JPS6238150B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a drive circuit for a thermal printer.

FIG. 1 shows a part of a conventional constant temperature control circuit for a thermal pen, which is known from Japanese Patent Publication No. 53-15378. By slightly modifying the circuit shown in FIG. 1, it is possible to apply it to a thermal printer, and the problems in this case will be described in detail below. 1 is a power supply with variable elements such as batteries, and the voltage V ₀
occurs. 2 is a resistor, 3 is a capacitor, the capacitor is connected in series to the power supply 1 via the resistor 2, and a switch 4 is connected in parallel to the capacitor 3, forming a charging/discharging circuit 11 by the above circuit. do. Reference numeral 5 indicates a switch drive circuit for opening and closing the switch 4 in constant synchronization, 6 a voltage comparison circuit, 7 a reference power supply, and 8 a variable voltage divider. 9 is a thermal pen; 10 is a switch for the thermal pen 9, which is turned on and off by the output of the voltage comparator circuit 6, and operates to maintain the temperature of the thermal pen 9 at a constant temperature regardless of fluctuations in the power supply voltage _V0 . In order to apply Fig. 1 to a thermal printer, it is necessary to increase the number of heating elements corresponding to the thermal pen 9 and to provide a circuit to selectively control the switch 10. The principle of obtaining remains unchanged. Switch 4 in FIG. The temperature of the thermal pen is controlled by turning on the switch 10 at regular intervals T for a heating time t.

Figure 2 is a diagram showing the relationship between heating time t and power supply voltage V _c . When the power supply voltage is high V ₁ , the charging time is faster, and when the heating time is short t ₁ and the power supply voltage is low V ₂ , the charging time is faster. , the heating time is long t ₂ .

When the resistance value of the thermal pen is r, the energy (PW) per unit time applied to the thermal pen is expressed by the following equation (1).

PW=V _c ² /r ₁ ×t/T ………(1) In order for the temperature to be constant, it is sufficient if the energy is constant, so the relationship between t and V _c at that time is V ² _c xt = constant ……(2) would be good. The characteristics of a circuit having such a voltage and pulse width will be referred to as equal energy characteristics.

The heating time t of the circuit shown in FIG.
It is expressed by the formula.

t=-C·Rln(1-E/V _c )...(3) This is shown in FIG. 3 with V _c /E on the x-axis and t/cR on the y-axis on a logarithmic scale. 12 is equation (3)
It is a characteristic curve divided by CR, and 13 has a slope of -2
is a straight line. The relational expression of the locus of points parallel to this straight line 13 is expressed by the following equation (4).

y=d1/x d=constant...(4) In other words, the relational expression (2) between t and V _c of isotemperature characteristics is also expressed as equation (4)
The temperature is constant along the trajectory parallel to the straight line 13. The portion of the characteristic curve 12 that can be approximated in parallel to the straight line 13 is from point P to point Q, and the value of x is about 1.3 to 1.6. Assuming that the power source is four manganese batteries, the initial value is about 6.5V, and at this time, x = VE / E = 1.6, so E = 4.06.
(V). If x=1.3 and E=4.06, then V _c will be approximately 5.3 (V). That is,
This means that constant temperature driving is only possible at about 6.5 (V) to 5.3 (V). Generally, manganese batteries have an internal resistance of about 0.5Ω in SUM3, and when there are multiple heat generating elements equivalent to thermal pens 9, such as in a thermal printer, and they are energized at the same time, the voltage drop will be significant. Therefore, a circuit with equal energy characteristics over a wide voltage range is required. Also, like a thermal printer,
A method that instantly converts applied energy into heat to form dots requires characteristics close to a straight line 32 because even a small difference in energy greatly affects print quality. In addition, the direction of curvature of the characteristic line 31 is such that there is too much energy for both high and low voltages, making it unsuitable for controlling easily-destroyable heating elements such as those of thermal printers. . In order to solve these problems, a constant voltage circuit is used.
A method of driving the thermal printer is also considered, but if the power source is driven by a battery, the power loss due to the power supply circuit is large, and battery driving is not suitable.

Also, in many serial type thermal printers, due to the power storage effect of the heat generating head, characters are written lightly at the beginning and become darker as the head moves, and in some cases, the dots become blurred and difficult to read. It was hot too. The present invention attempts to solve these various problems.

SUMMARY OF THE INVENTION An object of the present invention is to provide a thermal printer drive circuit that eliminates variations in the shading of characters from digit to digit and enables uniform print density in a serial drive system for a thermal printer. Still another object is to provide a thermal printer drive circuit that consumes less power and is inexpensive.

A driving circuit for a thermal printer according to the present invention will be described in detail below.

FIG. 4 is a schematic diagram of a conventionally used serial drive type thermal printer. Fourth
Figure A is a front view of the thermal printer, and 21
is a heat generating head in which a heat generating element 22 is arranged.
24 is a platen, 25 is a carriage for fixing the heat generating head 21, and 27 is a grooved cam 28. This grooved cam and the projections provided on the carriage 25 allow the carriage 25 to be moved in the digit direction (lateral direction in the figure). Reference numeral 29 represents a motor serving as a drive source for operating the drive shaft, and reference numeral 32 represents a gear that transmits the rotation of the motor 29. FIG. 4B is a side view of the thermal printer, and 23 is printing paper;
26 is a platen pressing spring that presses the platen 24 against the heat generating head 21; 30 is a paper feed roller;
Reference numeral 1 indicates a paper holding roller, and each time one line of printing is completed, a paper feed roller engaged with a motor 29 rotates to feed the paper. Heat generating element 22
is a serial drive type thermal printer.
Generally, it has 5 or 7 vertical dots of heating elements, and the heating head on which these are arranged is moved laterally to form characters in a 4 x 5 or 5 x 7 matrix. In such conventional thermal printers, when printing on the same line, there is often a difference in shading between the characters in the starting digit and the characters in the last digit. That is heat generating head 2
This is because heat accumulates in the printer, and in order to eliminate this, measures such as reducing the printing speed are required, which inevitably lowers the product value. The present invention eliminates these disadvantages.

FIG. 5 is a schematic circuit diagram of an embodiment of an electronic desktop calculator (hereinafter abbreviated as a calculator) using a circuit that is the basis of a driving circuit for a thermal printer according to the present invention. 41 is a power source that generates a voltage V _c , 42 is a Zener diode which is a type of constant voltage means, 43 is an adjustment resistor, and 44 is a capacitor. Zener diode 42, resistor 43,
A charging circuit is formed by combining a series circuit 46 whose basic component is a capacitor 44 and a resistor 45 provided to add the zener voltage V _Z of the zener diode, and is inserted in series with the power supply 41. has been done. The connection point between the Zener diode 42 and the adjustment resistor 43 is defined as a voltage dividing point S. A transistor 47 is a type of switch means for instantaneously discharging the charge in the capacitor 44, and is connected in parallel to the capacitor 44 via a resistor 48 for overcurrent prevention. A discharge circuit is formed by the transistor 47 and the resistor 48. A diode 49 is a type of constant voltage means, and forms a reference voltage generation circuit 57 through a series circuit with a resistor 50 to generate a reference voltage E. 51 is a voltage comparison circuit which is turned on and off depending on the charge level of the capacitor 44;
Turn off. Reference numeral 21 denotes a heat generating head shown in FIG. 4, in which a heat generating element 22 is arranged. Reference numeral 52 denotes a power supply transistor that controls the energization of the heating element 22, and is turned on for the energization time determined by the gate circuit 53 between the output of the voltage comparator circuit 51 and the output of the character signal generator 54, and is turned on for a predetermined period. Enables printing of characters. 55 is a discharge transistor 47
It is a timing signal generator that controls
A signal is generated to instantaneously turn on the discharge transistor 47 in synchronization with the printing position. The discharge time may be several + _microseconds . The charging circuit, the discharging circuit, the reference voltage generating circuit, and the voltage comparing circuit 51 form an energization width control circuit 58. The pulse width of the output waveform of the energization width control circuit 58, that is, the energization time to the heat generating element is assumed to be t.

Next, the operation of the energization width control circuit 58 will be explained in the sixth section.
This will be explained in detail using figures.

If the potential at the voltage dividing point S in FIG. 5 is V _S , it is expressed by equation (5).

V _S =V _c -V _Z ......(5) Pulse width t of the output of the voltage comparator circuit 51 at this time
is expressed by equation (6).

t=-C·Rln (1-E/V _c -V _Z ) (6) C is the value of the capacitor 44, and R is the value of the adjustment resistor 43.

FIG. 6 is a graph showing the relationship between the pulse width t of the energization width control circuit 58 and the power supply voltage _Vc .
The same scale as in the figure is used. x= _Vc /E
The characteristic curve is drawn by applying the y value obtained from the actual charging potential V _S at the _time of has a maximum of 2.5 (V). Components that are the same as those in FIG. 3 are designated by the same numbers. A characteristic curve 61 shown by a solid line shows the characteristics of the energization width control circuit shown in FIG. 5, and 62 is a straight line with a slope of -2. Characteristic curve 6
The range approximated by the straight line 62 of 1 is from point J to point K, and the value of X is 6.6 to 92. Converting this to power supply voltage is 6.4 (V) to 4.6 (V),
The range of equal energy characteristics has been significantly expanded compared to conventional circuits. Generally, the longer the current is applied to the heat generating element 22, the greater the heat dissipation, so it is necessary to input more energy.Adjustment of such characteristics can be done by changing the constant voltage value of the Zener diode 42 or by changing the standard voltage value. This can be easily corrected by adjusting the voltage E, etc.

FIG. 7 shows an embodiment of a drive circuit for a thermal printer according to the present invention, and the same parts as in FIGS. 4 and 5 are designated by the same numbers. The transistor 71 and the resistors 72 and 73 constitute a constant voltage means 76 having characteristics similar to those of the Zener diode 42 in FIG. 75 is a transistor which is a type of constant voltage means, and this transistor 75
A series circuit consisting of the constant voltage means 76 and the resistor 74 is inserted between the terminals of the power source 41. A series circuit of at least a resistor 78 and a first capacitor 44 is inserted between a connection point between the constant voltage means 76 and the resistor 74 and one terminal of the power supply 41. A charging circuit whose main components are the constant voltage means 76, the resistor 78, and the first capacitor 44 is formed. Further, a discharge transistor 47 is connected in parallel to the first capacitor 44, and the above circuit is collectively referred to as a charge/discharge circuit. The diode 77 is
This is inserted when you want to fine-tune the characteristics.

50 is a resistor, and 149 is a Zener diode which is a type of constant voltage means. A series circuit of the resistor 50 and the Zener diode 149 forms a constant voltage generating circuit. A second capacitor 80 is connected in parallel to this Zener diode 149 via a resistor 79. Further, a discharge transistor 82 is connected in parallel with the second capacitor via a resistor 81, and the resistor 5
A reference voltage generation circuit is formed mainly by a series circuit of 0 and a zener diode 149 as a constant voltage means, and a capacitor connected in parallel with the zener diode 149 as a constant voltage means.
51 is a voltage comparison circuit that compares the charge levels of the first capacitor 44 and the second capacitor 80. A permanent magnet 83 is fixed on the shaft of the motor 29, and together with a head 90 including a coil, constitutes a tachogenerator 86 (hereinafter abbreviated as TG 86). 85 is a timing signal generator that converts the waveform of TG 86 into a waveform that instantly turns on the discharge transistor 47. The TG 86 and the timing signal generator 85 constitute a position detector 87. The heat generating head 21 is arranged separately into heat generating elements of group A and heat generating elements of group B, which are shifted in position, to reduce the number of dots that are energized at the same time, and to reduce the voltage drop of manganese batteries, etc.

FIG. 8 shows changes in the reference voltage E as seen on the time axis, with reference numeral 91 indicating changes in the reference voltage E and reference numeral 92 indicating a signal from the CPU 57 to the transistor 82. Vd is the maximum value of the reference voltage E, which is equal to the voltage of the Zener diode 49, and the final potential of Vl is calculated by the following formula, assuming that the value of the resistor 79 is R ₁ and the resistance value of the resistor 81 is R ₂ It is expressed as (7).

Vl=Vd× _R2 / _R1 + _R2 ...(7) tp is the time of the printing process, tr is the time of the return process of returning the heat generating head 21 to the home position, and when printing is started, An H level signal is output from the CPU 57, the charge in the capacitor 80 starts to discharge, and the reference voltage E gradually decreases. The heating head 21 changes depending on the charging time of the capacitor 44.
Since the time for energizing is determined, the heating head 2
As the number 1 moves, the energizing time is reduced, preventing heat from accumulating in the heat generating head 21, and eliminating shading in printing for each digit. The value of the capacitor 80 is adjusted to match the time constant of heat accumulation in the heat generating head 21.
Determine the values of resistors 79 and 81. In the above explanation, the transistor 82 is turned on continuously during the printing process, but there is also a method in which the transistor 82 is turned on at the same time when the heat generating head 21 is actually energized. It's practical.

As described above, the present invention is an extremely useful thermal printer drive circuit that eliminates the drawbacks of conventional thermal printers and eliminates the difference in print density between digits. Furthermore, it is possible to realize a low power consumption thermal printer drive system that suppresses the power consumption of the power supply.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional constant temperature control circuit for a thermal pen, in which 1 indicates a power source, 3 a capacitor, 6 a voltage comparison circuit, and 9 a thermal pen.
FIG. 2 shows the relationship between the heating time t and the power supply voltage V _c of the circuit shown in FIG. 1. FIG. 3 is a characteristic diagram showing the relationship between power supply voltage and energization time of the conventional constant temperature control circuit shown in _FIG .
The vertical axis is scaled with t/C·R. FIG. 4 is a schematic diagram of a thermal printer, in which 21 indicates a heat generating head and 29 indicates a motor. FIG. 5 is a schematic diagram of a calculator using the basic circuit of the thermal printer drive circuit according to the present invention.
42 is a constant voltage means, 44 is a capacitor, 49 is a constant voltage means, 51 is a voltage comparison circuit, 22 is a heating element, and 58 is a current width control circuit. FIG. 6 is a graph showing the relationship between the power supply and pulse width of the energization width control circuit according to the present invention. FIG. 7 shows an embodiment of a driving circuit for a thermal printer according to the present invention, in which 44 is a first capacitor, 80
76 indicates a second capacitor, and 76 indicates a constant voltage means. FIG. 8 is a diagram showing changes in the reference voltage of the drive circuit of the thermal printer according to the present invention.

Claims (1)

[Claims] 1. In a drive circuit for a thermal printer that has a plurality of heat generating elements and selectively energizes the heat generating elements to print on printing paper, an energization control means that turns on and off the energization of the heat generating elements; It has an energization width control circuit that determines the energization time to the heat generating element,
The energization width control circuit connects in parallel to the first capacitor a charging circuit including a series circuit of at least one resistor and a first capacitor inserted between power supply terminals, and discharges the first capacitor. a constant voltage generating circuit connected to at least one of the power supply terminals, at least one resistor connected to the constant voltage generating circuit, and a second
a reference voltage generation circuit comprising a series circuit with a capacitor and a discharge circuit connected in parallel to the second capacitor to discharge the second capacitor;
It comprises a voltage comparison circuit that compares the charge level of the first capacitor and the charge level of the second capacitor, and the reference voltage generating circuit has a voltage that corresponds to the time constant of heat accumulation of the heat generating head in which the heat generating element is disposed. 1. A driving circuit for a thermal printer, characterized in that the driving circuit has a constant, and the voltage comparison circuit determines the energy applied to the heating element.