JP2895946B2 - Fluorescent display tube filament drive circuit - Google Patents

Description

The present invention relates to a circuit for driving a filament of a fluorescent display tube.

[Prior Art] In a conventional filament driving circuit of a fluorescent display tube for a vehicle, there are a DC type and an AC type in which a current flowing through the filament is used. The configuration is simpler than the mold.

As shown in FIG. 6, the conventional DC filament driving circuit has a resistance in order from battery Vb to ground.
11 and a Zener diode 12 (constant voltage applying means) are connected in series. The filament F of the fluorescent display tube is connected in parallel to the Zener diode 12.

In the above configuration, even if the voltage of the battery Vb fluctuates, the resistance
Only the current at 11 and the resulting voltage drop change, the applied voltage and current at filament F do not change. Thereby, the calorific value of the filament F can be made constant, and the fluorescent display tube can be displayed with stable luminance.

[Problem to be Solved by the Invention] In the drive circuit having the above configuration, the voltage of battery Vb (for example,
12V) and the set voltage of Zener diode 12 (eg 3V)
In order to obtain the difference from the above, it is necessary to cause a voltage drop and power consumption at the resistor 11, and the following disadvantages are caused by the voltage drop and power consumption. In other words, if the resistance value of the resistor 11 is reduced to keep the filament applied voltage constant even when the battery voltage is greatly reduced, a large amount of current flows through the resistor 11, so that it is necessary to use a capacitor having a large rated power and a large shape. Not only that, heat generation is increased, and thermal stress is applied to surrounding semiconductors and the like. Conversely, if the resistance value of the resistor 11 is increased to suppress the power consumption and the amount of heat generation, the voltage drop increases, so that it becomes difficult to stabilize the filament voltage when the battery voltage changes. That is, when the battery voltage decreases, the applied voltage to the filament voltage may not be maintained constant.

In order to solve the above-mentioned drawbacks and / or other drawbacks, the inventor first connects a switching element in series with a filament, and operates the switching element by a pulse from a pulse generation circuit to supply the filament. Was thought to be performed intermittently. Further, in such a configuration, if the output is fixed at a level that turns on the switching element due to a failure of the pulse generation circuit, there is a risk of overheating of the filament and accompanying burnout. A high-pass filter composed of a capacitor and the like was arranged between the generation circuit and the control terminal of the switching element.

However, since a pulse voltage is applied to the filament, noise may be generated by the high frequency component.

Means for Solving the Problems In order to solve the above problems, the present invention has a gist of a filament driving circuit for a fluorescent display tube, which has the following configuration.

(A) A pulse generation circuit that outputs a rectangular pulse periodically.

(B) A preceding-stage switching element which receives a pulse of this pulse generation circuit at a control terminal and turns on / off.

(C) A high-pass film disposed between the pulse generating circuit and the control terminal of the preceding switching element.

(D) A constant voltage circuit arranged between the DC power supply and the ground. In this constant voltage circuit, a resistor on the DC power supply side and a Zener diode on the ground side are connected in series, and a connection point between the resistor and the Zener diode is also connected to a power supply terminal of the preceding switching element. I have.

(E) A post-stage switching element disposed between the DC power supply and the filament.

(F) A noise prevention filter arranged between the connection point between the resistance of the constant voltage circuit and the Zener diode and the post-stage switching element. In this filter, the resistor on the connection point side and the capacitor on the ground side are connected in series, and the connection point between the resistor and the capacitor is connected to the control terminal of the post-stage switching element.

[Operation] The noise prevention filter can smoothly change the voltage applied to the filament both when turning on from off and when turning off from on. by this,
Generation of noise can be prevented.

[Reference Example] Prior to describing an embodiment of the present invention, a related art that the inventor has invented up to the present invention will be described as a reference example. FIG. 2 shows a fluorescent display tube 1 used in a vehicle. The fluorescent display tube 1 includes a filament F, a grid G, and a segment anode A housed in the tube 2. The filament F is driven by the filament driving circuit 3, the grid G is driven by the grid driving circuit 4, and the segment anode A is driven by the anode driving circuit 5. The grid drive circuit 4 and the anode drive circuit 5 receive a constant voltage from a constant voltage circuit connected to the battery Vb (DC power supply), and control the application of the constant voltage to the grid G and the anode A, respectively.

As shown in FIG. 1, the filament drive circuit 3 includes a transistor 13 (switching element), a resistor 11, and a Zener diode 12 connected in order from the battery Vb to the ground. The resistor 11 and the Zener diode 12 constitute a constant voltage circuit 10. Transistor 13 is of the PNP type, the emitter of which is battery Vb
And the collector is connected to the resistor 11. The filament F is connected in parallel to the Zener diode 12.

The filament driving circuit 3 further includes a pulse generation circuit 15. The output stage of this pulse generation circuit 15 and the battery
A capacitor 21 and two resistors 22, 23 are connected in series with Vb. The connection point between the resistors 22 and 23 is connected to the base of the transistor 13. The capacitor 21 and the resistors 22 and 23 constitute a high-pass filter 20.

A diode 24 having a cathode facing the battery Vb is connected between the base of the transistor 13 and the battery Vb. The diode 24 is for releasing a reverse bias applied between the base and the emitter of the transistor 13 when the output level from the pulse output circuit 15 switches from a low level to a high level.

In the above configuration, for example, 20 KH
Each time a low-level pulse is output at z, the battery V
A current flows from b to the pulse generation circuit 15 through the high-pass filter 20, and the transistor 13 is biased and turned on by a voltage drop at the resistor 23. When the transistor 13 is turned on, a current from the battery Vb flows through the resistor 11 and further flows through the filament F and the Zener diode 12. At this time, even if the voltage of the battery Vb fluctuates, only the current flowing through the resistor 11 and the resulting voltage drop fluctuate, and the voltage across the Zener diode 12, in other words, the voltage applied to the filament F, is kept constant. Dripping. Therefore, the filament F can maintain a constant heating value according to the set voltage of the Zener diode 12 and the duty ratio of the pulse.

The heat value of the resistor 11 is calculated by the following (a) and (b).
For this reason, the size can be reduced as compared with the conventional DC filament driving circuit shown in FIG.

B) No current always flows through the resistor 11, and current flows only when a low-level pulse is output from the pulse generation circuit 15. Therefore, the time during which the current flows can be shortened according to the duty ratio of the pulse.

B) Since the transfer time of the current supply to the filament F becomes shorter in accordance with the duty ratio, in order to maintain an appropriate heat generation amount of the filament F, the voltage applied to the filament F, that is, the set voltage of the Zener diode 12 is increased. There is a need. As a result, the voltage drop required by the resistor 11 can be small.

Also, since the expected voltage drop of the resistor 11 is low, the Zener diode can be sufficiently biased even if a resistor having a small rated power is used. The action can be secured and the filament voltage can be kept constant.

Furthermore, even if the output of the pulse generation circuit 15 is fixed at a low level, that is, a level at which the transistor 13 is turned on due to a failure of the pulse generation circuit 15,
Since the high-pass filter 20 is disposed between the transistor 15 and the base of the transistor 13, the base voltage of the transistor 13 can be set to the voltage of the battery Vb,
13 can be kept off. As a result, it is possible to prevent the filament F from being overheated and the resulting burnout, and to prevent the surrounding elements from being subjected to thermal stress due to the overheating of the resistor 11 and the Zener diode 12.

In addition, in the configuration of FIG. 1, the following advantage is also obtained by disposing the transistor 13 on the power supply Vb side of the constant voltage circuit 10. When the output from the pulse generation circuit 15 is at a low level and the transistor 13 is on, like the conventional drive circuit,
Due to the voltage drop of the resistor 11, the voltage of the filament F is lower than the voltage of the battery Vb and lower than the voltages of the grid G and the anode A. When the output from the pulse generation circuit 15 is at a high level and the transistor 13 is on, the voltage of the filament F becomes the ground voltage. Thus, when the transistor 13 is turned on,
The voltage of filament F is always on grid G,
Since the voltage is lower than the voltage of the anode A, the transfer of electrons from the filament F to the anode A is always and reliably performed. As a result, a good display function of the fluorescent display tube can be maintained.

FIG. 3 shows another reference example. In this reference example, the first
The same reference numerals are given to constituent members corresponding to the drawings, and detailed description thereof will be omitted. In the example of FIG. 3, transistor 13 'is NP
It is of an N type, and is arranged on the ground GD side with respect to the constant voltage circuit 10, and its emitter is grounded. In this reference example, when the output from the pulse generation circuit 15 goes high, the transistor 13 'turns on. Important functions are the same as in the reference example of FIG.

By the way, in the reference examples of FIGS. 1 and 3, since a pulse voltage is applied to the filament F, noise may be generated by the high-frequency component. The present invention prevents such noise.

Example An example of the present invention will be described below with reference to FIG. The embodiment shown in the figure is the same as that of FIGS. 1 and 3 in that it has a pulse generating circuit 15, a high-pass filter 20, and a diode 24, but differs in the following points. The embodiment shown in FIG. 4 includes two-stage transistors 13A and 13B. The pre-stage transistor 13A turns on and off in response to the output from the pulse generation circuit 15. The collector of the first-stage transistor 13A is connected to the base of the second-stage transistor 13B. The latter transistor 13B is used to control the power supply of the filament F. The filament F is connected to the emitter side of the rear transistor 13B, and the collector of the rear transistor 13B is connected to the battery Vb via the resistor 100. Further, a constant voltage circuit 110 including a resistor 111 and a Zener diode 112 is interposed between the battery Vb and the ground.
A connection point a between the reference 11 and the Zener diode 112 is connected to the collector (current side terminal) of the preceding transistor 13A.
A noise prevention filter 120 is arranged between the connection point a and the base of the subsequent transistor 13B. In the filter 120, a resistor 121 on the connection point a side and a capacitor 122 on the ground side are connected in series, and a connection point b between the resistor 121 and the capacitor 122 is connected to the base of the post-stage transistor 13B. .

The operation of the above embodiment will be described with reference to the time chart of FIG. When a low-level pulse is output from the pulse generation circuit 15, the preceding transistor 13A is turned off in response to this, and the constant voltage of the Zener diode 112 of the constant voltage circuit 110 is applied to the base of the following transistor 13B. As a result, the latter-stage transistor 13B is turned on, and power is supplied to the filament F from the battery Vb. At this time, the applied voltage of the filament F is
This is a voltage obtained by subtracting the base-emitter voltage of the subsequent transistor 13B from the constant voltage of 112, and can be maintained substantially constant. Therefore, the calorific value of the filament F is also constant.

When the battery Vb changes, the voltage between the collector and the emitter of the transistor 13B changes. At this time, the current flowing through the resistor 100 and the filament F is constant. Since the high-frequency component of the pulse voltage applied to the filament F is cut by the filter 120, the pulse voltage does not become a complete rectangular wave as shown by a broken line but has a gentle waveform as shown by a solid line. That is, when the front-stage transistor 13A is turned off, a current flows from the connection point a of the constant voltage circuit 110 to the capacitor 122 via the resistor 121 of the filter 120, and is stored in the capacitor 122. Along with this, the voltage of the capacitor 122, and eventually the voltage applied to the filament F, gradually rises with time. When the front-stage transistor 13A is turned on, the electric charge stored in the capacitor 122 is discharged to the ground via the resistor 121 and the front-stage transistor 13A. The capacitor 12
The voltage of 2 and thus the voltage applied to the filament F gradually decreases with time. As a result, it is possible to prevent a noise from being generated in a radio or the like.

The resistor 100 partially plays a role of a voltage drop to be borne between the collector and the emitter of the transistor 13B, but may not be in principle. The constant voltage circuit 110 is a filament F
, The resistors 111 and the zener diodes 112 having a small rated power can be used.

The present invention is not limited to the above embodiments, and various modes are possible.

[Effects of the Invention] As described above, in the present invention, the voltage applied to the filament can be smoothly changed by the noise prevention filter both when the filament is turned on from off and when it is turned off from on. Thus, generation of noise can be prevented.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a filament drive circuit showing a reference example,
FIG. 2 is a schematic configuration diagram of a fluorescent display tube, and FIG. 3 is a circuit diagram of a filament drive circuit showing another reference example. FIG. 4 is a circuit diagram of a filament drive circuit showing one embodiment of the present invention, and FIG. 5 is a time chart for explaining the operation of the filament drive circuit of FIG. FIG. 6 is a circuit diagram of a conventional DC filament driving circuit. Vb: DC power supply (battery), 13A: front-stage switching element (front-stage transistor), 13B: rear-stage switching element (back-stage transistor), 15: pulse generation circuit,
20: High-pass filter, 110: Constant voltage circuit, 111:
Resistor, 112: Zener diode, 120: Noise prevention filter, 121: Resistor, 122: Capacitor, a ...
Connection point between the resistance of the constant voltage circuit and the Zener diode, b
…… Connection point between the resistor and capacitor of the noise prevention filter.

Claims (1)

(57) [Claims] 1. A filament driving circuit for a fluorescent display tube having the following configuration. (A) A pulse generation circuit that outputs a rectangular pulse periodically. (B) A preceding-stage switching element which receives a pulse of this pulse generation circuit at a control terminal and turns on / off. (C) A high-pass film disposed between the pulse generating circuit and the control terminal of the preceding switching element. (D) A constant voltage circuit arranged between the DC power supply and the ground. In this constant voltage circuit, a resistor on the DC power supply side and a Zener diode on the ground side are connected in series, and a connection point between the resistor and the Zener diode is also connected to a power supply side terminal of the preceding switching element. I have. (E) A post-stage switching element disposed between the DC power supply and the filament. (F) A noise prevention filter disposed between a connection point between the resistance of the constant voltage circuit and the Zener diode and a control terminal of the post-stage switching element. This filter is
The resistor on the connection point side and the capacitor on the ground side are connected in series, and the connection point between these resistors and the capacitor is connected to the control terminal of the latter-stage switching element.