A Circuit for the Automatic Stabilization of X-ray Tube Current

The electron emission of the filament in a continuously evacuated X-ray tube is liable to change very considerably over a long exposure, and the circuit described here automatically controls it to a constant value. The 50 cycle current in the 230 V. primary of the X-ray tube filament transformer is passed through a diode which carries the negative half-cycle, and a thyratron which carries the positive half-cycle. A resistor inserted in the h.t. circuit develops a voltage proportional to the X-ray tube current, and this is applied to the grid of the thyratron, so that increased X-ray tube current causes the thyratron to fire later in the half-cycle and thus reduce the filament current. Adequate sensitivity is obtained with no amplification. Late firing of the thyratron introduces a d.c. component and some saturation of the transformer core. The same basic circuit is also suitable for ionization gauge control.