GB2318038A

GB2318038A - Automatic degaussing circuit for a color picture tube

Info

Publication number: GB2318038A
Application number: GB9721274A
Authority: GB
Inventors: Kwang Ho Lee
Original assignee: LG Electronics Inc
Current assignee: LG Electronics Inc
Priority date: 1996-10-07
Filing date: 1997-10-07
Publication date: 1998-04-08
Anticipated expiration: 2017-10-07
Also published as: ID18640A; MX9707682A; KR19980026117A; BR9702980A; KR100237958B1; GB9721274D0; GB2318038B

Abstract

An automatic degaussing circuit for a colour picture tube (CPT) is capable of automatically degaussing the CPT when it is released from a power conservation function. The degaussing circuit includes a degaussing coil L1 for eliminating electromagnetic waves produced from the CPT, a power conservation mode detecting section 10 for detecting the power conservation mode signal of the CPT and providing a degaussing control signal for controlling the operation of the degaussing coil L1. A degaussing coil driving section drives the degaussing coil for a predetermined time when the power conservation mode is released.

Description

AUTOMATIC DEGAUSSING CIRCUIT FOR COLOR PICTURE TUBE The present invention relates generally to a colour picture tube of a monitor. In particular, the present invention relates to a degaussing circuit for a colour picture tube (CPT) which can automatically degauss the OPT when a power conservation mode of the monitor is released.

Generally, in a CPT, electron beams which have been emitted from a cathode heated by a heater are controlled, accelerated, focused, and then strike phosphor dots on a fluorescent screen of the OPT to display a video signal.

During these processes, electromagnetic waves are produced from the CPT, and exert harmful influence upon the peripheral circuits of the OPT and the human body. Accordingly, in order to eliminate such electromagnetic waves, the OPT is provided with a degaussing device for degaussing the electromagnetic field which is instantaneously produced especially when the power is initially supplied.

According to a conventional degaussing device, if a power switch is turned on and an initial power is supplied to the CPT, the degaussing device operates for a predetermined time to degauss the electromagnetic waves produced from the CPT. As a result, the degaussing device performs the degaussing operation only when the power is initially supplied, and then will not perform the degaussing operation until the initial power is supplied again after the power is off.

Recently, most video display appliances such as monitors employ a power conservation function for preventing unnecessary power consumption of the appliance. Thus, during a normal operation of the appliance wherein a power conservation mode is released, the degaussing operation is not performed without actuating a separate degaussing switch.

If such a degaussing switch is not provided on the appliance, the electromagnetic waves produced from the OPT cannot be eliminated at all during normal operation of the CPT.

It is an object of the present invention to solve the problems involved in the prior art, and to provide a degaussing circuit which can automatically perform the OPT degaussing operation when the power conservation mode is released regardless of the on/off state of the degaussing switch.

In order to achieve the above object, there is provided a degaussing circuit for a OPT having a degaussing coil for eliminating electromagnetic waves, the degaussing circuit comprising a power conservation mode detecting section for detecting a release of a power conservation mode in accordance with a power conservation mode signal of the OPT inputted thereto and providing a degaussing control signal for controlling an operation of the degaussing coil if the release of the power conservation mode is detected, and a degaussing coil driving section for driving the degaussing coil for a predetermined time in accordance with the degaussing control signal inputted from the power conservation mode detecting section when the power conservation mode is released.

The above object, other features and advantages of the present invention will become more apparent by describing the preferred embodiment thereof with reference to the accompanying drawings, in which: FIG. l is a schematic circuit diagram of the automatic degaussing circuit for a OPT in which the present invention is embodied.

FIG. 2A to 2F are waveform diagrams illustrating the waveforms appearing at various points of FIG. 1.

FIG. 1 shows the construction of the automatic degaussing circuit for a CPT.

Referring to Fig. 1, the automatic OPT degaussing circuit includes a degaussing coil Li, installed on a proper portion of the CPT, for degaussing electromagnetic waves produced from the CPT, a thermistor THi, one terminal of which is connected to one terminal of the degaussing coil Li, for controlling the amount of current flowing to the degaussing coil Li, a bridge rectifying section Bud1, coupled between the other terminal of the degaussing coil Li and one terminal of a power switch SWl, for rectifying an input AC power supply, a power conservation mode detecting section 10 for detecting a release of a power conservation mode in accordance with a power conservation mode signal inputted thereto from a power conservation circuit (not illustrated) of the CPT, and outputting a degaussing control signal for controlling the operation of the degaussing coil Li if the release of the power conservation mode is detected, and a degaussing coil driving section for driving the degaussing coil Li for a predetermined time in accordance with the degaussing control signal inputted from the power conservation mode detecting section 10 when the power conservation mode is released.

The power conservation mode detecting section 10 includes a transistor Ql for performing a switching operation according to the power conservation mode signal, a Zener diode ZDl for passing therethrough a voltage supplied by the switching operation of the transistor Ql only when the voltage is higher than the Zener voltage thereof to prevent the erroneous operation of the power conservation mode detecting section 10, a transistor Q2 for performing a switching operation in accordance with the voltage supplied through the Zener diode ZDl, a resistor R8 and a capacitor C3, connected between the anode of the Zener diode ZDl and the base of the transistor Q2, for determining a switching timing of the transistor Q2. A bias voltage of 12V is supplied to the collector of the transistor Ql and the cathode of the Zener diode ZDl through a resistor R6.

Meanwhile, the degaussing coil driving section 20 includes a relay RLl, whose movable contacts are connected to one terminal of the thermistor THi and one terminal of the power switch SWl, respectively, for controlling the voltage supplied to the degaussing coil Li through the thermistor THl, a Zener diode ZD2, coupled to the output terminal of the power conservation mode detecting section 10, for passing therethrough a voltage supplied by the output of the power conservation mode detecting section 10 only when the voltage is higher than the Zener voltage thereof, and a transistor Q3, connected to a driving coil of the relay RLl, for controlling the operation of the relay RLl in accordance with the voltage supplied through the Zener diode ZD2. The bias voltage of 12V is supplied to the other terminal of the driving coil of the relay RLi and the cathode of the Zener diode ZD2.

In FIG. 1, the reference numeral Rl denotes a resistor for the prevention of rush current, Rl to R7 denote resistors, and C1, C2 and C4 denote capacitors.

The operation of the OPT degaussing circuit as constructed above will now be explained in detail with reference to FIGs.

1, 2A to 2F.

Referring to Fig.l, if the power switch swi is turned on, the common contact c of the relay RLl is in contact with the movable contact a at an initial state, and thus, as shown in FIG. 2A, a degaussing coil driving voltage is supplied to the degaussing coil Li through the thermistor THi. Accordingly, the degaussing coil Li is driven to perform the degaussing operation.

Specifically, at the initial state of the CPT, the transistor Q2 in the power conservation mode detecting section 10 is turned off, while the transistor Q3 in the degaussing coil driving section 20 is turned on. Accordingly, a driving voltage is supplied to the driving coil of the relay RLl, causing the common contact c of the relay RL to become in contact with the movable contact a. Thus, the degaussing coil driving voltage is supplied to the degaussing coil Li through the contacts of the relay RLi and the thermistor THi, so that the degaussing coil LI performs the degaussing operation.

In this state, if the resistance value of the thermistor THi increases as a predetermined time elapses, no more operating current flows to the degaussing coil Li through the thermistor THi, causing the operation of the degaussing coil Li to stop.

On the other hand, if the power conservation mode of the CPT is on, a 'high' level power conservation mode signal is inputted from the power conservation circuit of the CPT to the base of the transistor Ql in the power conservation mode detecting section 10 through dividing resistors R4 and R5. Thus, the transistor Ql is turned on, and the voltage of 12V pass through the resistor R6 and the collector-emitter path of the transistor Ql. Accordingly, no voltage is applied to the cathode of the Zener diode ZDl, and a 'low' level signal is applied to the base of the transistor Q2, causing the transistor Q2 to be turned off.

As the transistor Q2 in the power conservation mode detecting section 10 is turned off, the driving voltage is inputted to the base of the transistor Q3 through the Zener diode ZD2 in the degaussing coil driving section 20, causing the transistor Q3 to be turned on, and thus the driving voltage flows to ground through the driving coil of the relay RLl and the transistor Q3. Accordingly, the common contact c of the relay RLl becomes in contact with the movable contact b, so that the power supply to the degaussing coil LI through the thermistor THl is intercepted. As a result, during the power conservation mode of the CPT, the degaussing coil is not driven.

In this state, if the power conservation mode is released or turned off, a 'low' level power conservation mode signal as shown in FIG. 2A is applied to the base of the transistor Ql in the power conservation mode detecting section 10, causing the transistor Ql to be turned off. Thus, the collector voltage of the transistor Ql increases as shown in FIG. 2C in accordance with the time constant of the resistor R6 and the capacitor C2.

Specifically, if the collector voltage of the transistor Ql increases over the Zener voltage of the Zener diode ZDl, the Zener diode ZDl is turned on, and the voltage having passed through the Zener diode ZDl is applied to the base of the transistor Q2 as shown in FIG. 2D. Accordingly, the transistor Q2 is turned on, and thus the driving voltage supplied to the cathode of the Zener diode ZD2 in the degaussing coil driving section 20 flows through the transistor Q2 in the power conservation mode detecting section 10.

At this time, if the voltage charged in the capacitor Cl has been substantially discharged, the Zener diode ZD2 and the transistor Q3 are sequentially turned off as shown in FIG. 2E, so that no driving current flows to the driving coil of the relay RLl, causing the common contact c of the relay RLl to become in contact with the movable contact a. Accordingly, the degaussing coil driving voltage is supplied to the degaussing coil Li through the contacts of the relay RLl and the thermistor THi, and thus the degaussing coil Li is driven to perform the degaussing operation.

Here, the time period tl as shown in FIG. 2C is determined by the resistor R6 and the capacitor C2, and the time period t2 as shown in FIG. 2D is determined by the resistor R8 and the capacitor C3.

Thereafter, if the time period t2 has passed from the release of the power conservation mode, the transistor Q2 in the power conservation mode detecting section 10 is turned off since the capacitor C3 is fully charged by the current flowing to the capacitor C3 in the power conservation mode detecting section 10.

As the transistor Q2 is turned off, the capacitor Cl in the degaussing coil driving section 20 is gradually charged in accordance with the time constant of the resistor R3 and the capacitor Cl as shown in FIG. 2E.

At this time, if the charged voltage of the capacitor becomes higher than the Zener voltage of the Zener diode ZD2, this Zener diode ZD2 is turned on, and the voltage through the Zener diode ZD2 is applied to the base of the transistor Q3, so that the transistor Q3 is turned on and the common contact c of the relay RLi becomes in contact with the movable contact b. Accordingly, the degaussing coil driving voltage which is supplied to the degaussing coil Li through the thermistor THi is intercepted to stop the degaussing operation of the degaussing coil Li. As a result, when the predetermined time elapses after the release of the power conservation mode, the operation of the degaussing coil Li is automatically stopped.

Here, the Zener diode ZDl in the power conservation mode detecting section 10 prevents the degaussing coil driving section 20 from its erroneous operation due to an instantaneous input of an abnormal pulse.

As described above, according to the present invention, the OPT is automatically degaussed for a predetermined time when the power conservation mode is released in a monitor having a power conservation function, thereby improving the capability and reliability of the monitor.

Claims

CLAIMS:

1. A degaussing circuit for a color picture tube (CPT) having a degaussing coil for eliminating electromagnetic waves, the degaussing circuit comprising: a power conservation mode detecting section for detecting a release of a power conservation mode in accordance with a power conservation mode signal of said OPT inputted thereto and providing a degaussing control signal for controlling an operation of said degaussing coil if said release of said power conservation mode is detected; and a degaussing coil driving section for driving said degaussing coil for a predetermined time in accordance with said degaussing control signal inputted from said power conservation mode detecting section when said power conservation mode is released.

2. A degaussing circuit as claimed in claim 1, further comprising a thermistor, connected in series to said degaussing coil, for controlling current flowing to said degaussing coil.

3. A degaussing circuit as claimed in claim 1 or claim 2, wherein said power conservation mode detecting section comprises: a first transistor for performing a switching operation according to said power conservation mode signal; a Zener diode for passing therethrough a voltage supplied in accordance with said switching operation of said first transistor only when said voltage is higher than a Zener voltage of said Zener diode to prevent an erroneous operation of said power conservation mode detecting section; and a second transistor for performing a switching operation in accordance with said voltage supplied through said Zener diode.

4. A degaussing circuit as claimed in any one of the preceding claims, wherein said degaussing coil driving section comprises: a relay for controlling said voltage supplied to said degaussing coil; a Zener diode coupled to said power conservation mode detecting section, for passing therethrough a voltage supplied in accordance with an output of said power conservation mode detecting section only when said voltage is higher than a Zener voltage of said Zener diode; and a transistor, connected to said relay, for controlling an operation of said relay in accordance with said voltage supplied through said Zener diode.

5. A degaussing circuit for a color picture tube (CPT) having a degaussing coil for eliminating electromagnetic waves, the degaussing circuit comprising: a thermistor, connected to said degaussing coil, for controlling an amount of current flowing to said degaussing coil; a bridge rectifying section, coupled to said degaussing coil and a power switch, for rectifying an input AC power; a relay, coupled to said thermistor and said power switch, for controlling a driving voltage supplied to said degaussing coil through said thermistor; a power conservation mode detecting section for detecting a release of a power conservation mode in accordance with a power conservation mode signal of said OPT inputted thereto and providing a degaussing control signal for controlling an operation of said degaussing coil if said release of said power conservation mode is detected; and a degaussing coil driving section for driving said degaussing coil by driving said relay for a predetermined time in accordance with said degaussing control signal inputted from said power conservation mode detecting section when said power conservation mode is released.

6. A degaussing circuit for a colour picture tube (CPT) substantially as described hereinbefore with reference to the accompanying drawings and as shown in Figure 1.