DC Switching circuit

A bidirectionally conductive switching circuit for selectively coupling a plurality of dc high voltages to a beam penetration CRT, the switching circuit comprising:

• a beam penetration CRT possessing a high voltage terminal having a capacitance that can be charged and discharged and also having a resistance high enough to limit the current due to the resistance to a value less than either of the first and second holding currents for the first and second serial strings of triacs recited below:
• a first dc voltage supply;
• a first serial string of triacs, each triac thereof having first and second conduction terminals and a gate terminal, the conduction terminals coupled serially between the first dc voltage supply and the high voltage terminal of the beam penetration CRT, the first serial string having a first holding current beneath whose value the string will no longer conduct in the absence of a signal applied to the gate terminals;
• a first trigger means having a plurality of outputs each individually coupled to an associated gate terminal in the first serial string of triacs, for synchronously applying to the gate terminals a periodic signal switching the triacs conductive to charge the capacitance of the beam penetration CRT toward the voltage of the first dc voltage supply until the charging current falls below the value of the first holding current and for thereafter maintaining the voltage at that level;
• a second dc voltage supply of voltage different in value than the voltage of the first dc voltage supply;
• a second serial string of triacs, each triac thereof having first and second conduction terminals and a gate terminal, the conduction terminals coupled serially between the second dc voltage supply and the high voltage terminal of the beam penetration CRT, the second serial string having a second holding current beneath whose value the string will no longer conduct in the absence of a signal applied to the gate terminals;
• and
• a second trigger means having a plurality of outputs each individually coupled to an associated gate terminal in the second serial string of triacs, for synchronously applying to the gate terminals a periodic signal switching the triacs conductive to discharge the capacitance of the beam penetration CRT toward the voltage of the second dc voltage supply until the discharging current falls below the value of the second holding current and for thereafter maintaining the voltage at that level.