GB2247384A - Automatic degaussing circuit for a picture tube - Google Patents

Abstract

Motion sensors 5. 6 detect when the picture tube cabinet 1 is moved. In response to this detection the degaussing circuit 7 is automatically operated. The motion sensors may be pendulum devices, trembler switches, tilt switches or gyroscopic devices. Alternatively the sensors 5, 6 may be magnetic sensors that respond to a change in the local magnetic field to initiate degaussing. <IMAGE>

Description

DEGAUSSING CIRCUIT FOR A PICTURE TUBE The invention relates to a degaussing circuit for a picture tube in a television device. According to prior art for degaussing operation an alternating current with amplitude smoothly decreasing to zero is fed to a degaussing coil mounted on the picture tube. Generally the degaussing operation is initiated at receiver switch on. After the degaussing current has decreased to zero or nearly to zero the degaussing circuit does not operate during operation of the device.

If the magnetic environment of the picture tube is disturbed during operation then the shadow mask of the picture tube is magnetized causing disturbances within the reproduced picture. These disturbances are not eliminated because there is no degaussing operation before the device is switched off and switched on again.

It is an object of the invention to modify the degaussing circuit in such a way that the degaussing operation is initiated automatically as required independently of the device being switched off and on.

According to the invention means are provided for detecting a movement of the cabinet of the device and/or changes of magnetic field within the device said means initiating degaussing operation.

The invention is based on the following considerations. If the cabinet of the receiver is moved then the relationship between magnetic earth field and the receiver cabinet is changed and consequently the picture tube will be magnetized so that the colour purity of the displayed picture is degraded. In order to compensate for such disturbances said movement or said changes in magnetic field are detected to initiate the degaussing operation. It may be that during the degaussing operation a brief visible disturbance arises due to the degaussing current. However, this short disturbance is more acceptable than a degraded picture for the whole time of operation after movement or changes in magnetic field.For initiating the degaussing operation a standby circuit may be used so that degaussing operation upon detection of movement of the cabinet or upon detection of change of magnetic field is initiated even if the receiver is in the standby mode.

Preferably said means for detecting movement of the cabinet is in addition or instead of detection of changes in the local magnetic field. This is based on the fact that generally in case of movement the relationship between the local magnetic field of the earth and a magnetic sensor is changed so that output signal of said magnetic sensor varies with movement of the cabinet. This variation can be used for initiating degaussing operation.

Magnetic detectors will also detect changes in the local magnetic field due the proximity of magnets such as those contained in external loudspeakers or toys.

Sensors for detecting movement are well known and may take the form of what are generally called "gyros". Such gyros and their application for detecting movement of a camera are more fully described in US-PS 4 612 575. Examples of other movement detecting sensors are pendulum devices and trembler switches, for instance mercury tilt switches. A sensor for detecting magnetic field even in case receiver is not moved is for example a so-called electronic compass as described in WO 88/06274.

In order that the invention may more readily be understood, a description is now given by way of example only, reference being made to the drawing. Within the drawing Figure 1 shows the principle of the invention in a simplified manner, Figure 2 shows a detailed circuit for initiating the degaussing operation and Figure 3 shows waveforms for describing the operation of the circuit according to Figure 2.

Figure 1 shows the cabinet 1 of a television receiver including a picture tube 2 carrying a degaussing coil DC. The receiver is connected to antenna 3 and a power supply 4. One or more movement detecting sensors are mounted within cabinet 1. In Figure 1, as an example only, two sensors 5, 6 are shown within cabinet 1.

If only one sensor is present, then movement in one direction is detected. In Figure 1, said sensors 5, 6 detect movement of the cabinet 1 in two orthogonal directions. Output signals of sensors 5, 6 are fed to degaussing circuit 7 generating degaussing current im.

In each case the sensors 5, 6 detect a movement of the cabinet 1 in the corresponding direction. The degaussing circuit 7 is controlled in such a way that degaussing current im is initiated which decreases to zero or nearly zero within a short interval, which is typically about 1 second. Initiating of the degaussing operation is indicated symbolically by switch S. For initiating degaussing operation a standby circuit may be used. This has the advantage that degaussing upon movement of the cabinet or upon change of magnetic field within the cabinet is initiated even if the receiver is in the so called "standby" condition. In the case of standby operation, minor adaptations of the power supply circuits of the receiver may by required.

Figure 2 shows a circuit by which degaussing current with amplitude decreasing to zero can be initiated by actuating switch S. A line flyback pulse 8 with an amplitude of about 1.200 V is derived from the collector of the line output switching transistor not shown. This pulse 8 is fed from terminal 9 via diode D1 and resistor R1 to capacitor C1. Capacitor C1 is charged to a voltage U1 of about 1.000 V. Capacitor C2 is charged via resistor R5 from a positive voltage U2 of approximately +150 V. The time constant C2 * R3 is sufficiently long to allow C1 to reach 1.000 V before the increasing voltage Us at capacitor C2 can fire via resistor R3 diac Di and thereby switch on transistor T1.When diac Di fires, T1 conducts so that C1 discharges through the degaussing coil DC connected parallel to C1 via the first switch constituted by transistor T1 and the antiparallel shunted diode D2. The degaussing coil DC resonates with C1 so that an oscillatory current im flows via D2 and T1. This current im decays to zero, or nearly zero, whereby the degaussing action occurs. Time constant R2 * C2 is long enough to supply a base current pulse to T1 which is sufficiently long to allow the degaussing current to decay smoothly to zero, or nearly to zero. The switch S parallel to C2 is closed to terminate the degaussing operation and to prevent further degaussing action.

Figure 3 shows different waveforms in Figure 2.

Assume that C1 is already charged to U1 and switch S is closed so that Us is zero and Di does not fire. T1 is cut off and no degaussing current im can flow through DC. At time tl switch S is opened for initiating degaussing operation. Voltage Us at C2 increases by the charging current from U2 via R5. At time t2 voltage Us reaches threshold value Usl. In this moment Di fires and transistor T1 becomes conductive so that degaussing current im is initiated.

At t3 degaussing current im decays to zero due to discharge of C1. For finishing degaussing operation and preventing further degaussing current switch S is closed so that Us decays to zero and T1 is turned off.

Claims (13)

1. Degaussing circuit for a picture tube in a television device wherein for degaussing operation an alternating current with amplitude smoothly decreasing to zero is fed to a degaussing coil mounted at the picture tube, characterized in that means (5, 6) are provided for detecting a movement of the cabinet (1) of the device and/or changes of magnetic field within the device said means initiating degaussing operation.

2. Circuit according to Claim 1, characterized in that said means include one or more gyros (5, 6).

3. Circuit according to Claim 1, characterized in that said means include one or more pendulums with electrical contacts to detect movement.

4. Circuit according to Claim 1, characterized in that said means include one or more mercury tilt switches.

5. Circuit according to Claim 1, characterized in that said means include one or more switches actuated by movement.

6. Circuit according to Claim 1, characterized in that said means for detecting changes of magnetic field include an electronic compass.

7. Circuit according to Claim 1, characterized in that said means detects movement and is also sensitive to changes in magnetic field.

8. Circuit according to Claim 1, characterized in that degaussing operation is initiated by means of a standby circuit of the device.

9. Circuit according to Claim 1, characterized in that said degaussing coil (CD) is connected via a switch (T1, D2) to a first capacitor (C1) charged to a dc-voltage degaussing operation being initiated by closing said switch (Fig. 2).

10. Circuit according to Claim 9, characterized in that said switch is constituted by an antiparallel circuit of a transistor (T1) and a diode (D2).

11. Circuit according to Claim 10, characterized by a second capacitor (C2) being connected to the base of said transistor (T1) and via a resistor (R5) to a dc voltage (U2) a second switch (S) being connected parallel to said second capacitor (C2) and being opened for initiating degaussing operation (Fig. 2).

12. Circuit according to Claim 11, characterized by a diac (Di) connected between said second capacitor (C2) and the base of said transistor (T1).

13. Circuit as described within the specification and the drawing.