JP3170110B2 - Thermal printer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal printer having a function of measuring a resistance value of a heating element constituting a thermal head.

[0002]

2. Description of the Related Art Thermal printers include a thermal transfer printer using an ink film and a thermal printer that records an image by directly heating a thermal recording material. These thermal printers use a thermal head in which a large number of heating elements (resistance elements) are arranged in a line.

For example, in a color thermal printer, as described in JP-A-61-213169, a magenta thermosensitive coloring layer, a cyan thermosensitive coloring layer, and a yellow thermosensitive coloring layer are sequentially formed on a support. A color thermosensitive recording material is used. In this color thermosensitive recording material, in order to selectively color each thermosensitive coloring layer, its coloring heat energy is different, and a deeper thermosensitive coloring layer requires higher coloring heat energy. Also, when the next heat-sensitive coloring layer is thermally recorded, the heat-fixed heat-sensitive coloring layer is irradiated with a specific electromagnetic wave so as to prevent the heat-sensitive coloring layer therefrom from being recorded again. Is performed.

A thermal printer is provided with a thermal head in which a large number of heating elements are arranged in a line, and applies a coloring heat energy corresponding to a heat-sensitive coloring layer to be recorded to a color thermosensitive recording material. This heat energy for color development is obtained by adding heat energy for color development to a desired density (hereinafter, heat energy for gradation expression) to heat energy immediately before color development (hereinafter, this is called bias heat energy). is there. This bias heat energy is a constant value determined according to the coloring characteristics of the thermosensitive coloring layer. On the other hand, for the gradation expression thermal energy, it is necessary to perform fine heat generation control in order to express high gradation. In general, the heating element is energized for several milliseconds to several tens of ms in bias heating, and the energization of the heating element is controlled in units of several μs to several tens of μs in gradation expression heating.

Incidentally, in order for such fine heat control to be accurately reflected on a print result, it is necessary that all the heat generating elements constituting the thermal head have uniform resistance values. However, the resistance value of the heating element generally has a variation of about 5%, which causes inconvenient phenomena such as uneven color in a recorded image.

In order to improve this, for example, Japanese Patent Laid-Open No. 2-2
As described in Japanese Patent No. 48262, there has been proposed a thermal printer that measures all the resistance values of several hundred heating elements provided in a thermal head, corrects image data based on the measurement results, and prints. ing. In this thermal printer, a noise absorbing capacitor is connected via a switch between a pair of power supply terminals for driving a heating element constituting a thermal head. After the inactive state, the power supply voltage E is applied to one heating element.
Then, the voltage V by the heating element was measured, and the equation r = [V /
(E−V)] · R is used to calculate the resistance value r of the heating element. This is performed for each heating element, and image data is corrected based on the obtained resistance value of each heating element and printed. R is the value of the reference resistance connected between the power supply and the thermal head.

However, in this thermal printer, when the voltage V generated by the heating element is measured, the switch is required to be turned off to disconnect the noise absorbing capacitor from the pair of power supply terminals. In addition, there is a drawback that the measurement result is apt to be affected by extraneous noise and the measurement result tends to vary. The voltage E
For example, an A / D conversion circuit is required to measure, so that the structure is complicated.

Accordingly, the present applicant has filed Japanese Patent Application No. 4-2336.
No. 26 proposed a thermal printer having a function of measuring the resistance of each heating element with a simple circuit. This thermal printer discharges a capacitor charged to a certain voltage by a heating element and measures the time until the terminal voltage of the capacitor drops to a certain value, thereby obtaining the resistance value of each heating element. And corrects image data to perform printing without unevenness. Further, the absolute value of the resistance value of each heating element can be measured by performing the charging / discharging measurement using a reference resistance having a known resistance value.

[0009]

However, when the absolute value of the resistance value of each heating element is actually measured, the resistance value is determined by a switch for turning ON / OFF the power supply to the heating element, for example, the saturation voltage of the transistor. It was found that an error occurred in the measured value of. Assuming that the power supply voltage is E, the discharge time is t, the capacitance of the capacitor is C, the resistance value of the heating element is R, and the saturation voltage of a predetermined heating element is V ₀ , the potential V of the capacitor is obtained.
Is represented by the following equation. V = (E−V ₀ ) exp (−t / CR) + V ₀ ... If t = T and V = V1, V1 = (E−V ₀ ) exp (−T / CR) + V ₀ ∴R = -T / C / ln {(V1 -V 0) / (E-V 0)} when idealized ··· V ₀ = _0, 'putting a, R' R = R = - T / C / ln (V1 / E) ··· ∴R '/ R = ln {(V1-V 0) / (E-V 0)} / ln (V1 / E) ··· eg E = 20 [V], V1 = 15 [V], V ₀ = 0.
In the case of 3 [V], R ′ / R = 1.0177, and an error occurs.

The present invention provides a thermal printer which can measure a heating element resistance value with high accuracy without a measurement error caused by the saturation voltage of a transistor while having a simple circuit, and thereby can perform printing without unevenness. The purpose is to do so.

[0011]

In order to achieve the above object, a thermal printer according to the present invention comprises a plurality of parallel-connected thermal printers.
Heating elements and multiple heating elements connected in series with each heating element.
A heat control switch is provided.
Connected to a power supply to generate heat.
In a thermal printer, a capacitor connected in parallel with the plurality of heating elements, the plurality of heating elements and the
A charge switch for charging the capacitor , and a charge switch for charging the capacitor, turning off the charge switch after charging the capacitor, and being connected in series with a heating element to be measured. A control switch is turned on, a first discharge time measuring means for measuring a time from when the capacitor starts discharging through the heating element to when the capacitor reaches the first predetermined potential, and after the capacitor starts discharging. A second discharge time measuring means for measuring a time required to reach a second predetermined potential lower than the first predetermined potential;
Resistance value calculation means for calculating a resistance value based on the first and second discharge times measured for each heating element, and correction means for correcting image data based on the obtained resistance value of each heating element. It is provided with.

[0012]

After the capacitor is charged by turning on the charge switch, the charge switch is turned off, and the control switch connected in series with the heating element to be heated is turned on, so that the capacitor starts discharging through the heating element. After that, the time until the first predetermined potential is reached is measured. Further, the time until the discharge is continued and reaches the second predetermined potential is measured. The resistance value of the heating element is calculated based on the two discharge times. Then, the resistance values of the other heating elements are similarly calculated. Correction data is calculated from a difference between these resistance values and, for example, an ideal resistance value, and thereby the image data is corrected. Since the heating elements are driven by the corrected image data, accurate printing without unevenness is performed.

[0013]

FIG. 2 shows an embodiment of a color thermal printer. In FIG. 2, a platen drum 10 holds a color thermal recording material 11 on its outer periphery, and is rotated by a pulse motor (not shown) during thermal recording. A clamp member 12 is attached to the platen drum 10, and at least one portion, for example, the tip of the color thermosensitive recording material 11 is fixed to the platen drum 10. The clamp member 12 has a U-shape, and elongated holes 12a and 12b provided at both ends are fitted to the platen drum shaft 15 and the guide pins 16, respectively.
The clamp member 12 is pressed against the platen drum 10 by a spring 17 and the color thermosensitive recording material 1 is pressed.
When the clamp 1 is released or when the clamp is released, the solenoid 18
Is moved in a direction away from the platen drum 10.

On the outer periphery of the platen drum 10, a thermal head 20 and an optical fixing device 21 are provided.
The thermal head 20 is provided with a heating section 22 for sequentially generating constant bias thermal energy and thermal energy for gradation expression corresponding to the color density of the pixel. Optical fixing device 2
1, approximately 365 nm and 42 nm as shown by the solid line in FIG.
A rod-shaped ultraviolet lamp 23 having an emission peak at 0 nm
And a cut filter 24 having transmission characteristics as indicated by a dotted line. This cut filter 24
Transmits near-ultraviolet rays of about 420 nm when inserted in front of the ultraviolet lamp 23 by a solenoid or the like. A conveyance roller pair 28 is disposed in the paper supply / discharge passage 27, and the color thermosensitive recording material 11 is conveyed through the conveyance roller pair 28. Also, on the platen drum side of the paper supply / discharge passage 27,
At the time of paper ejection, the rear end of the color thermosensitive recording material 11 is
A separation claw 29 is provided for guiding the cradle 7. In this embodiment, one passage is used for both the paper supply passage and the paper discharge passage, but these may be provided separately.

FIG. 4 shows an example of a color thermosensitive recording material. On a support 32, a cyan thermosensitive coloring layer 33,
A magenta thermosensitive coloring layer 34, a yellow thermosensitive coloring layer 35, and a protective layer 36 are sequentially provided. Each of the thermosensitive coloring layers 33 to 35 is provided from the surface in the order of thermal recording. For example, when thermal recording is performed in the order of magenta, yellow, and cyan, the yellow thermosensitive coloring layer 35 and the magenta thermosensitive layer are arranged. The position with the color forming layer 34 is exchanged. As the support 32, opaque coated paper or plastic film is used, and when an OHP sheet is produced, a transparent plastic film is used.

The cyan thermosensitive coloring layer 33 contains an electron-donating dye precursor and an electron-accepting compound as main components, and develops cyan when heated. Magenta thermosensitive coloring layer 3
No. 4 contains a diazonium salt compound having a maximum absorption wavelength of about 365 nm and a coupler which reacts with the diazonium salt compound to develop magenta. When the magenta thermosensitive coloring layer 34 is irradiated with ultraviolet light near 365 nm after thermal recording, the diazonium salt compound is photolyzed and loses its coloring ability. The yellow thermosensitive coloring layer 35 has a maximum absorption wavelength of about 42
It contains a diazonium salt compound having a thickness of 0 nm and a coupler which thermally reacts with the compound to develop a yellow color. When the yellow thermosensitive coloring layer 35 is irradiated with near-ultraviolet rays of 420 nm, it is light-fixed and loses its coloring ability.

FIG. 1 shows an electric circuit of a color thermal printer. In the frame memory 40, one frame of image data is written in a state of being separated for each color. At the time of the gradation expression heating, the image data of the color to be printed is read line by line from the frame memory 40 and written into the line memory 41. The image data of the line memory 41 is read out for each pixel and is read by the comparator 42.
Sent to The comparator 42 compares the image data of each pixel with the gradation data (comparison data), and outputs a signal of “1” when the image data is larger.

The print controller 43 includes, for example, 64
In the case of gradation, gradation data of "0" to "3F" is sequentially generated in hexadecimal notation. When the gradation data of “0” is sent from the print controller 43, the comparator 42 compares the gradation data with the image data of each pixel in order. As a result, the comparison result for one line is output from the comparator 42 as a serial signal, and sent to the shift register 44 via the switch Sa for switching between the print mode and the resistance measurement mode. When the comparison of the image data for one line is completed, the print controller 43 generates the gradation data of “1” and sends it to the comparator 42. Therefore,
By using the gradation data of “0” to “3F”, the image data of each pixel is compared 64 times and converted into 64-bit drive data. The 64-bit drive data is sent to the shift register 44 in 64 times.

The serial drive data is shifted in the shift register 44 by a clock and converted into a parallel signal. The drive data converted into the parallel signal by the shift register 44 is latched by the latch array 45 in synchronization with the latch signal. When the strobe signal is input, the AND gate array 46 outputs a signal of “H” when the input drive signal is “1”. The latch array 45 and the AND gate array 46 are provided with circuit elements for each pixel.

The transistors Tr1 to TrN are connected to the output terminals of the AND gate array 46, respectively. When the output signal is "H", the transistors are turned on. The heating elements R1 to RN constituting the thermal head 20 are connected in series to the transistors Tr1 to TrN, respectively.

A noise absorbing capacitor 50 is connected in parallel with the heating elements R1 to RN.
0 is connected to the power supply 51 via the switch Sb.
The switch Sb is always closed in the print mode, and in the resistance measurement mode, the opening and closing of the switch Sb is controlled by the print controller 43 each time the resistance values r _{1 to} rn of the heating elements R _{1 to} RN are measured.

The non-inverting input terminals of the two comparators 52 and 53 are connected to one terminal of the capacitor 50, and the reference voltages V1 and V2 of the comparators 52 and 53 are connected.
Are the resistances Ra, Rb of the resistance values ra, rb, rc, respectively.
It is obtained by resistance voltage division by b and Rc. That is, V1 = {(rb + rc) / (ra + rb + rc)} E V2 = {rc / (ra + rb + rc)} E After the capacitor 50 is charged in the resistance measurement mode, the switch Sb is opened, and only the transistor Tr1 of the heating element R1 to be measured is turned on.

As shown in FIG. 5, when the capacitor 50 is fully charged, the potentials of the non-inverting input terminals of the comparators 52 and 53 are E, respectively. As the heating element R1 discharges, the potentials of the non-inverting input terminals of the comparators 52 and 53 decrease, and eventually match the reference potential V1 of the comparator 52. Immediately after this, the comparator 52
Of the output terminal changes from positive to negative. The resistance measuring unit 43a of the print controller 43 measures the time T1 from immediately after the switch Sb is opened to the time when the output voltage from the comparator 52 changes from positive to negative, and measures this time as RA.
Write to M43b. When the potential at the non-inverting input terminal of the comparator 53 matches the reference potential V2 after a further time, the potential at the output terminal of the comparator 53 changes from positive to negative. The resistance measuring unit 43a measures the time T2 from the time immediately after the switch Sb is opened to the time when the output voltage from the comparator 53 changes from positive to negative.
Write to 3b.

The resistance measuring section 43a obtains the obtained T1, T2
Calculating the resistance value r ₁ of the heating element R1 by solving the following formula 1 from and writing to RAM43c with.

[0025]

[Formula 1]

In this formula 1, V1, V2, E and the capacitance C of the capacitor 50 are predetermined values, and T1 and T1 are measured values. Therefore, by solving the formula 1 for R, the resistance value of the heating element is calculated. be able to.

Equation 1 is obtained as follows. The power supply voltage is E, the discharge time is t, the capacitance of the capacitor is C, the resistance value of the heating element is R, and the saturation voltage of a predetermined heating element is V _0.
Then, the potential V of the capacitor is expressed by the following equation (2). V−V ₀ = (E−V ₀ ) exp (−t / CR) (2) If t = T1 and V = V1, V1−V ₀ = (E−V ₀ ) exp (−T1 / CR) ∴V1-Eexp (-T1 / CR) = V 0 {1-exp (-T1 / CR)} ··· (3) If at t = T2 V = V2, V2 -V 0 = (E- V ₀₎ from exp (-T2 / CR) ∴V2- Eexp (-T2 / CR) = V 0 {1-exp (-T2 / CR)} ··· (4) equation (3) / formula (4) Equation 1 is obtained by canceling the saturation voltage V ₀ . In the print mode, the image data is corrected and printed based on the resistance values r _{1 to} rn calculated according to Equation 1.

Next, the operation of the above embodiment will be described with reference to FIGS. During the initial setup of the color thermal printer, the mode is switched to the resistance measurement mode by the switch Sa, and the shift register 44 is connected to the print controller 43. Print controller 43
Means that the transistor Tr1 is ON and the transistors Tr2 to Tr2
One line of data in which TrN is turned off is output. Then, the switch Sb is output by the resistance measuring unit 43a.
Is turned on, and charging of the capacitor 50 is started. After the charge voltage of the capacitor 50 reaches E, the switch Sb is turned off. As a result, the capacitor 50 starts to be discharged via the heating element R1, and gradually the comparator 50
The potential applied to each of the non-inverting input terminals 2 and 53 is reduced.

When the potential of the non-inverting input terminal of the comparator 52 matches the reference potential V1, the discharge time T
1 is measured by the resistance measuring unit 43a,
43b. Subsequently, the potential of the non-inverting input terminal of the comparator 53 matches the reference potential V2, and the required discharge time T2 is measured by the resistance measuring unit 43a.
This is written to the RAM 43b.

Next, the resistance measuring section 43a is connected to the RAM 43b.
Reads the discharge time T1, T2 from calculates the resistance value r ₁ of the heating element R1 by Equation 1, it RAM4
Write to 3c. Hereinafter Similarly, the resistance value r ₂ ~rn heating elements R2~RN is written is calculated RAM43c.

In the print mode, the shift register 44 is connected to the comparator 42 by the switch Sa. In the print mode, first, image data of three colors is taken into the frame memory 40. These image data are correction data is calculated from the difference between the ideal resistance value and the actually measured resistance value r ₁ ~rn when the heat element R1~RN is completely uniform, the heating element R1~RN The image data is corrected by the correction data so that the image to be recorded is accurately printed.

At the time of sheet feeding, the platen drum 10 is stopped with the clamp member 12 being vertical in FIG. 2, so that when the solenoid 18 is energized, the clamp member 12 is set to the clamp release position. The conveying roller pair 28 nips the color thermosensitive recording material 11 supplied from a cassette (not shown) and conveys it toward the platen drum 10. The transport roller pair 28 stops once when the leading end of the color thermosensitive recording material 11 enters between the platen drum 10 and the clamp member 12. Thereafter, when the solenoid 18 is turned off, the clamp member 12 is turned off.
Is returned by the spring 17 to clamp the front end of the color thermosensitive recording material 11. After this clamping, the platen drum 10 and the conveying roller pair 28 rotate, so that the color thermosensitive recording material 11 is wound around the platen drum 10.

When the platen drum 10 rotates intermittently at regular steps and the leading end of the recording area of the color thermosensitive recording material 11 reaches the thermal head 20, thermal recording is started. At the time of this thermal recording, the image data of the yellow image for one line is read from the frame memory 40 and once written to the line memory 41.

Next, the corrected image data of each pixel is sequentially read from the line memory 41 and sent to the comparator 42.
Here, it is compared with the gradation data of the gradation level “0”. The output of the comparator 42 is “1” for a pixel that records a yellow image, and is “0” for a pixel that does not record a yellow image. The comparison result of each pixel is sent to the shift register 44 as serial drive data, and is shifted in the shift register 44 by a clock to be converted into parallel drive data. This parallel drive data is latched by the latch array 45, and
It is sent to the D gate array 46.

The print controller 43 generates a bias heating pulse having a long width and sends it to the AND gate array 46 as a strobe signal. AND gate array 4
6 outputs the logical product of the strobe signal and the output signal of the latch array 45, so that among the output terminals of the AND gate array 46, the output terminal of the latch array 45 is "1".
Output "1". For example, AND
When the first output terminal of the gate array 46 is “1”, the transistor Tr1 is turned on, and thus the heating element R
1 is energized and generates heat. As a result, the heating element R1 is energized for a time corresponding to the bias heating pulse, and gives the bias heat energy to the color thermosensitive recording material 11.

Before the end of the bias heating, the print controller 43 generates gradation data having a gradation level of "0", sends it to the comparator 42, and compares it again with the image data of each pixel. By this comparison, serial drive data is formed, and this drive data is written to the shift register 44. When the bias heating is completed, the print controller 43 generates a tone expression pulse having a short pulse width. This gradation expressing pulse is A as a strobe signal.
It is sent to the ND gate array 46. The heating element is energized for a short time by the strobe signal, and the yellow thermosensitive coloring layer 35 is colored to the density of the gradation level “1”. Less than,
In order for the print controller 43 to sequentially change the gradation levels from “1” to “3F”, the comparator 42 outputs drive data corresponding to each gradation level. As a result, the heating elements R1 to RN are energized by the number of times corresponding to the corrected image data, and apply the gradation expression heat energy to the color thermosensitive recording material 11 to develop a color at a desired density.
For example, in the case of 64 gradations, for a pixel having the maximum density, 64 pulse currents are supplied to the heating element for gradation expression.

When the first line of the yellow image is recorded, the platen roller 10 rotates stepwise by one pixel, and the second line of the yellow image is read from the frame memory 40.
The image data of the line is read. The second line is thermally recorded on the color thermosensitive recording material 11 based on the image data of the second line of the yellow image. When the portion where the yellow image is thermally recorded reaches the optical fixing device 21, the yellow thermosensitive coloring layer 35 is optically fixed here. In the optical fixing device 21, the cut filter 24 is set in front of the ultraviolet lamp 23, so that near-ultraviolet light near 420 nm is irradiated on the color thermosensitive recording material 11. As a result, the diazonium salt compound contained in the yellow thermosensitive recording material 11 is decomposed, and the coloring ability is lost.

When the platen drum 10 makes one rotation and the recording area comes to the position of the thermal head 20 again, a magenta image is recorded on the magenta thermosensitive coloring layer 34 line by line. The coloring heat energy of the magenta image is larger than the coloring heat energy of the yellow image. However, since the yellow heat-sensitive coloring layer 35 has already been optically fixed, the yellow heat-sensitive coloring layer 35 does not recolor. The color thermosensitive recording material 11 on which the magenta image has been recorded is optically fixed by the fixing device 21 as described above. In this case, since the cut filter 24 is retracted from the front of the ultraviolet lamp 23, all the electromagnetic waves radiated from the ultraviolet lamp 23 are applied to the color thermosensitive recording material 11. Of the electromagnetic waves, the magenta thermosensitive coloring layer 34 is optically fixed by ultraviolet rays near 365 nm.

When the platen drum 10 makes one more rotation and the recording area comes to the position of the thermal head 20 again, a cyan image is recorded on the cyan thermosensitive coloring layer 33 line by line. The cyan heat-sensitive coloring layer 33 does not have light fixability to the cyan heat-sensitive coloring layer 33 because the coloring heat energy has a value that does not cause coloring in a normal storage state. Therefore, in the thermal recording of the cyan thermosensitive coloring layer 33, the optical fixing device 21 is in the OFF state.

After the completion of the thermal recording of the yellow, magenta and cyan images, the platen drum 10 and the conveying roller pair 28 rotate in reverse. The reverse rotation of the platen drum 10 causes the rear end of the color thermosensitive recording material 11 to
The sheet is guided to the sheet supply / discharge passage 27 and is nipped by the conveying roller pair 28. Thereafter, when the platen drum 10 reaches the sheet feeding position, the solenoid 18 is energized and the platen drum 10 stops. Solenoid 18
As a result, the clamp member 12 moves against the spring 17 so that the clamp at the front end of the color thermosensitive recording material 11 is released. As a result, the thermally recorded color thermosensitive recording material 11 is discharged to the tray via the paper supply / discharge path 27.

The discharge times and resistance values r ₁ -rn of the heating elements R ₁ -RN written in the RAMs 43 b, 43 c are determined by incorporating a backup battery or by supplying a backup power supply from, for example, the power supply 51. Is held. When power is supplied from the power supply 51, the RAM 43b,
Since 43c is data lost, the measurement of the resistance value r ₁ ~rn every set up the color thermal printer.
In addition, the RAM 43c is connected to an R
It may be replaced with OM. In this case, the previously measured resistance values r _{1 to} rn are burned into the ROM, and this RO
M may be incorporated in the print controller when assembling the color thermal printer.

In the above-described embodiment, when calculating the resistance value of each heating element, two discharge times T1 and T2 are measured, and then these values are substituted into the above-mentioned formula 1 to calculate each time. However, for example, Equation 1 may be replaced with an approximate equation, or a table may be prepared in advance, and the discharge times T1 and T2 may be given to obtain the resistance value of each heating element.

It is also possible to make only the function of measuring the resistance value of the heating element of the thermal head independent of the color thermal printer described above. In this case, for example, it can be used as a tester when inspecting a thermal head. In addition, although a color thermal printer has been described as an example, the present invention can be applied to a monochrome thermal printer, a color thermal transfer printer, and the like. Also, the line printer that moves the color thermal recording material and the thermal head relatively one-dimensionally has been described. However, the present invention can also be used for a serial printer whose relative movement is two-dimensional.

[0044]

As described in detail above, the thermal printer of the present invention measures two discharge times for one heating element and calculates the resistance value of the heating element based on this measurement. Although it is a simple circuit, it is possible to accurately measure the resistance value of the heating element without including a measurement error caused by the saturation voltage of each switch means. Then, the correction data is calculated based on the resistance value of each heating element and the image data is corrected by this, so that printing without unevenness can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an electric circuit of a color thermal printer.

FIG. 2 is a schematic view illustrating an example of a color thermal printer.

FIG. 3 is a graph showing characteristics of an ultraviolet lamp and a cut filter of the optical fixing device.

FIG. 4 is an explanatory diagram showing a layer structure of a color thermosensitive recording material.

FIG. 5 is a graph showing discharge characteristics.

FIG. 6 is a flowchart of a resistance measurement mode.

FIG. 7 is a waveform diagram of a signal supplied to each unit shown in FIG.

FIG. 8 is a flowchart of a print mode.

[Explanation of symbols]

 Reference Signs List 20 thermal head 43 print controller 43a resistance measuring section 43b, 43c RAM 50 capacitor 51 power supply 52, 53 comparator R1 to RN heating element Rs resistance Sa, Sb switch T1, T2 discharge time Tr1, TrN, Trs transistor

Claims (1)

(57) [Claims] 1. A heating device comprising: a plurality of heating elements connected in parallel; and a plurality of heating control switches connected in series with each heating element. In the thermal printer, a capacitor connected in parallel with the plurality of heating elements,
Connected in series between the plurality of heating elements and the power supply;
It is a charge switch for charging the capacitor, ON the connected control switch heating element in series to be the measurement as well as to the OFF charging switch after charging the capacitor in the ON the charging switch, the capacitor A first discharge time measuring means for measuring a time from the start of discharging via the heating element to reaching a first predetermined potential; and a first discharging time measuring means which is lower than the first predetermined potential after the capacitor starts discharging. A second discharge time measuring means for measuring a time required to reach a predetermined potential of 2, a resistance value calculating means for calculating a resistance value based on the first and second discharge times measured for each heating element, A thermal printer, comprising: a correcting unit that corrects image data based on the obtained resistance value of each heating element.