JPH1120218A - Resistor-driving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a constant heat generation amount (consumption power) even when a resistance value of a heat-generating body varies, by switching constant current driving and constant voltage driving with time, and making constant a sum of consumption power by the constant voltage driving and consumption power by constant current driving. SOLUTION: When a signal driving sufficiently a transistor 7 is input to an input of a resistor 8 while a transistor 9 is kept off, a voltage Vcc between a collector and an emitter of the transistor 7 decreases sufficiently, and a heat- generating body 1 is driven with a constant voltage. A consumption power at this time is inversely proportional to a resistance of the heat-generating body 1. On the other hand, when a driving voltage Vb is input to the transistor 9 while the transistor 7 is kept off, a collector current of the transistor 9 is nearly equal to an emitter current and consequently not influenced by a value of the resistance of the heat-generating body 1. The consumption power is proportional to the resistance value of the heat-generating body 1. A time of the constant voltage driving and a time of the constant current driving are controlled at a driving control circuit to make a total heat generation amount constant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resistor driving circuit for driving a resistor having a variable resistance so as to have a constant power consumption, and more particularly to a thermal head in which a plurality of heating elements each operate as a resistor.

[0002]

2. Description of the Related Art A thermal head has a structure in which a large number of heating elements are arranged and a signal is applied to a driving circuit provided for each heating element to control whether or not each heating element generates heat. Widely used in thermal printers and thermal transfer printers. FIG. 1A is a diagram illustrating a configuration of a heating element of a thermal head and a driving circuit thereof. As shown, each of the heating elements 1-1, 1-2,..., 1-n is connected to a drive circuit composed of transistors 2-1, 2-2,. Here, each heating element 1-1,
, 1-n are connected to the power supply line 3 on the high potential side and the collector of the transistor, and control the signals a1, a2,..., An applied to the base of the transistor. -1, 1-2,..., 1-n can be controlled whether or not current flows. Each heating element acts as a resistor. FIG. 1B is an example showing the structure of the thermal head. The heating element 1 has a configuration in which the required number of dots are arranged side by side, and the heating control is performed by the drive IC 4. A plurality of transistors 2 (64, 128, etc.) are incorporated in the drive IC 4 and are connected to the heating element 1 by wire bonding. From the connector 5, a control signal and a power supply of the drive IC 4 are input.

There is a sublimation type thermal transfer printer as a printer using a thermal head. This sublimation type thermal transfer printer is about to be used as a color printer because a relatively uniform print density is obtained and the image quality is good. However, when used as a color printer, there is a problem that print density unevenness is not sufficient at present. There are various possible causes of the print density unevenness,
One of the major causes is a variation in the resistance value of the heating element.
If there is such a variation, stripes extending in the paper traveling direction are generated.

[0004]

In order to reduce the variation in the resistance value of the heating element, the resistance value of each heating element is actually measured in the manufacturing process of the thermal head, and a predetermined resistance value is set based on the measurement result. Countermeasures such as laser trimming have been taken. However, even if such measures are taken, the variation in the resistance value of the heating element is not at a sufficiently small level. Therefore, when the thermal head is completed, the resistance value of each heating element is measured and stored, and the current and energization time are controlled for each heating element so that the amount of heat generated by each heating element is constant during use. Has been done. However, in order to control the current and conduction time for each heating element,
A ROM that records the resistance value of each heating element, a circuit that reads data from the ROM, and a circuit that controls the current and conduction time of each heating element according to the read data are required.
There is a problem that the circuit configuration is complicated and expensive.

[0005] Further, since the heating element itself generates heat,
It deteriorates with use, and the resistance value changes over time. Moreover, the aging of the resistance value differs for each heating element, and the variation in resistance value of each heating element increases. Such variation in resistance value over time for each heating element is not so large,
It is still a problematic level. The above measures cannot eliminate the influence of the aging of the resistance value of the heating element.

The present invention is intended to solve such a problem. Even if the resistance value of the heating element varies, the driving of the heating element (resistor) so that the heating value (power consumption) is constant. It is intended to realize a circuit.

[0007]

In order to achieve the above object, in the resistor driving circuit according to the first aspect of the present invention, when driving at a constant voltage, the power consumption is inversely proportional to the resistance value, and the driving at a constant current is performed. In this case, paying attention to the fact that the power consumption is proportional to the resistance value, switching between constant voltage drive and constant current drive over time is performed so that the sum of the power consumption by the constant voltage drive and the power consumption by the constant current drive becomes constant. It is characterized by doing.

That is, a resistor driving circuit according to a first aspect of the present invention is a resistor driving circuit for driving a resistor,
A constant voltage driving circuit for applying a constant voltage to the resistor; a constant current driving circuit for flowing a constant current to the resistor; a constant voltage driving circuit for a first predetermined time; and a constant current driving circuit for a second predetermined time. And a drive control circuit for switching to operate.

Also, as a modification of the first aspect of the present invention, when driving a resistor by a transistor, a constant-current driving circuit becomes a constant-voltage driving by lowering the power supply voltage. Are prepared, and the drive circuit is set to be a constant current drive when using one power supply and to be a constant voltage drive when using the other power supply. Is switched over in time, so that the sum of the power consumption by the constant voltage driving and the power consumption by the constant current driving becomes constant.

That is, a resistor driving circuit according to a modification of the first embodiment of the present invention is a resistor driving circuit for driving a resistor, wherein a first power supply for outputting a first voltage and a second power supply are provided. A power supply for outputting a voltage of the first power supply, a switch for switching the power supply from one of the first power supply and the second power supply to the resistor, and a switch for supplying power from the first power supply. A driving circuit that operates as a constant voltage driving circuit that applies a constant voltage to the body, and that operates as a constant current driving circuit that causes a constant current to flow through the resistor when power is supplied from the second power supply. And The resistor driving circuit according to the second aspect of the present invention focuses on the point that the power consumption of the resistor is determined by the product of the square of the current and the resistance. When a constant resistor having a predetermined resistance value approximate to the resistance value of the resistor is connected in series with the resistor, the current is inversely proportional to the sum of the resistance values of the resistor and the constant resistor. Since the resistance values are similar, the square of the current flowing through the resistor is almost inversely proportional to the resistance of the resistor, so that the product of this and the resistance of the resistor can be regarded as substantially constant. Therefore, the power consumption is almost constant.

That is, a resistor driving circuit according to a second aspect of the present invention is a resistor driving circuit for driving a resistor,
A constant resistor connected in series to the resistor and having a predetermined resistance value approximating the resistance of the resistor; a constant voltage drive circuit for applying a constant voltage to the resistor and the constant resistor connected in series; It is characterized by having.

To accurately set the resistance value of the heating element,
Difficult as mentioned above. However, the resistance value of the constant resistor connected to the drive circuit can be set accurately. Therefore, such a resistor driving circuit is possible.

If the above-described resistor driving circuit is used for a driving circuit of a heating element of a thermal head, the amount of heat generated can be kept constant even if the resistance value of the heating element varies. The present invention can be applied to a device other than a thermal head as long as it drives a large number of resistors, and the resistance of each resistor varies.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention described below are embodiments in which the present invention is applied to a thermal head.
FIG. 2 is a basic configuration showing an embodiment (first embodiment) of the first aspect of the present invention, and FIG. 3 is a diagram for explaining the effect. FIG. 2 shows a circuit for switching the heating element 1 between a constant voltage drive and a constant current drive in a time-division manner. The transistor 7 and the resistor 8 drive the heating element 1 at a constant voltage, and the transistor 9 and the resistor 10 drive the heating element 1 at a constant current. Drive.

First, when a signal for sufficiently driving the transistor 7 is applied to the input of the resistor 8 while the transistor 9 is off, the voltage Vce between the collector and the emitter of the transistor 7 becomes sufficiently low, and the heating element 1 is a constant voltage drive, the power consumption P1 is the resistance of the heating element Rth, the power supply voltage is Vcc, the P1 ≒ ^Vcc 2 / Rth. Therefore, the power consumption P1 is equal to the resistance value Rt of the heating element 1.
It is inversely proportional to h. On the other hand, when the drive voltage Vb is input to the transistor 9 with the transistor 7 turned off, a current of Ie = (Vb−Vbe) / Re flows through the resistor 10. Here, Vbe is the voltage between the base and the emitter of the transistor 9, and Re is the resistance value of the resistor 10. Since the collector current Ic of the transistor 9 is substantially the same as the emitter current Ie and is not affected by the resistance value Rth of the heating element 1, the heating element 1 is driven at a constant current and the power consumption P2 is P2
≒ Ie ² · Rth. Therefore, the power consumption P2 is proportional to the resistance value Rth of the heating element 1. In a driving circuit for a heating element of a conventional thermal printer, it is common to use either constant voltage driving or constant current driving. In either method, the amount of heat generated is greatly affected by the variation in the resistance value Rth. Was gone. In the present invention, the constant voltage driving and the constant current driving are performed on one print pixel in a time division manner, and the constant voltage driving time T1 is set so that the total heat generation becomes substantially constant.
And the time T2 of the constant current drive is controlled by the drive control circuit.

The driving times T1 and T2 are determined by the resistance R of the heating element.
It is set so that P1 · T1 = P2 · T2 at approximately the center value of th. Therefore, the average power consumption P3 of the heating element
The P3 = (P1 · T1 + P2 · T2) / (T1 + T2) = (Vcc 2 / Rth + Ie 2 · Rth) / (T1 + T2) , and the heating value of the constant voltage drive and the constant current drive at the center value of the resistance value Rth of approximately the same become. When the resistance value Rth is smaller than the center value, P1 increases and P2 decreases, so that the fluctuation of the heat generation amount decreases. Conversely, when the resistance value Rth is larger than the center value, P1 decreases and P2 increases, so that the variation of the heat generation amount similarly decreases. FIG.
Is the resistance R of the power consumption P1 at the time of constant voltage driving, the power consumption P2 at the time of constant current driving, and the average power consumption P3 according to the present invention.
The effect of the variation of th is shown. From this, it can be seen that the variation of P3 due to the resistance value Rth is much smaller than that of P1 and P2.

FIG. 4 shows a basic configuration of an embodiment (second embodiment) of a modification of the first embodiment of the present invention, and FIG. 5 is a view for explaining the effect. FIG. 4 is also a circuit for switching between constant voltage driving and constant current driving in a time sharing manner as in FIG. The power supply voltage is switched between Vi and Vv by the changeover switch 14, and the power supply voltage Vi during the constant current drive and the power supply voltage Vv during the constant voltage drive are set so as to satisfy the following conditions. First, consider the constant current drive. When the set current flowing through the heating element 1 is Ie, the power supply voltage Vi is set so that the following condition is satisfied even when the resistance value Rth of the heating element 1 is maximum. (Re + Rth) · Ie ≦ Vi Here, Re is a resistance value of the resistor 13. Further, when the drive voltage Vb of the transistor 11 is set as follows, the heating element 1 always flows with the current Ie and is driven at a constant current. Vb = Re · Ie + Vbe Here, Vbe is a base-emitter voltage of the transistor 11. The power consumption P4 of the heating element 1 at this time is
It is found that P4 ・ Ie ² · Rth, which is proportional to the resistance value Rth.

Next, the constant voltage driving will be considered. The power supply voltage is switched to Vv, and Vv is set such that the following condition is satisfied even when the resistance value of the heating element 1 is the minimum value. (Re + Rth) · Ie ≧ Vv Here, Ie is a current value set by the constant current drive. In this case, the driving circuit of FIG. 4 cannot perform constant current driving because the power supply voltage is too low, and the collector-emitter voltage Vce of the transistor 11 becomes almost zero.
Is I ≒ Vv / (Re + Rth). Here, if it is set so that Re << Rth, the current I
It can be seen that the constant voltage driving is substantially in inverse proportion to the resistance value Rth of the heating element 1.

At this time, the power consumption P5 of the heating element 1 is P
5 = I ² · Rth ≒ Vv ² / Rth, and the resistance value Rt
It is inversely proportional to h. Switching of the changeover switch 14
The time of the constant current driving is T4, and the time of the constant voltage driving is T5.
, The resistance value Rth of the heating element is approximately the center value,
P4 · T4 = P5 · T5. . Therefore, the average power consumption P6 of the heating element is P6 = (P4 · T
4 + P5 · T5) / (T4 + T5) ≒ (Ie ² · Rth
^{· T4 + Vv 2 · T5 /} Rth) / (T4 + T5) , and the heating value of the constant voltage drive and the constant current drive at the center value of the resistance value Rth becomes substantially the same. When the resistance value Rth is smaller than the central value, P5 increases and P4 decreases, so that the fluctuation of the heat generation amount decreases. Conversely, when the resistance value Rth is larger than the center value, P5 decreases and P4 increases, so that the variation in the heat generation amount similarly decreases. FIG. 5 shows the power consumption P4 at the time of constant current driving and the power consumption P at the time of constant voltage driving.
5 and the resistance value Rth of the average power consumption P6 according to the present invention.
The effect of fluctuations in From this, P6 becomes P
4, it can be seen that the variation due to the resistance value Rth is very small as compared with P5.

FIG. 6A shows a basic configuration of an embodiment (third embodiment) according to the second aspect of the present invention, and FIG. 7 is a diagram for explaining the effect thereof. FIG. 6A shows a circuit in which the heating element 1 and the resistor 17 are connected in series and the transistor 18 is driven at a constant voltage. The resistance value Rc of the resistor 17 has a sufficiently small variation compared to the heating element. I do. Here, the resistance value Rc of the resistor 17 is set substantially equal to the center value of Rth, and R
When the variation of th is Δ, Rth = Rc (1 + Δ)
Becomes The value of Δ is about −0.2 to +0.2 in a normal thermal head, even if aging is included. The power consumption P of the heating element 1 when the power supply voltage is 2 Vp in this circuit
7 is a ^{P7 ≒ (2Vp / (Rth +} Rc)) 2 · Rth = (Vp 2 / Rc) / {1 + Δ 2 / (4 · (1 + Δ))}.

On the other hand, FIG. 6B shows a conventional general thermal head drive circuit, in which a heating element 1 is driven at a constant voltage by a transistor 2. Power consumption P8 of heating element 1 in this case
P8 ＰVp ² / Rth = (Vp ² / Rc) / (1 + Δ) Since the variation in Rc is negligible, in the circuit of FIG. 6A according to the present invention, the influence of the change in the resistance value Rth of the heating element 1 is smaller than that of the conventional driving circuit of FIG.
From this, it can be seen that Δ ² / (4 · (1 + Δ)). FIG. 7 shows P7 and P8 when Δ changes from −0.2 to +0.2. From this, FIG.
In the circuit of the present invention, it is understood that the influence of the fluctuation of the resistance value Rth is extremely small.

[0022]

As described above, according to the present invention,
The influence of the variation in the amount of generated heat due to the variation in the resistance value of the heating element can be significantly reduced as compared with the conventional driving circuit.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a driving circuit of a heating element of a conventional general thermal head, and FIG. 1B is an explanatory diagram showing a structural example thereof.

FIG. 2 is a circuit diagram showing a first embodiment of the present invention.

FIG. 3 is an explanatory diagram showing an effect of the first embodiment of the present invention.

FIG. 4 is a circuit diagram showing a second embodiment of the present invention.

FIG. 5 is an explanatory diagram showing the effect of the second embodiment of the present invention.

FIG. 6 is a circuit diagram showing a third embodiment of the present invention.

FIG. 7 is an explanatory diagram showing the effect of the third embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heating element 2 Transistor 3 Power supply line 4 Drive IC 5 Connector 6 Heat sink 7 Transistor 8 Resistance 9 Transistor 10 Resistance 11 Transistor 12 Resistance 13 Resistance 14 Changeover switch 15 Power supply line 16 Power supply line 17 Resistance 18 Transistor 19 Resistance 20 Power supply line 21 Resistance 22 Power line

Claims (4)

[Claims] 1. A resistor driving circuit for driving a resistor, comprising: a constant voltage driving circuit for applying a constant voltage to the resistor; a constant current driving circuit for flowing a constant current to the resistor; A drive control circuit for switching the constant voltage drive circuit to operate for a first predetermined time and the constant current drive circuit to operate for a second predetermined time. 2. A resistor driving circuit for driving a resistor, comprising: a first power supply for outputting a first voltage; a second power supply for outputting a second voltage; and the first power supply. A switch for switching power to be supplied from any one of the second power supply to the resistor; and a constant voltage drive circuit for applying a constant voltage to the resistor when power is supplied from the first power supply. Works,
And a drive circuit that operates as a constant current drive circuit that supplies a constant current to the resistor when power is supplied from the second power supply. 3. A resistor driving circuit for driving a resistor, the resistor driving circuit being connected in series with the resistor, and having a predetermined resistance close to the resistance of the resistor, and being connected in series. And a constant voltage driving circuit for applying a constant voltage to the constant resistor. 4. A thermal head, comprising: a plurality of heating elements acting as resistors; and a plurality of driving circuits provided for each of the heating elements for supplying electric power so that each heating element generates heat. A thermal head using the resistor driving circuit according to any one of claims 1 to 3.