Protection circuitry for cathode ray tubes

Claim 1

Circuitry for protecting a cathode ray tube against excessive beam intensity, the circuit comprising: sensing means for detecting movement of a cathode ray tube electron beam and for issuing a signal that is indicative of beam movement insufficent to allow continuous full beam intensity; and control means coupled to the sensing means and to an intensity line, the control means being responsive to the signal issued by the sensing means for clamping the intensity line to a predetermined maximum voltage limiting beam intensity to a level such that a visible indication of the electron beam is provided but such that damage to the cathode ray tube is prevented, the control means being further responsive to initiation of movement of the electron beam subsequent to a period of time during which the electron beam has been stationary at minimum intensity for providing increased beam intensity including full beam intensity, as selected by the user, for a predetermined period of time immediately following such initiation of movement of the electron beam, said predetermined period of time having a limited duration permitting the user to view movement of the electron beam while preventing damage to the cathode ray tube, and wherein the control means includes a limiting delay circuit for inhibiting limiting of the intensity of electron beam during said predetermined period of time.