JP2000164376A - Heater control/protection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heater control/protection circuit stopping the operation of a heater before the temperature becomes abnormally high when a drive circuit fails and capable of informing the user of the failure of the drive circuit. SOLUTION: When a heater drive circuit 3 fails, the output of the heater drive circuit 3 and the output of a heater control circuit 2 become the same logic, i.e., the same logical signal is inputted to two inputs of a monitor circuit 4, and a high-level signal is outputted from the monitor circuit 4. The PNP transistor 51 of a power supply cutoff circuit 5 is turned off, and the current flowing in a heater 1 is cut off to stop the operation of the heater 1. Upon receiving the high-level signal of the monitor circuit 4, the NPN transistor 62 of an alarm circuit 6 is turned on to light an LED 61, and the failure is shown to a user.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a heater control protection circuit, and more particularly to a heater control protection circuit for a liquid crystal display device.

[0002]

2. Description of the Related Art Generally, a fluorescent tube used for a backlight of a liquid crystal display device has a characteristic that its luminance is lowered at a low temperature. Therefore, as a means for compensating luminance at a low temperature, a method of operating a heater only at a low temperature to heat a fluorescent tube has been adopted. However, if the heater operates at a high temperature due to a failure in the heater circuit, a worst case fire may occur, and a heater control protection circuit is required to prevent the fire.

[0003] This conventional heater control protection circuit is shown in FIG.
As shown in FIG. 1, a heater 101 for heating a fluorescent tube of a backlight (not shown), a power supply 104 for supplying a current to the heater 101, and a heater control circuit 102 for outputting a control signal for operating the heater 101 in accordance with the temperature
A heater drive circuit 103 for receiving a control signal from the heater control circuit 102 to turn on or off a current flowing through the heater 101; a power supply 104 and the heater 101
And a temperature fuse 105 that cuts off the current flowing through the heater 101 when the temperature of the fluorescent tube (not shown) rises abnormally.

[0004] The heater control circuit 102 detects a voltage indicating the temperature around the fluorescent tube and controls the control unit 122 to be described later.
And a threshold voltage for determining a temperature for operating the heater 101 stored in advance, and a voltage detected by the temperature sensor 112, and flow to the heater 101 according to the comparison result. And a control unit 122 that outputs a control signal for turning on or off the current to the heater driving circuit 103. The control unit 122 includes, for example, a comparator.

[0005] Further, although not shown, the thermal fuse 105 is composed of a fusible material that melts at high temperature, two lead wires connected by the fusible material, and a case that covers them. Current flows between the two leads through the fusible.

The operation of such a heater control protection circuit will be described.

When the heater drive circuit 103 causes a short circuit due to a surge or the like, current continues to flow through the heater 101 regardless of the output of the heater control circuit 102, and the temperature of the fluorescent tube rises.

When the temperature rises, the fusible material in the thermal fuse melts and condenses at the end of each lead wire due to surface tension, and the current is cut off.

Therefore, the current flowing through the heater 31 is cut off, and the heater operation can be stopped.

As described above, in the conventional heater control and protection circuit, the current flowing through the heater is cut off by the temperature fuse after the temperature becomes high.

[0011]

However, since the conventional heater control protection circuit is protected by the thermal fuse, there is a problem that the thermal fuse does not operate unless the temperature becomes abnormally high for the thermal fuse to operate. .

Further, since it is not known whether the thermal fuse has been blown or not, the user cannot determine whether the protection circuit has been activated due to the failure, and as a result, the thermal fuse may be used with the failure. There was a problem that there is.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a heater control protection which can stop the operation of a heater before a temperature becomes abnormally high when a drive circuit fails, and at the same time notify the user that the drive circuit has failed. It is to provide a circuit.

[0014]

SUMMARY OF THE INVENTION Therefore, in order to achieve the above object, the present invention provides a heater for heating a fluorescent tube of a backlight and a heater control for outputting a control signal for operating the heater in accordance with the temperature. A circuit, a heater drive circuit for turning on or off a current flowing through the heater in response to a control signal from the heater control circuit, a monitor circuit for monitoring a control signal of the heater control circuit and an operation state of the heater drive circuit, and the monitor circuit And a power cutoff circuit that cuts off a current flowing through the heater when a signal indicating a failure is input from the power supply.

The heater control and protection circuit includes a heater for heating the fluorescent tube of the backlight, a heater control circuit for outputting a control signal for operating the heater in accordance with the temperature, and a control signal from the heater control circuit. A heater drive circuit for turning on or off the current flowing through the heater, a control signal for the heater control circuit and a monitoring circuit for monitoring the operation state of the heater drive circuit, and a signal indicating a failure is input from the monitor circuit to the heater. A power supply shutoff circuit for interrupting a current, and a warning circuit for prompting a warning when a signal indicating a failure is input from the monitoring circuit are provided.

[0016]

Next, an embodiment of a heater control protection circuit according to the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of a heater control and protection circuit according to the present invention.

As shown in FIG. 1, the heater control and protection circuit of the present invention includes a heater 1 for heating a fluorescent tube of a backlight (not shown), a power supply 7 for supplying an electric current to the heater 1, and a temperature-dependent heater. A heater control circuit 2 for outputting a control signal for operating the heater 1, a heater drive circuit 3 for receiving a control signal from the heater control circuit 2 to turn on or off a current flowing through the heater 1, and a control signal for the heater control circuit 2 A monitoring circuit 4 for monitoring the operating state of the heater driving circuit; a power supply shutoff circuit 5 inserted between the power supply 7 and the heater 1 for shutting off power supplied to the heater 1 by a signal from the monitoring circuit 4; A warning circuit 6 for prompting a warning when a signal indicating a failure is input from the controller 4
It consists of and.

The heater drive circuit 3 comprises an NPN transistor 31. The output of the heater control circuit 2 is provided at the base of the transistor, and the heater 1 is provided at the collector.
However, the negative side of the power supply 7 is connected to the emitter.

The heater control circuit 2 receives a temperature sensor 21 such as a thermistor, a voltage detected by the temperature sensor 21 and a previously stored threshold voltage for determining a temperature for operating the heater 1. And a unit 22. The temperature sensor 21 detects a voltage indicating the temperature around the fluorescent tube and outputs the voltage to the control unit 22. The control unit 22 compares the input threshold voltage with the voltage detected by the temperature sensor 21, and outputs a control signal for turning on or off the current flowing through the heater 1 to the heater drive circuit 3 according to the comparison result. I do. The control unit 22 includes, for example, a comparator.

Further, the monitoring circuit 4 has an exclusive OR 41
And the inverter 42 and the heater control circuit 2
And the signal from the heater driving circuit 3 is directly applied to the input of the exclusive OR 41. Then, the exclusive OR 41
Is inverted by the inverter 42 and becomes the output of the monitoring circuit 4.

In addition, the power supply cutoff circuit 5 comprises a PNP transistor 51. The output of the monitor circuit 4 is provided at the base of the PNP transistor 51, the heater 1 is provided at the collector, and the power supply 7 of the power supply 7 is provided at the emitter. The sides are connected.

The warning circuit 6 comprises an LED 61, an NPN transistor 62 and a resistor 63.
The anode of 61 is connected to the power supply 7 via the resistor 63, the collector of the NPN transistor 62 is connected to the cathode of the LED 61,
The base is connected to the output of the monitoring circuit 4 and the emitter is connected to the ground.

Next, the operation of the heater control protection circuit having the above configuration will be described with reference to FIG.

First, the operation when the heater control protection circuit is in a normal state will be described.

When the temperature of the fluorescent tube is low, the control unit 22 of the heater control circuit 2 determines that the voltage from the temperature sensor 21 is smaller than the threshold voltage, and as a result, outputs a high-level control signal to the heater drive circuit. Output to 3. As a result, the NPN transistor 31 of the heater drive circuit 3 is turned on, and the heater drive circuit 3 outputs a low level signal.

Since a low-level signal and a high-level signal are input to the respective inputs of the monitoring circuit 4, the monitoring circuit 4 outputs a low-level signal. As a result, the PNP transistor 51 of the power supply cutoff circuit 5 is turned on, and a voltage is applied to the heater 1.

Next, when the temperature of the heater 1 rises due to the operation of the heater 1, the control unit 22 determines that the voltage from the temperature sensor 21 is higher than the threshold voltage, and as a result, sends a low-level control signal to the heater driving circuit. Output to 3. As a result, the NPN transistor 31 of the heater drive circuit 3 is turned off, and the current flowing through the heater 1 is cut off.

Second, the operation when the heater drive circuit 3 has a short-circuit fault will be described.

When the short circuit occurs in the heater drive circuit 3, the output signal of the heater drive circuit 3 is fixed at a low level.

On the other hand, when the temperature rises, the heater control circuit 2 outputs a low-level control signal for shutting off the heater drive circuit 3 to stop the heater 1. Therefore, a low-level signal is input to the two inputs of the monitoring circuit 4, and a high-level signal is output from the monitoring circuit 4.

As a result, the PNP transistor 51 of the power supply cutoff circuit 5 is turned off, the current flowing through the heater 1 is cut off, and the operation of the heater 1 is stopped.

At the same time, the warning circuit 6 receives the high level signal of the monitoring circuit 4, turns on the NPN transistor 62 of the warning circuit 6, turns on the LED 61, and informs the user that the heater driving circuit 3 has failed. Inform.

Third, the operation when the heater drive circuit 3 has an open failure will be described.

When the heater drive circuit 3 has an open failure, the output signal of the heater drive circuit 3 is fixed at a high level.

At a low temperature, the heater control circuit 2 outputs a high-level control signal for turning on the heater drive circuit 3 in order to operate the heater 1. Therefore, a high-level signal is input to the two inputs of the monitoring circuit 4, and a high-level signal is output from the monitoring circuit 4.

As a result, the PNP transistor 51 of the power supply cutoff circuit 5 is turned off, the current flowing through the heater 1 is cut off, and the operation of the heater 1 is stopped, as in the case where the heater drive circuit 3 is short-circuited.

At the same time, the warning circuit 6 receives the high level signal of the monitoring circuit 4, turns on the NPN transistor 62 of the warning circuit 6, turns on the LED 61, and informs the user that the heater driving circuit 3 has failed. Inform.

Therefore, even if the heater drive circuit 3 fails, the operation of the heater 1 can be stopped immediately, and the LED 61 is turned on to notify the user of the failure.

Although the LED is used as a warning means for the user, the present invention is not limited to this, and it is only necessary for the user to know that a failure has occurred. Is obtained.

[0041]

As described above, according to the heater control protection circuit of the present invention, the state of the drive circuit is monitored by the monitoring circuit, and if there is an abnormality, the power supply to the heater is immediately cut off by the power cutoff circuit. Then, the operation of the heater can be stopped. Therefore, there is an effect that the safety becomes higher than when the temperature fuse is blown and the operation of the heater is stopped after the temperature rises abnormally.

Further, since the warning circuit indicates that the protection circuit is operating due to a failure, the user can be notified of the failure. To play.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of a heater control protection circuit according to the present invention.

FIG. 2 is a block diagram showing a conventional heater control protection circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heater 2 Heater control circuit 3 Heater drive circuit 4 Monitoring circuit 5 Power cutoff circuit 6 Warning circuit 7 Power supply

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G09G 3/36 G09G 3/36 // F21V 8/00 601 F21V 8/00 601D G09F 9/00 337 G09F 9 / 00337Z H03K 17/94 H03K 17/94 J H05B 3/00 365 H05B 3/00 365N

Claims (2)

[Claims] 1. A heater for heating a fluorescent tube of a backlight, a heater control circuit for outputting a control signal for operating the heater in accordance with the temperature, and turning on a current flowing to the heater in response to a control signal from the heater control circuit. Or a heater drive circuit for turning off, a monitor circuit for monitoring a control signal of the heater control circuit and an operation state of the heater drive circuit, and a power supply for shutting off a current flowing to the heater when a signal indicating a failure is input from the monitor circuit. And a circuit for controlling the heater control. 2. A heater for heating a fluorescent tube of a backlight, a heater control circuit for outputting a control signal for operating the heater in accordance with the temperature, and turning on a current flowing to the heater in response to a control signal from the heater control circuit. Or a heater drive circuit for turning off, a monitor circuit for monitoring a control signal of the heater control circuit and an operation state of the heater drive circuit, and a power supply for shutting off a current flowing to the heater when a signal indicating a failure is input from the monitor circuit. A heater control protection circuit, comprising: a circuit; and a warning circuit that prompts a warning when a signal indicating a failure is input from the monitoring circuit.