Abstract:
A display apparatus with a cathode ray tube has a degaussing circuit fed by a voltage. The degaussing circuit includes at least a PTC resistor and at least a degaussing coil. Further circuit elements are provided for selecting an impedance of the degaussing circuit based on said voltage. A process Degaussing the cathode ray tube is also described occurs according to a specific process.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a display apparatus having a cathode ray tube, to a degaussing circuit for such a display apparatus, and to a process for degaussing a cathode ray tube.  
       BACKGROUND OF THE INVENTION  
       [0002]     Display apparatus using a cathode ray tube are generally equipped with a degaussing circuit in order to remove the residual magnetic field from the metallic parts. Generally, the degaussing circuit is operated when turning on the display apparatus or upon command from the user.  
         [0003]     The operating parameters of the degaussing operation (intensity, pseudo-period of the pulses, decay time, etc.) are determined to best fit a given apparatus and the components of the degaussing circuit are chosen to obtain these parameters. However, in a conventional apparatus, these choices are made for a given standard of the mains voltage to which the apparatus is to be connected.  
         [0004]     As a consequence, a problem occurs when an apparatus has to be connected to several standards of mains voltage, notably when the AC input voltage may be either 110 V or 230V.  
       SUMMARY OF THE INVENTION  
       [0005]     The invention proposes a display apparatus with a cathode ray tube and a degaussing circuit fed by a voltage and comprising at least a PTC resistor, at least a degaussing coil and means for selecting an impedance of the degaussing circuit based on said voltage.  
         [0006]     The impedance of the degaussing circuit can therefore be selected, notably so that the operating parameters best fit the display apparatus when it is plugged to mains with a given voltage standard.  
         [0007]     In a possible embodiment, the display apparatus comprises a voltage sensor circuit for measuring said voltage and at least one switch controlled by said voltage sensor circuit.  
         [0008]     According to a possible solution, said switch selectively connects a first PTC resistor or a second PTC resistor in the degaussing circuit.  
         [0009]     When said degaussing circuit comprises a first degaussing coil and a second degaussing coil, it is particularly advantageous and simple that said first degaussing coil and said second degaussing coil be selectively connected in series or in parallel.  
         [0010]     The invention also proposes a process for controlling such a display apparatus, which solves the above problem in a particularly safe way. This proposed process has the successive steps of 
        sensing said voltage;     selecting said impedance based on said voltage;     operating said degaussing circuit.        
 
         [0014]     The invention therefore also proposes a degaussing circuit for a display apparatus with a cathode ray tube, the degaussing circuit being fed by a voltage and comprising at least a PTC resistor, at least a degaussing coil and means for changing the impedance of the degaussing circuit based on said voltage.  
         [0015]     The effect of the degaussing circuit can thus be optimal even though the feeding voltage may vary.  
         [0016]     According to preferred embodiments 
        the degaussing circuit has a voltage sensor circuit for measuring said voltage and at least one switch controlled by said voltage sensor circuit;     said switch selectively connects a first PTC resistor or a second PTC resistor in-circuit;     the degaussing circuit comprises a first degaussing coil and a second degaussing coil, and said first degaussing coil and said second degaussing coil are selectively connected in series or in parallel. Lastly, the invention proposes a process for degaussing a cathode ray tube with a degaussing circuit fed by a voltage, comprising the successive steps of:     sensing said voltage;     selecting an impedance of said degaussing circuit based on said voltage;     supplying said voltage to said degaussing circuit.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     Other features of the invention will appear from the following description of several embodiments of the invention made in the light of the appended drawings, where:  
         [0024]      FIG. 1  represents a degaussing circuit of a first embodiment of the invention;  
         [0025]      FIGS. 2   a  to  2   d  are timing diagrams showing operation of various elements of the circuit of  FIG. 1 ;  
         [0026]      FIG. 3  represents a degaussing circuit of a second embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]      FIG. 1  depicts a degaussing circuit of a first embodiment of the invention. The display apparatus having the degaussing circuit of  FIG. 1  is connected to the mains via a plug P. The plug P is connected to the electrical circuitry of the display apparatus through a mains switch  1 . When the apparatus is switched on, the two electrical inputs E 1 , E 2  (outputs of the mains switch  1 ) thus carry an AC voltage of 110 V or 230 V.  
         [0028]     The first electrical input E 1  is connected to a first pin of a first PTC resistor R 1  and to a first pin of a second PTC resistor R 2 . The second pin of the first PTC resistor R 1  and the second pin of the second PTC resistor R 2  are respectively connected to a first input of a selector switch  6  and to a second input of this selector switch  6 .  
         [0029]     The first PTC resistor R 1  is suitable when the mains voltage (voltage between the electrical inputs E 1 , E 2 ) is 110 V whereas the second PTC resistor R 2  is suitable when the mains voltage is 230 V.  
         [0030]     An output of the selector switch  6  is connected to an input of a control switch  8 . The output of the control switch  8  is connected to a first extremity of a degaussing coil DGC, which second extremity is connected to the second electrical input E 2 .  
         [0031]     The control switch  8  is controlled by a control circuit  4 , for instance a micro-processor of the display apparatus.  
         [0032]     A voltage sensor circuit  2  is also connected to the electrical inputs E 1  and E 2  in order to measure the voltage difference between them (and thus the mains voltage supplying the apparatus) and to control the selector switch  6  depending on the measured voltage.  
         [0033]     For instance, R 1  is a 5 Ω PTC resistor referenced BC96686 whereas R 2  is a 26 Ω PTC resistor referenced BC96688 (for use with a degaussing coil DGC having a resistance of 25 Ω).  
         [0034]     Operation of the degaussing circuit will now be explained with reference to  FIGS. 2   a  to  2   d.    
         [0035]     As shown by  FIG. 2   a , the mains switch  1  is closed by the user of the display apparatus at time t 0 . At that time, a lot of transients may occur. The voltage sensor circuit  2  may not be able to operate correctly, on the one hand because it may not be correctly initialised yet and on the other hand because the voltage between the electrical inputs E 1  and E 2  may not be stabilised.  
         [0036]     Normal operation of the voltage sensor circuit  2  thus only starts at time t 1 , about 100 ms later than t 0 , as represented on  FIG. 2   b.    
         [0037]     At time t 1 , the voltage sensor circuit  2  sends a control signal to the selector switch  6  depending on the voltage measured between electrical inputs E 1  and E 2 . If the measured voltage is below 170 V, the voltage sensor circuit  2  controls the selector switch  6  so that it connects the first PTC resistor R 1  to the control switch  8  (first input of the selector switch  6  selected). If the measured voltage is above 170 V, the voltage sensor circuit  2  controls the selector switch  6  so that it connects the second PTC resistor R 2  to the control switch  8  (second input of the selector switch  6  selected).  
         [0038]     At time t 2 , after a short delay, due for instance to the activation time of the selector switch  6  (e.g. when this selector switch is a relay), the selector switch  6  is in the required state as illustrated on  FIG. 2   c  and the desired PTC resistor (R 1  or R 2 ) is thus actually connected to the input of the control switch  8 .  
         [0039]     Between times t 3  and t 4 , the control circuit  4  activates the degaussing operation by closing the control switch  8  as shown on  FIG. 2   d . Time t 3  is later than time t 2  so that the correct PTC resistor (R 1  or R 2 ) is connected to the degaussing coil DGC from the beginning of the degaussing operation triggered by the control circuit  4  via the control switch  8 .  
         [0040]     This can realised by triggering the control switch  8  after a pre-determined time T (T=t 3 −t 0 ), stored as a parameter of the control circuit  4  and computed based on the maximum expected delay to have the selector switch  6  in the requested state (delay t 2 −t 0 ).  
         [0041]     The degaussing operation lasts for a pre-determined period of time (t 4 −t 3 ) necessary for the decay of the degaussing current to take place thanks to the used PTC resistor (R 1  or R 2 ).  
         [0042]     A second embodiment of the invention is represented on  FIG. 3 . Electrical inputs E 1  and E 2  are fed from the mains via a plug P and a mains switch  11 . Two PTC resistors R are connected in parallel between electrical input E 1  and a first pin of a control switch  18 . In the described example, two identical PTC resistors R (Murata 9 Ω) are used. Instead of 2 parallel PTC resistors, 2 PTC resistors connected in series or even a single PTC resistor could be used.  
         [0043]     The second pin of the control switch  18  is connected to a first input of a first path switch  17  through a first degaussing coil DGC 1 , to a second input of the first path switch  17  and to a first pin of a capacitor C.  
         [0044]     The first input of the first path switch  17  is also connected to a second input of a second path switch  15 . The first input of the second path switch  15  is left unconnected.  
         [0045]     Electrical input E 2  is connected to the output of the second path switch  15 , to the output of the first path switch  17  through a second degaussing coil DGC 2  and to the second pin of the capacitor C.  
         [0046]     Capacitor C (0.1 μF) is meant to reduce the impedance of the degaussing coils DGC 1 , DGC 2  at the horizontal scanning frequency.  
         [0047]     Preferably, the first degaussing coil DGC 1  is a top degaussing coil whereas the second degaussing coil DGC 2  is a bottom degaussing coil. As a possible variation, DGC 1  is the bottom degaussing coil and DGC 2  the top degaussing coil.  
         [0048]     A voltage sensor  12  is connected to electrical inputs E 1  and E 2  in order to measure the voltage therebetween, determine the mains voltage and accordingly control the first and second path switches  15 ,  17  as further explained below.  
         [0049]     A controller  14  (for instance a micro-processor of the display apparatus) controls the control switch  18  to trigger the degaussing operation.  
         [0050]     If the voltage sensor  12  detects a mains voltage lower than 180 V, the first and second path switches  15 ,  17  are controlled so that their second input contacts their output. The first degaussing coil DGC 1  is then connected to electrical input E 2  via the second path switch  15  and the second degaussing coil DGC 2  is connected to the second pin of the control switch  18 . The first and second degaussing coils DGC 1 , DGC 2  are thus connected in parallel.  
         [0051]     If the voltage sensor  12  detects a mains voltage higher than 180 V, the first and second path switches  15 ,  17  are controlled so that their first input contacts their output. The first and second degaussing coils DGC 1 , DGC 2  are then connected in series between electrical input E 2  and the second pin of the control switch  18 .  
         [0052]     Thanks to this connection modification of the degaussing coils DGC 1  and DGC 2 , the impedance of the degaussing circuit is modified to best fit the mains voltage, i.e. to keep a slow decay time whatever the mains voltage.  
         [0053]     The degaussing operation is made by having the controller  14  close the control switch  18  for a predetermined period. This period can be determined as in the first embodiment so that the adapted connection is made between the degaussing coils DGC 1 , DGC 2  (by use of the first and second path switches  15 ,  17 ) before the degaussing operation is triggered by the controller  14 .  
         [0054]     The invention is of course not limited to the above embodiments. For instance, other values for the threshold voltage than 170 V or 180 V can be used.