Method and circuit for limiting CRT X-ray production

Abstract: A method and circuit for preventing excessive CRT X-ray production by continually measuring operating parameters of the CRT that are correlated with X-ray production during this operation. The measured parameters are continually compared with predetermined limit values, and various CRT operation controls are adjusted in response to the comparison to maintain the measured parameter values below the predetermined limit values.

Description: Field of The Invention The present invention relates to a method and a circuit for limiting X-ray production in operating cathode ray tubes, and more particularly for adjusting the CRT operation to limit X-ray production while allowing operation of the CRT to continue. During operation of a cathode ray tube, electrons are accelerated through a high potential difference, on the order of tens of kilovolts. As a consequence, they acquire large kinetic energy and will cause X-ray production when they undergo collision with matter. In order to guard against X-ray overexposure of people in proximity to apparatus which has a CRT, the United States government has promulgated regulations concerning the permissible limits to X-ray production. As they apply to television receivers, these regulations are presently set forth at Title 21 of the US Code of Federal Regulations, Section 1020. Description of The Related Art In order to meet the Federal regulations, it is an established practice to incorporate in television receivers a control circuit which turns the receiver off when operating conditions which may result in excessive X-ray production occur. It is not practicable to measure the X-ray flux directly; instead the operating parameters of the cathode ray tube are measured and when they reach values which might result in excessive X-ray production, the CRT is rendered inoperative. The type of protection circuits currently in use are analog latching circuits which disable the CRT anode supply in the receiver in the event that the anode power or voltage exceeds predetermined limits for the particular receiver CRT. Although this type of protection circuit does ensure compliance with the Federal regulations, the conventional protection circuit has drawbacks. US-A-4,841,406 describes a prior art X-ray radiation protection circuit for use in a television receiver. In the prior art protection circuit, a first detector is interconnected with an electron beam power source and monitors the beam current therefrom. The first detector produces a first voltage proportional in magnitude to the electron beam current. A second detector monitors a high voltage transformer that produces a potential for operating on the electron beam and the second detector produces a second voltage that is proportional in magnitude to the high voltage potential. A monitoring circuit constantly monitors both the first and second voltages and produces a trigger signal whenever the additive sum of the first and second voltages exceeds a selected magnitude. The trigger signal indicates an excessively high combination of electron beam current and high voltage. The trigger signal is supplied to a trigger responsive device which disables the power supply for the entire television set. The prior art protection circuit often is rendered operative and shuts down the CRT during input signal transitions or during momentary horizontal oscillator excursions caused by non-standard signal reception. Additionally, this analog circuit requires very tight component tolerances to ensure its proper operation. Notwithstanding the use of precision components, it is still necessary to design the prior art protection circuit to operate the CRT with a margin in its operating parameters to ensure that excessive X-ray production does not occur under worst case conditions. As a result, the CRT may have to be operated at operating parameter values for which there is a considerable comprise in performance. It would be desirable to have the protection circuit adjust the operation of a CRT to maintain it operating under permissible conditions, and not shut it off under transient conditions that will not result in excessive X-ray production. SUMMARY OF THE INVENTION It is an object of the invention to provide a method of and an apparatus for operating a CRT which continually monitors the operating parameters of an operating CRT and adjusts various operation controls of the CRT in order to maintain the operating parameters within permissible ranges. It is another object of the invention to provide a method and circuit for avoiding excessive x-ray production in a operating CRT which is digital and reduces the requirements for precision analog components. In accordance with an aspect of the invention there is provided an apparatus as set out in Claim 1. In accordance with an other aspect of the invention there is provided a method as set out in Claim 10. Additional features, which may optionally be used to implement the invention to advantage, are defined in the dependent claims. According to the invention, the operating parameters of an operating CRT that are correlated with x-ray production are continually measured. The measured operating parameter values are continually compared with predetermined values of the operating parameters during operation of the CRT. As a result of the comparison, various operation controls for the CRT are adjusted to maintain the operating parameters below certain predetermined limit values corresponding to the limits of permissible x-ray production by the CRT. This in contradistinction with the prior art X-ray radiation protection circuit which provides a yes/no shut off decision in the form of a trigger pulse, rather than an adjustment to maintain the CRT operating under permissible conditions. Preferably, the comparison of the measured parameter values and the predetermined parameter values is carried out under microprocessor control. The microprocessor compares each of the measured operating parameter values in succession with a corresponding limit value and continually repeats the sequence of successive comparisons. The various operation controls for the cathode ray tube are then adjusted by the microprocessor within a hierarchical sequence. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a protection circuit according to the invention; and Fig. 2 is a flowchart of the protection method according to the invention. DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 illustrates a circuit of part of an apparatus having a cathode ray tube (CRT) and the X-ray protection circuit according to the invention. The CRT 1 is operated by an anode voltage supplied from the secondary winding of a high voltage transformer 2. The primary winding of the transformer 2 receives the high voltage supply voltage B+ which is stepped up by the transformer 2 to the anode voltage. The anode voltage is sensed by a circuit including diode 5, capacitor 6 and resistor 7. This circuit operates as a simple half-wave rectifier, and the voltage developed across the resistor 7 has a value which is indicative of the anode voltage of the CRT 1. The CRT beam current is sensed by a circuit including resistors 10 and 13, diode 11 and capacitor 12. The resistor 10 is grounded and a current flowing through it is proportional to the CRT beam current. Consequently, the voltage developed across the resistor 10 is also proportional to the beam current, and this voltage is sensed by a half-wave rectifier formed by the diode 11, capacitor 12 and resistor 13. The sensed analog signals representative of the beam current, the anode voltage and the high voltage supply voltage B+ are applied to an analog multiplexer 20. These analog voltages are sequentially applied by the multiplexer 20 to an analog-to-digital (A/D) converter 21. The output of the A/D converter 21 is a sequence of digital values of samples of successive ones of the analog signals representative of the CRT operating parameters: beam current, anode voltage and high voltage supply voltage. The sequence of digital values is applied to a microprocessor 22. The microprocessor 22 is shown connected to read-only memory (ROM) 23 and electrically erasable programmable read-only memory (EEPROM) 24. It will be understood that either ROM or EEPROM, or both, could be used, and they can be separate circuit elements or contained in the integrated circuit comprising the microprocessor 22. Moreover, other components could be included in the same integrated circuit. For example, Signetics type 8050 IC's which include a microprocessor, A/D converter and a multiplexer on one integrated circuit could be used. A set of predetermined limit values of the CRT operating parameters are stored in the ROM 23, EEPROM 24 or a memory internal to the microprocessor 22. The memory also stores instructions for the microprocessor 22. The operating parameters sensed by the circuit are correlated with X-ray production by the CRT during its operation. The stored predetermined limit values of the operating parameters include certain predetermined limit values corresponding to the limits of permissible X-ray production by the CRT. The comparison is continually carried out during CRT operation, and when the measured values of the operating parameters approach the predetermined limit values, the microprocessor 22 develops an appropriate output signal for initiating some adjustment of various operation controls for the CRT. The possible various CRT operation controls include a hierarchy of actions, with the most severe being a complete shutdown of the apparatus. First, the microprocessor 22 controls the picture being shown. Next, it reduces the picture brightness. The next level of control is to shutdown the CRT anode voltage, and after that the B+ high voltage supply voltage. The most extreme action is to turn off all power. In an advantageous implementation, the hierarchy of adjusting CRT operation controls is as follows: Lowering brightness control, lowering supply voltage, changing color video broadcast signal (CVBS) input, switching off high voltage, and switching apparatus off. An additional feature of the invention is the provision of a timer 30 which periodically applies a timing signal to the microprocessor 22. The microprocessor 22 then tests whether a predetermined number of parameter value comparisons have been carried out during an interval determined by the timer 30. If sensor signals are not being applied to the microprocessor 22, or if some other failure has prevented the microprocessor 22 from carrying out parameter value comparisons, then the microprocessor 22 detects this condition and shuts down CRT operation, for example through a B+ controller 35. Thus, the CRT cannot continue operating without being subject to the control of the protection circuit. Another feature of the invention is that it can be used to regulate the operating parameters with respect to criteria other than X-ray production. For example, samples of the B+ voltage value are applied to the microprocessor 22. The controller 35 for the B+ voltage, which is responsive to a control signal, receives the control signal from the microprocessor 22 over the signal path 36. Thus, variations in B+ voltage can be regulated by the protection circuit. It should be noted that this function may require a more powerful microprocessor 22 and more memory than if only the X-ray protection function is carried out. Fig. 2 illustrates a flowchart of the operations carried out in order to control CRT operations. Initially, a sensor input is read and compared with a corresponding limit value (steps 101, 103 and 105). If the comparison indicates that the measured parameter value is within the predetermined limit, then the process continues of reading inputs and comparing them (steps 107, 109 and 101-105). If the comparison indicates that a measured value exceeds its corresponding limit value, then a determination is made as to whether further operation will be allowed (step 111). If further operation is allowed, the CRT operation controls are adjusted (step 113), and then the process of reading and comparing operating parameter values continues (steps 107, 109 and 101-105). On the other hand, if no further operation is to be permitted after an out-of-limit condition is detected, for example, because the out-of-limit condition has continued after a predetermined number of comparisons/adjustments, no further adjustment is made. Instead, the system is shut down (step 115). Another feature of the invention includes a provision for displaying on the CRT a visual image to alert the viewer that adjustments are being made to the operation controls and that there is need for servicing. The displayed message can be simple for the purpose of merely alerting that servicing is required, or can include the identity of the parameter which requires correction and the out-of-range values in order to assist a technician in diagnosing the problem even before the apparatus containing the CRT is opened. It will be appreciated that the particular adjustments made in response to an out-of-limit condition can be varied from the particular example given here, and different parameters can be sensed and compared in order to determine the need for adjustment. For example, instantaneous values of the beam current can be compared against a predetermined limit value, and a running average of the beam current can be determined by the microprocessor and the running average value can be compared with a different predetermined limit. Thus, the particular embodiment disclosed herein should be understood as being exemplary and not limiting, and that the scope of the invention is determined by the following claims.

Claims: An apparatus having a cathode ray tube (1) and a high voltage power supply (2, 35) for supplying high voltage to operate the cathode ray tube, comprising: measuring means (5-7, 10-13, 20, 21) for continually measuring operating parameters of said cathode ray tube, said operating parameters including at least a voltage on an anode of the cathode ray tube, and a beam current of the cathode ray tube; and protection means (22, 23, 24, 30) for preventing an excessive X-ray production by the cathode ray tube on the basis of the continually measured operating parameters, characterized in that the protection means comprise: a memory (23, 24) for storing predetermined values of operating parameters of said cathode ray tube corresponding to the limits of permissible X-ray production by said cathode ray tube; a controller (22) for comparing the values of the continually measured operating parameters of said cathode ray tube with said predetermined limit values of operating parameters and for adjusting various operation controls for said cathode ray tube to maintain said operating parameters below said predetermined limit values corresponding to the limits of permissible X-ray production, the various operation controls including at least brightness control. An apparatus according to Claim 1, wherein said measuring means (5-7, 10-13, 20, 21) comprises: an analog-to-digital converter (21) for converting analog signals to digital signals; an analog multiplexer (20) for receiving multiple analog signals and applying them to said analog-to digital converter; and a plurality of sensors (5-7, 10-13), each sensor sensing an operating parameter of said cathode ray tube and developing an analog signal representative of the sensed parameter and applying the analog signal to said multiplexer. An apparatus according to Claim 2, wherein said sensors (5-7, 10-13) comprise: means (5-7) for sensing the cathode ray tube anode voltage; and means for (10-13) sensing the beam current of said cathode ray tube. An apparatus according to Claim 3, wherein said measuring means further include means for sensing the high voltage supply (2, 35) of said cathode ray tube. An apparatus according to Claim 1, wherein said controller (22) comprises: a microprocessor programmed to compare the values of the continually measured operating parameters with said predetermined limit values and for developing output signals for adjusting the various operation controls for said cathode ray tube within a hierarchical sequence to maintain said operating parameters below said predetermined limit values. An apparatus according to claim 5, wherein said hierarchical sequence of adjusting operation controls for said cathode ray tube comprises: lowering brightness control, lowering supply voltage, changing color video broadcast signal (CVBS) input, switching off high voltage, and switching apparatus off. An apparatus according to Claim 1, wherein said apparatus further comprises: means for displaying on the cathode ray tube (1) a message indicating that operation controls for the cathode ray tube are being adjusted and that a condition requiring correction exists. An apparatus according to Claim 7, wherein said means for displaying displays at least the measured values of the operating parameters which exceed the predetermined limit values to permit viewing thereof. An apparatus according to Claim 1, wherein said controller (22) develops a control signal (36) for regulating the high voltage supply voltage (2, 35). A method of preventing excessive X-ray production from a cathode ray tube (1) operating in an apparatus, said method comprising the steps; continually measuring (5-7, 10-13, 20, 21) operating parameters of the cathode ray tube that are correlated with X-ray production by the cathode ray tube during its operation, said operating parameters including at least a voltage on an anode of said cathode ray tube, and a beam current of said cathode ray tube; and preventing an excessive X-ray production by the cathode ray tube on the basis of the continually measured operating parameters; characterized in that said preventing step comprises the steps of: continually comparing (22) the measured operating parameter values during cathode ray tube operation with predetermined values of the operating parameters; and adjusting (22) various operation controls for the cathode ray tube maintain the operating parameters below certain predetermined limit values corresponding to the limits of permissible X-ray production by the cathode ray tube, said operation controls including at least brightness control.

Inventors: JERNIGAN FOREST, JERNIGAN, FOREST, Jernigan, Forest, c/o INT. OCTROOIBUREAU B.V.
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H04N17
IPC codes: H04N3, G09G1, H04N5