JPH07314756A - Method and device for measuring value of resistance of thermal head and thermal equipped therewith - Google Patents

Abstract

PURPOSE:To measure the resistances of heating elements composing a thermal head under the condition that the accident from electric shock and the breakage of the heating element are prevented from occurring. CONSTITUTION:A reference resistance having a known value of resistance is connected in parallel to a plurality of heating elements composing a thermal head. By turning a charging switch Sb, ON, the charging of a capacitor is started. After the charging of the capacitor, by turning the charging switch Sb OFF, the capacitor starts to discharge through the reference resistance. When the terminal voltage of the capacitor coincides with the reference voltage Vref, the discharging time Ts required for coincides is measured. Similarly, the discharging times T1-TN by respective heating elements are measured. By multiplying the ratios between the discharging times T1-TN and the discharging time Ts with the value of the resistance of the reference resistance, the values of the resistances of the respective heating elements are calculated. After the measurement of the discharging time TN by the final heating element, the discharging of the capacitor through the reference resistance is performed so as to discharge the remaining charge of the capacitor. At printing, image data are corrected on the basis of the values of the resistances of the respective heating elements in order to carry out the printing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for measuring the resistance value of a heating element which constitutes a thermal head, and a thermal printer equipped with the method and apparatus.

[0002]

2. Description of the Related Art Thermal printers include a thermal transfer printer that uses an ink film and a thermal printer that directly heats a thermal recording material to record an image. In these thermal printers, a thermal head in which a large number of heating elements (resistive elements) are arranged in a line is used.

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, the coloring heat energy is different in order to selectively develop the color of each thermosensitive coloring layer, and the deeper thermosensitive coloring layer requires higher coloring heat energy. Also, when the next thermosensitive coloring layer is heat-recorded, the heat-sensitive thermosensitive coloring layer on it is irradiated with electromagnetic waves peculiar to the heat-recorded thermosensitive coloring layer so as not to be recorded again. Is done.

The thermal printer is provided with a thermal head in which a large number of heating elements are arranged in a line, and imparts coloring heat energy corresponding to the thermosensitive coloring layer to be recorded to the color thermosensitive recording material. This coloring heat energy is obtained by adding heat energy immediately before coloring (hereinafter, referred to as bias heat energy) to heat energy for causing a desired density (hereinafter, referred to as gradation expression heat energy). is there. This bias heat energy is a constant value that is determined according to the color development characteristics of the thermosensitive color development layer. On the other hand, the gradation expression heat energy requires detailed heat generation control in order to express high gradation. Generally, the heating element is energized for several ms 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 μs in gradation expression heating.

In order to accurately reflect such fine heat generation control on the printing result, it is necessary that the resistance values of all the heat generating elements constituting the thermal head are uniform. However, the resistance value of the heating element generally varies by about 5%, which causes an inconvenient phenomenon such as color unevenness 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, a thermal printer has been proposed which measures all the resistance values of several hundreds of heating elements provided in a thermal head, corrects image data based on the measurement result, and prints. ing. In this thermal printer, a capacitor for noise absorption is connected via a switch means between a pair of power supply terminals that drive a heating element that constitutes a thermal head. After the inoperative state, the power supply voltage E is supplied to one heating element.
Then, the voltage V due to the heating element is measured, and the equation r = [V /
(EV)] · R, the resistance value r of the heating element is calculated. Then, this is performed for each of the heating elements, and the image data is corrected based on the obtained resistance value of each heating element and printed. Note that R is the value of the reference resistance connected between the power source and the thermal head.

However, in this thermal printer, the switch is turned off when the voltage V due to the heating element is measured.
This switch is necessary because the capacitor for absorbing noise is separated from the pair of power supply terminals, and this switch is required, and there is a drawback in that it is easily affected by external noise and the measurement result tends to vary. Further, in order to measure the voltage V, for example, an A / D conversion circuit is required, which has a drawback that the structure is complicated.

Therefore, the present applicant has filed Japanese Patent Application No. 4-2336.
No. 26 proposed a device that can measure the resistance value of each heating element with a simple circuit. This device obtains the resistance value of the heating element by discharging the capacitor charged to a certain voltage by the heating element and measuring the time until the terminal voltage of the capacitor drops to a certain value. Also, connect a reference resistance with a known resistance value in parallel with each heating element, and measure the time until the terminal voltage of the capacitor charged to a constant voltage drops to a certain value with the reference resistance and each heating element. By doing so, the absolute value of the resistance value of each heating element can be measured.

[0009]

In the measuring method as described above, since the measurement of the discharge time by each heating element is completed in the middle of discharging, the resistance value of each heating element can be measured quickly, but the final heating After measuring the discharge time of the device, the charge remains on the capacitor. Therefore, if this capacitor is touched carelessly, an electric shock may occur and it may be dangerous. Further, when the driving of the thermal head is started, the residual charge of the capacitor flows to the heating element that is driven first, and this heating element may be damaged, which is a problem.

It is an object of the present invention to provide a method for measuring the resistance value of a thermal head which prevents the above-mentioned electric shock and damage to the heating element.

[0011]

In order to achieve the above object, a resistance value measuring method according to a first aspect of the present invention is such that a plurality of heating elements arranged in a line on a thermal head have a reference resistance of a known resistance value. Connect the capacitors in parallel with each other, measure the discharge time until the capacitors reach the predetermined potential after charging the capacitors, and measure the discharge time of the reference resistance and each heating element. From the time, while calculating the resistance value of each heating element,
After the discharge time has been measured, the reference resistor discharges the residual charge of the capacitor.

According to a second aspect of the present invention, there is provided a method for measuring a resistance value, wherein a plurality of heating elements arranged in a line on a thermal head are connected in parallel with a reference resistance, a capacitor, and a discharging resistor, the resistance of which is known. The discharge time until the capacitor reaches the predetermined potential after charging is measured for each reference resistance and each heating element, and the resistance value of each heating element is calculated from the discharge time of the reference resistance and each heating element. And the residual charge of the capacitor is discharged by the discharging resistor after the discharge time measurement is completed.

According to a third aspect of the present invention, a plurality of heating elements linearly connected in parallel to the thermal head, a plurality of heating control switches connected in series with each heating element, and the heating control switch are turned on. In order to heat the power source that heats the heating elements, the reference resistance that is connected in parallel with each heating element and has a known resistance value, and one heating element selected when measuring the resistance value of each heating element. Capacitor of
The charge switch for charging this capacitor and the charge switch after charging the capacitor are turned off to start discharge through the heating element to be measured, and the time required for the capacitor to drop to a predetermined potential from the start of discharge is measured. Discharge time measuring means, resistance value calculating means for calculating the resistance value of each heat generating element from the discharge time of each heat generating element and the discharge time of the reference resistance, and measuring the discharge time of each heat generating element and the reference resistance And discharging means for discharging the residual charge of the capacitor by the reference resistance.

A thermal printer according to a fourth aspect is a thermal printer including a thermal head in which a plurality of heating elements are arranged in a line, and a plurality of heating control switches connected in series with each of the plurality of heating elements. , A power source for heating the heating elements whose heating control switch is turned on, a reference resistance with a known resistance value connected in parallel with each heating element, and 1 selected when measuring the resistance value of each heating element A capacitor for heating each heating element,
The charge switch for charging this capacitor and the charge switch after charging the capacitor are turned off to start discharge through the heating element to be measured, and the time required for the capacitor to drop to a predetermined potential from the start of discharge is measured. Discharge time measuring means, resistance value calculating means for calculating the resistance value of each heating element from the discharging time of each heating element and the discharging time of the reference resistance, and an image based on the obtained resistance value of each heating element It is provided with a correcting means for correcting data and a discharging means for discharging the residual charge of the capacitor by the reference resistance after the discharge time of each heating element and the reference resistance has been measured.

[0015]

The reference resistance and the capacitor having known resistance values are connected in parallel to each heating element of the thermal head. After charging the capacitor, the time from when the capacitor starts discharging through the reference resistance to when it drops to a predetermined potential is measured. Similarly, the discharge time by each heating element is measured, and the ratio with the discharge time by the reference resistance is obtained. The resistance value of each heating element is calculated by multiplying each time ratio by the resistance value of the reference resistance. Then, after measuring the discharge time of the final heating element, a switch connected in series with the reference resistance or the discharge resistance is turned on to discharge the residual charge of the capacitor through the reference resistance or the discharge resistance.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 2 showing a color thermal printer in which the measuring method of the present invention is carried out, a platen drum 10 is rotationally driven by a stepping motor 12 via a platen shaft 11. The platen drum 10 is provided with a clamper 15 for pressing and fixing the front end portion 13a of the color thermosensitive recording material 13 against the outer peripheral surface 10a of the platen drum 10. The clamper 15 is attached to the platen shaft 11 via a long hole 15a. The clamper 15 is shifted in the radial direction of the platen drum 10 by the cam mechanism 16 when the color thermosensitive recording material 13 is clamped or released.

On the outer periphery of the platen drum 10, the thermal head 2 is provided at the thermal recording position of the color thermal recording material 13.
0, the optical fixing device 21 is arranged on the downstream side (in the normal direction of the platen drum 10). The thermal head 20 is
A large number of heating elements are arranged in a line in the main scanning direction (direction perpendicular to the drawing), and each heating element generates heat energy according to the density of the pixel.

As shown by the solid line in FIG. 3, the optical fixing device 21 includes a rod-shaped ultraviolet lamp 23 having emission peaks at about 365 nm and 420 nm, and a cut filter 24 having a transmission characteristic shown by a dotted line. It is configured. The cut filter 24, when placed in front of the ultraviolet lamp 23 by a solenoid or the like, transmits near ultraviolet rays in the vicinity of 420 nm.

Upstream of the thermal head 20, there is provided a paper feed / discharge path 27 for both feeding and discharging. A transport roller pair 28 is arranged in the paper supply / discharge path 27, through which the color thermosensitive recording material 13 is transported. Further, on the platen drum side of the paper supply / discharge path 27, a separation claw 29 is provided for guiding the rear end of the color thermosensitive recording material 13 to the paper supply / discharge path 27 at the time of paper discharge. In this example, 1
Although one passage is used as both the paper feed passage and the paper discharge passage, they may be provided separately.

FIG. 4 shows an example of a color thermosensitive recording material. On the 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 formed. These thermosensitive coloring layers 33 to 35 are layered from the surface in the order of thermal recording. For example, in the case of thermal recording in the order of magenta, yellow, and cyan, the yellow thermosensitive coloring layer 35 and the magenta thermosensitive coloring layer 35 are used. The position of the coloring layer 34 is exchanged. An opaque coated paper or a plastic film is used as the support 32, and a transparent plastic film is used when an OHP sheet is prepared.

The cyan thermosensitive coloring layer 33 contains an electron-donating dye precursor and an electron-accepting compound as main components, and develops a cyan color 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 thermally reacts with this compound to develop magenta color. When the magenta thermosensitive coloring layer 34 is irradiated with ultraviolet rays in the vicinity of 365 nm after thermal recording, the diazonium salt compound is photodecomposed and the coloring ability is lost. 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 this yellow thermosensitive coloring layer 35 is irradiated with near-ultraviolet rays of 420 nm, it is photo-fixed and the coloring ability is lost.

FIG. 5 shows an electric circuit of the color thermal printer. Image data of one frame is written in the frame memory 40 in a state of being separated for each color. In 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 in the line memory 41. The image data of the line memory 41 is read out for each pixel and is compared 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 is, for example, 64
In the case of gradation, gradation data of "0" to "3F" is sequentially generated in hexadecimal system. When the gradation data of “0” is sent from the print controller 43, the comparator 42 sequentially compares the gradation data with the image data of each pixel. As a result, the comparison result for one line is output from the comparator 42 as a serial signal, and the mode changeover switch S
It is sent to the shift register 44 via a. 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. Then, the 64-bit drive data is sent to the shift register 44 in 64 times.

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 in the latch array 45 in synchronization with the latch signal. The AND gate array 46 outputs a signal of "H" when the input drive signal is "1" when the strobe signal is input. The latch array 45 and the AND gate array 46 are provided with circuit elements for each pixel.

Each output terminal of the AND gate array 46 has transistors Tr1 to T as heat generation control switches.
The rNs are connected to each other, and when the output signal is "H", the transistors Tr1 to TrN are turned on. The heating elements R1 to RN forming the thermal head 20 are connected in series to the transistors Tr1 to TrN, respectively. A reference resistor Rs, which is a reference when measuring the resistance values r _{1 to} rn of the heating elements R1 to RN, is connected so as to be in parallel with the heating elements R1 to RN. A transistor Trs is connected in series to the reference resistor Rs, and the transistor Trs is O when the REF signal output from the print controller 43 is “H”.
Become N. As the reference resistance Rs, one having a known resistance value rs and an error of about 1% is used. The reference resistance Rs is not used for printing.

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 charging switch Sb. The charging switch Sb is always closed in the print mode, and in the resistance measurement mode, the heating elements R1 to R1.
Opening and closing are controlled by the print controller 43 each time the resistance values r _{1 to} rn of the RN are measured.

The non-inverting input terminal of the comparator 52 is connected to one terminal of the capacitor 50, and the reference voltage Vref of the comparator 52 is obtained by the resistance voltage division by the resistors Ra and Rb. Capacitor 5 in resistance measurement mode
For example, the heating element R to be measured after 0 is charged
Only the first transistor Tr1 is turned on, and then the charging switch Sb is opened. In the state where the capacitor 50 is sufficiently charged, the potential of the non-inverting input terminal of the comparator 52 is E, but as the electric charge accumulated in the capacitor 50 is discharged by the heating element R1, the non-inverting input of the comparator 52 is input. The potential V of the terminal drops and eventually coincides with the reference voltage Vref. Immediately after this, the potential of the output terminal of the comparator 52 changes from positive to negative. The resistance measuring unit 43a of the print controller 43 measures the time T1 from immediately after the charging switch Sb is opened until the output voltage from the comparator 52 changes from positive to negative, by the timer 43b, and based on this measurement result, the resistance The value r ₁ is calculated and written in the RAM 43c.

From the discharge time T1, the resistance value r of the heating element R1
_The following formula 1 is used to calculate 1.

[0029]

## EQU1 ## r ₁ = T1 / Ts · rs

In this equation 1, Ts depends on the reference resistance Rs.
The discharge time, rs is the resistance value of the reference resistance Rs. Resistance values
Since rs has an error of about 1%, the resistance of the heating element R1
Value r ₁Can be calculated accurately. Discharge time T
s is recorded in the RAM 43d, and is recorded by the resistance measuring unit 43a.
When used.

Similar to the measurement of the discharge time T1 by the heat generating element R1, the discharge time T by the heat generating elements R2 to RN is measured.
2 to TN are sequentially measured, and the resistance value r ₂ to
rn is calculated and written in the RAM 43c. Then, after the discharging time TN by the final heating element RN is measured, the capacitor 50 is charged again. After the capacitor 50 is charged, the print controller 43 sets R
Output the EF signal to "H" and turn on the transistor Trs.
After that, the charging switch Sb is opened, and the discharging by the reference resistance Rs is started (see FIG. 1).

The resistance measuring section 4 of the print controller 43
3a measures the discharge time by the reference resistance Rs,
Even after the discharging time Ts has elapsed, the state where the charging switch Sb is opened is continued, and after the discharging time Ts ′ has elapsed, the charging switch Sb
Close. The discharge time Ts ′ is determined by actually measuring the time for discharging the electric charge of the capacitor 50 with the timer 43b, and is, for example, about 5 to 10 times the discharge time Ts. Also, charge switch S until the start of printing
b may be left open. The resistance values r _{1 to} rn written in the RAM 43c and the discharge time Ts written in the RAM 43d are held by the batteries 54 and 55. Then, in the print mode, the image data is corrected and printed by the resistance values r _{1 to} rn written in the RAM 43c.

Next, the operation of the above embodiment will be described with reference to FIGS. At the initial setup of the thermal printer, the keyboard 56 is operated to set the resistance measurement mode. In this measurement mode, the print controller 43 switches the mode selector switch Sa,
The shift register 44 is connected to the print controller 43. The charging switch Sb is turned on by the resistance measuring unit 43a, and the charging of the capacitor 50 is started. Then, after the charge voltage of the capacitor 50 reaches E, the print controller 43 outputs to the shift register 44 data for one line in which the transistors Tr1 to TrN are in the OFF state, and sets the REF signal to “H”. Turn on the transistor Trs.

Then, when the charge switch Sb is turned off, the electric charge of the capacitor 50 starts to be discharged through the reference resistor Rs, and the voltage applied to the non-inverting input terminal of the comparator 52 is gradually reduced. At the same time when the charging switch Sb is turned off, the resistance measuring unit 43a starts measuring the discharge time by the reference resistance Rs by the timer 43b.
When the potential of the non-inverting input terminal of the comparator 52 matches the reference voltage Vref, the resistance measuring unit 43a writes the discharge time Ts required for this in the RAM 43d and sets the REF signal to "L" to turn on the transistor Trs.
Set to FF.

Next, the charging switch Sb is turned on again,
After the charge potential of the capacitor 50 reaches E,
The transistor Tr1 is turned on and the other transistors Tr2 to TrN and
And the transistor Trs is turned off. Resistance measuring unit 4
3a measures the discharge time T1 by the heating element R1.
Then, the discharge time Ts by the reference resistance Rs is
The resistance of the heating element R1 read from d
Value r₁Is calculated and this is written in the RAM 43c.
It Similarly, the resistance value r of the heating elements R2 to RN ₂~ R
n is calculated and written in the RAM 43c.

After the resistance value rn of the final heating element RN is calculated and written in the RAM 43c, the print controller 43 further charges the capacitor 50, and then turns off the transistors Tr1 to TrN and turns on the transistor Trs. To do. Then, the resistance measuring unit 43a starts measuring the discharge time by the reference resistor Rs, but continues to keep the transistor Trs in the ON state even after the potential of the non-inverting input terminal of the comparator 52 matches the reference voltage Vref. After the discharge time Ts ′ has passed and the residual charge of the capacitor 50 is almost completely discharged by the reference resistor Rs,
The transistor Trs is turned off.

When the print mode is selected by operating the keyboard 56, the shift register 44 is connected to the comparator 42 by the mode changeover switch Sa. In this print mode, the image data of three colors is first captured in 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 printed accurately.

At the time of sheet feeding, the platen drum 10 is stopped with the clamper 15 being vertical in FIG. 2, and the clamper 15 is set to the unclamped position by the cam mechanism 16. The pair of transport rollers 28 are used for the color thermosensitive recording material 13 supplied from a cassette (not shown).
Is nipped and conveyed toward the platen drum 10. The conveying roller pair 28 is temporarily stopped when the front end of the color thermosensitive recording material 13 enters between the platen drum 10 and the clamper 15. Thereafter, the cam mechanism 16 shifts the clamper 15 to the clamp position, and the front end of the color thermosensitive recording material 13 is clamped. After the clamping, the platen drum 10 and the conveying roller pair 28 rotate, so that the color thermosensitive recording material 13 is wound around the platen drum 10.

Thermal recording is started when the platen drum 10 rotates intermittently by a constant step and the leading end of the recording area of the color thermosensitive recording material 13 reaches the thermal head 20. At the time of this thermal recording, the image data of the yellow image is read from the frame memory 40 for one line and once written in 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 the pixels that record the yellow image, and "0" for the pixels that do not record the 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 then the AN
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
Since 6 outputs the logical product of the strobe signal and the output signal of the latch array 45, among the output terminals of the AND gate array 46, the output terminal of the latch array 45 is "1".
Is output as "1". For example, AND
When the first output terminal of the gate array 46 is "1", the transistor Tr1 is turned on, so that the heating element R
1 is energized to generate heat. At this time, since no electric charge remains in the capacitor 50, an excessive current does not flow to the heating element R1 and the heating element R1 is not damaged.
Then, the heating element R1 is energized for a time corresponding to the bias heating pulse, and bias heat energy is applied to the color thermosensitive recording material 13.

Before the bias heating is completed, the print controller 43 generates grayscale data having a grayscale 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 in the shift register 44. When the bias heating is completed, the print controller 43 generates a gradation expressing 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. By this strobe signal, the heating element is energized for a short time, and the yellow thermosensitive coloring layer 35 is colored to the density of the gradation level "1". Less than,
Since the print controller 43 sequentially changes the gradation level from “1” to “3F”, the drive data corresponding to each gradation level is output from the comparator 42. As a result, each of the heating elements R1 to RN is energized the number of times corresponding to the corrected image data, and the gradation expressing heat energy is applied to the color thermosensitive recording material 13 to develop the color to the initial density.
For example, in the case of 64 gradations, 64 pulse currents are supplied to the heating element for gradation expression for the pixel having the maximum density.

When the first line of the yellow image is recorded, the platen roller 10 rotates stepwise by one pixel, and at the same time, 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 13 based on the image data of the second line of the yellow image. When the portion on which the yellow image is thermally recorded reaches the optical fixing device 21, the yellow thermosensitive coloring layer 35 is optically fixed here. In this optical fixing device 21, the cut filter 24 is set in front of the ultraviolet lamp 23, so that the near-ultraviolet light in the vicinity of 420 nm is applied to the color thermosensitive recording material 13. As a result, the diazonium salt compound contained in the yellow thermosensitive coloring layer 35 decomposes and the coloring ability disappears.

When the platen drum 10 makes one rotation and the recording area again reaches the position of the thermal head 20, a magenta image is recorded line by line on the magenta thermosensitive coloring layer 34. Although the coloring heat energy of the magenta image is larger than that of the yellow image, since the yellow heat-sensitive coloring layer 35 has already been fixed by light, the yellow heat-sensitive coloring layer 35 does not develop color again. The color thermosensitive recording material 13 on which the magenta image is 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 emitted from the ultraviolet lamp 23 are applied to the color thermosensitive recording material 13. Of the electromagnetic waves, the magenta thermosensitive coloring layer 34 is photo-fixed by the ultraviolet rays around 365 nm.

When the platen drum 10 makes one further rotation and the recording area again reaches the position of the thermal head 20, a cyan image is recorded line by line on the cyan thermosensitive coloring layer 33. The cyan thermosensitive coloring layer 33 has a value such that the coloring heat energy does not develop in a normal storage state, and therefore the cyan thermosensitive coloring layer 33 is not provided with optical fixing property. Therefore, in the thermal recording of the cyan thermosensitive coloring layer 33, the optical fixing device 21 is in the OFF state.

After the thermal recording of the yellow image, the magenta image, and the cyan image is completed, the platen drum 10 and the conveying roller pair 28 are reversed. By the reverse rotation of the platen drum 10, the rear end of the color thermosensitive recording material 13 is separated by the separation claw 29.
The sheet is guided to the sheet feeding / discharging path 27 and nipped by the pair of conveying rollers 28. After that, when the platen drum 10 reaches the sheet feeding position, the cam mechanism 16 causes the clamper 1 to move.
5 is shifted to the unclamped position, and the platen drum 10 is stopped. As a result, the thermally recorded color thermosensitive recording material 13 is discharged to the tray via the paper supply / discharge path 27.

In the embodiment described above, the reference resistor is used to discharge the residual charge of the capacitor, but as shown by the dotted line, another resistor Rd and switch Trd dedicated to discharging are used.
May be provided. Further, although the color thermal printer has been taken as an example, the present invention can be applied to a monochrome thermal printer, a color thermal transfer printer and the like. Further, although the line printer in which the color thermal recording material and the thermal head are moved relative to each other in one dimension has been described, the present invention can be applied to a serial printer in which relative movement is two-dimensional. In the description of the embodiments, the capacitor is first charged, and then the heating element or the reference resistor is turned on to be discharged. Conversely, after the heating element or the reference resistor is turned on in advance, the capacitor is charged. Alternatively, the resistance value may be measured from the start of charging.

[0048]

As described above, according to the present invention, after the measurement of the discharge time of the capacitor by the reference resistor and each heating element is completed, the residual charge of the capacitor is discharged by the reference resistor or the discharging resistor. Therefore, it is possible to prevent accidents where the capacitor is touched carelessly and an electric shock is caused, or the heating element is damaged at the start of printing.

[Brief description of drawings]

FIG. 1 is a waveform diagram of a signal supplied to each unit of an electric circuit of a color thermal printer.

FIG. 2 is a schematic diagram showing 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 block diagram showing an electric circuit of the color thermal printer.

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

[Explanation of symbols]

 13 Color Thermal Recording Material 20 Thermal Head 21 Optical Fixing Device 43 Print Controller 43a Resistance Measuring Section 43b Timer 43c, 43d RAM 50 Capacitor 51 Power Supply 52 Comparator R1 to RN Heating Element Rs Reference Resistance Sa, Sb Switches T1 to TN, Ts, Ts 'Discharge time Tr1 to TrN, Trs Transistor

Claims (4)

[Claims] 1. A reference resistor having a known resistance value and a capacitor are connected in parallel to a plurality of heating elements arranged in a line, and after the capacitor is charged, the capacitor is discharged until a predetermined potential is reached. The time is measured for each reference resistance and each heating element, and the resistance value of each heating element is calculated from the discharge time of the reference resistance and the discharging time of each heating element. A method for measuring a resistance value of a thermal head, characterized in that a residual charge of a capacitor is discharged by a resistance. 2. A reference resistor, a capacitor, and a discharging resistor, each having a known resistance value, are connected in parallel to a plurality of heating elements arranged in a line, the capacitor is charged, and then the capacitor is discharged to a predetermined potential. The discharge time until reaching is measured for each reference resistance and each heating element, the resistance value of each heating element is calculated from the discharge time of the reference resistance and each heating element, and the discharge time measurement is completed. Then, the residual value of the capacitor is discharged by the discharging resistor, and the resistance value measuring method of the thermal head is characterized. 3. A plurality of heat generating elements connected in parallel in a line, a plurality of heat generating control switches connected in series with each heat generating element, and a power source for generating heat from the heat generating elements whose heat generating control switches are turned on. A reference resistor having a known resistance value connected in parallel with each heating element, a capacitor for heating one heating element selected when measuring the resistance value of each heating element, and this capacitor are charged. Charge switch for turning off the charge switch after charging the capacitor
Discharge time measurement means for starting the discharge through the heating element to be measured, and measuring the time required for the capacitor to drop to a predetermined potential from the start of discharge, the discharging time of each heating element and the discharging time of the reference resistor, Therefore, a resistance value calculating means for calculating the resistance value of each heating element, and a discharging means for discharging the residual charge of the capacitor by the reference resistance after the discharge time of each heating element and the reference resistance have been measured are provided. Resistance value measuring device for thermal head. 4. A thermal printer including a thermal head having a plurality of heating elements arranged in a line, wherein a plurality of heating control switches connected in series with each of the plurality of heating elements and a heating control switch are turned on. Power source for heating the heating elements, a reference resistor connected in parallel with each heating element and having a known resistance value, and one heating element selected when measuring the resistance value of each heating element. Capacitor for charging, charging switch for charging this capacitor, and charging switch OF after charging the capacitor OF
Start discharge through the heating element to be set to F,
A discharge time measuring means for measuring the time required for the capacitor to drop to a predetermined potential from the start of discharge, and a resistance value calculation for calculating the resistance value of each heat generating element from the discharge time of each heat generating element and the discharge time of the reference resistance. Means, a correction means for correcting the image data based on the obtained resistance value of each heating element, and a discharge for discharging the residual charge of the capacitor by the reference resistance after the discharge time of each heating element and the reference resistance is measured. A thermal printer comprising means.