iron-closed choke coil is used. 3. Device according to claim i or 2, characterized characterized in that, with the tube voltage kept constant, that for measuring the primary voltage the measuring instrument used for the high-voltage transformer in values of the tube amperage is calibrated. : I. Device according to Claim 3, characterized by the use. of a measuring instrument with suppressed zero point. Establishment according to Claim r, z, 3 or q., Characterized in that with the tube voltage kept constant the device for regulating the primary voltage of the high-voltage transformer is so mechanically or electrically coupled to the timing relay that each Value of the tube amperage is the maximum permissible load duration for this value is set.