THERMAL PRINT HEAD HEATING CIRCUIT FAULT DETECTION DEVICE

Abstract: A thermal print head heating circuit fault detection device is provided with: a constant voltage circuit (42) consisting of a number of diodes (D₁,D₂,D₃) connected in series between a print head (41) provided with heating circuits (S₁,...S n ) and the power supply circuit (43) of the print head; a photocoupler (47) arranged so that the emitter portion of the photocoupler is driven by the difference in the potentials between the two ends (A,B) of the diodes; and a control circuit which applies electricity in sequence to the heating circuits of the print head and checks the output of the photocoupler collector to detect heating circuit faults.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a thermal print head heating circuit fault detection device, and particularly to a device circuit configuration which is simple and which can perform detection stably at high speed. Description of the Prior Art The print heads used in thermal printers are provided with a plurality of heating circuits, each of which is provided with heating elements, gate circuits, transistors, and so forth. The heating circuits will not function if a heating element, gate circuit or transistor fails, which will therefore cause part of a dot raster to be output with the printing in a non-functional state. The most frequent cause of such failure is circuit line breakage in the heating elements. As techniques for solving such problems there have been proposed, as in Japanese Laid-open Pat. Appln. No. 58 (1983)-28391, providing a current sequentially to the heating circuits which is of such a level that printing does not take place, and whether there is a faulty heating element is determined by detecting whether the current has passed through each of the heating circuits. With this technique, resistors having a low resistance value are connected in series on the common terminal side of the heating circuits, and when current is sequentially supplied to the heating circuits, it is detected whether the current has passed through the said resistors. However, in order to detect the very small currents that are used, it is necessary to provide an amplification circuit with a high amplification factor, and as a result the circuitry becomes complicated in addition to which the working speed of the amplification circuit is slow, so that a problem has been the time required for the checking process. Further, because there is a considerable manufacturing variation in the resistance values of the heating elements, it has been necessary to carry out fine adjustment of the amplification factor for each thermal print head, and it has not been possible to deal with variations in the resistance values of the heating elements of an entire thermal print head board. SUMMARY OF THE INVENTION The object of the present invention is to provide a thermal print head heating circuit fault detection device which has a simple circuit configuration, does not need adjustment even when there exists manufacturing error in the resistance values of the heating elements, and which can perform high-speed detection of circuit faults with high reliability. To achieve this object the present invention comprises providing between the print head possessing the heating circuits and the power supply circuit of the print head a required number of diodes connected in series to form a constant voltage circuit, a photocoupler arranged so that the emitter of the photocoupler is driven by the difference in the potentials arising between the two ends of the diodes, and a control circuit which applies a sequential electrical condition to the heating circuits of the print head and checks the output of the photocoupler collector to thereby perform detection of heating circuit faults. The invention will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of the circuit of a printer employing the thermal print head heating circuit fault detection device according to the present invention; Fig. 2 is a circuit diagram showing the principal parts of the present invention; and Fig. 3 is a flowchart showing the operation of the thermal print head heating circuit fault detection device according to this invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS In Fig. 1, connected to a CPU 30 are a program ROM 31, a data RAM 32 and an I/O port 33. The CPU 30 performs overall control of printer functions, such as printing and heating circuit fault detection, in accordance with a program stored in the program ROM 31. Stored in the data RAM 32 are printing data, the location of faulty heating circuits determined on the basis of the results of heating circuit fault detection, and the like. Also connected to the I/O port 33 are a print data input circuit 34 for the input of the data to be printed, a drive circuit 35 for driving a pulse motor 36 used to transport the printing paper, a drive circuit 37 for driving a display 38, such as a CRT, a drive circuit 39 for driving a buzzer 40, and a thermal print head 41. A print head power supply circuit 43 is arranged so that on the one hand it supplies power directly to the print head 41 and on the other hand it supplies power to the print head through a fault detection circuit 42. This fault detection circuit 42 and power supply circuit 43 are each connected to the I/O port 33 so as to be suitably controlled by the CPU 30 in accordance with the program. Fig. 2 shows details of the print head 41, the fault detection circuit 42 and the power supply circuit 43. The print head 41 is provided with a data register 44 comprised of shift registers, a latch circuit 45 and n heating circuits S₁, S₂, S₃, ..., S n . Each of the heating circuits S₁, S₂, S₃, ..., S n is comprised of AND gates G₁, G₂, G₃, ..., G n , transistors Tr₁, Tr₂, Tr₃, ..., Tr n , and heating elements R₁, R₂, R₃, ..., R n . The data register 44 is for storing one dot-line of printing data, the data input DI being input one bit at a time via I/O port 33 by means of a clock signal CLK and output to the latch circuit 45. The latch circuit 45 is arranged so that when a latch signal LAT is input via the I/O port 33 the data stored in the data register 44 is read out. The output terminals of the latch circuit 45 are connected to one of the input terminals of the AND gates G₁, G₂, G₃, ..., G n . The other input terminal of each of the AND gates G₁, G₂, G₃, ..., G n is connected to the input terminal of the I/O port 33 for the input of a strobe signal STR. The output terminal of each of the AND gates G₁, G₂, G₃, ..., G n is connected to the base of the corresponding transistor Tr₁, Tr₂, Tr₃, ..., Tr n . The emitter of each of the transistors Tr₁, Tr₂, Tr₃, ..., Tr n is grounded, and the collector is connected to one side of the corresponding heating element R₁, R₂, R₃, ..., R n . The other side of each of the heating elements R₁, R₂, R₃, ..., R n is connected to a common terminal 46 of the print head 41. The power supply circuit 43 is provided with two power supplies each having the same voltage. Power supply P is for printing operations and is supplied directly to the common terminal 46 of the print head 41 via a diode D4, while power supply C is for fault detection purposes and is connected to the common terminal 46 of the print head 41 via the fault detection circuit 42. The fault detection circuit 42 consisting of diodes D₁, D₂ and D₃, fixed resistances R a and R b , a variable resistance VR, and the photocoupler 47 will now be described. The diodes D₁, D₂ and D₃ are silicon diodes connected in series in that order between the power supply circuit 43 and the print head 41. Connected to the anode side of the diode D₁ is the variable resistance VR, and connected to the variable resistance VR is the fixed resistance R a . Connected between the fixed resistance R a and the cathode side of the diode D₃ is a photodiode PD that forms the emitter portion of the photocoupler 47. That is, a circuit consisting of the variable resistance VR, the fixed resistance R a and the photodiode PD of the photocoupler 47 is connected in parallel with the diodes D₁, D₂ and D₃ which are connected in series. The voltage reduction produced by the said three diodes D₁, D₂ and D₃ connected in series is constant. Therefore, as the diodes D₁, D₂ and D₃ form a constant voltage circuit, the voltage between point A and point B remains constant. When fault detection power supply C is now supplied, a prescribed voltage is produced between point A and point B, there is a flow of current in the variable resistance VR, the fixed resistance R a and the photodiode PD, and the photodiode PD lights. The number of diodes is not limited to three, but may be any number that is sufficient to produce enough of a voltage drop to cause the photodiode PD to light. The variable resistance VR is provided for adjusting the value of the current flow through the photodiode PD. The emitter of the phototransistor PTr which forms the collector of the photocoupler 47 is grounded and the collector is connected to the power source via resistor R b . The collector of the phototransistor PTr is also connected to the I/O port 33, and the CPU 30 performs the detection of faults in the heating circuits S₁, S₂, S₃, ..., S n by detecting the potential of the said collector. That is, when the fault detection power supply C comes from the power supply circuit 43, it goes to the print head 41 via the fault detection circuit 42, but if there is any fault in the heating circuits S₁, S₂, S₃, ..., S n that are the object of the detection process, there will be no difference in potential produced between point A and point B, and hence no emission by the photodiode PD, and accordingly, the collector side of the phototransistor PTr goes High. Again, if there is no fault a potential difference will be produced between point A and point B, the photodiode PD lights, and the collector side of the phototransistor PTr goes Low. If the CPU 30 which is monitoring the phototransistor PTr collector potential detects that the potential has gone High, the CPU 30 determines that a circuit fault has occurred, while if the potential is Low the circuit is determined to be normal. The heating circuit fault detection operation in the thermal printer shown in Fig. 1 and Fig. 2 will now be described with reference to Fig. 3. In step 1, the number of heating circuits S₁, S₂, S₃, ..., S n , that is, the total number of dots N of the print head 41, is placed into a specific address of the data RAM 32. Following this, in step 2, by turning off printing power supply P and turning on fault detection power supply C, power is supplied to the common terminal 46 of the print head 41 via the fault detection circuit 42. In step 3, by inputting a single clock signal CLK with the input data DI in the High state, a binary "1" signal is set into the first stage of the data register 44. Next, in step 4, a latch signal LAT is input to latch the contents of the data register 44 with the latch circuit 45, and a strobe signal STR is input to obtain a current flow only in heating circuit S₁. In this state, the collector-side potential of the phototransistor PTr is checked, and if it is Low, it is determined that the heating circuit S₁ is normal, while if it is High it is determined that it is faulty. That is, when heating circuit S₁ is in a faulty condition owing to a circuit line break or the like in the heating element R₁, current does not flow in the heating circuit S₁ or the fault detection circuit 42, so no difference in potential between point A and point B is produced. Accordingly, there is no emission by the photodiode PD and the potential on the collector side of the phototransistor PTr goes High. When the heating circuit S₁ is working normally, current flows through the fault detection circuit 42 to the heating circuits, producing a potential difference between point A and point B, so there is emission by the photodiode PD and the collector-side potential of the phototransistor PTr goes Low. During the heating circuit fault detection operation the pulse of the strobe signal STR is set to a narrow enough width that the heating elements R₁, R₂, R₃, ..., R n do not print. If it is determined that there is no fault in the heating circuit S₁, the process moves on to step 8. If it is determined that there is a fault, in step 7 a value N corresponding to the number of the faulty heating circuit is stored in a specific address of the data RAM 32, after which the N value is decremented by just one in step 8. Next, in step 9, by inputting a single clock signal CLK with the input data DI of the data register 44 in the Low state, the binary "1" signal is shifted from the first to the second position of the data register 44. In step 10 it is determined whether N equals zero or not. If N does not equal zero it is determined that checking of all of the heating circuits S₁, S₂, S₃, ..., S n has not yet been completed, and the procedures of step 4 through step 10 are repeated. Thus, by applying an electrical condition in sequence to each of the heating elements S₁, S₂, S₃, ..., S n and also checking the collector-side potential of the phototransistor PTr, when the checking for circuit faults has thus been completed for all of the heating circuits S₁, S₂, S₃, ..., S n , N becomes zero and the process advances to step 11. In step 11 it is determined whether there is a faulty heating circuit. If it is determined that there is no faulty heating circuit, the print head 41 heating circuit fault detection operation is terminated and printing or other such operations are proceeded with. When it is determined that there is a faulty heating circuit, reference is made to the faulty heating circuit number stored in the data RAM 32 to determine the extent of the fault in terms of printing capability, i.e. whether printing is possible. Then, in step 13, the display 38 is used to indicate whether printing is possible and also to show the number of the faulty heating circuit or circuits, and a buzzer sounds to signal the completion of the heating circuit fault detection operation. As the printing operation does not form part of the gist of the present invention, details thereof will be omitted, other than to say that after printing power supply P is turned on and fault detection power supply C is turned off, printing proceeds as the print data is input into the data register 44. In the printing operation, the pulse of the strobe signal STR is set to a wide enough width to permit printing by the heating elements R₁, R₂, R₃, ..., R n . Although in the above embodiment the fault detection power supply C and the printing power supply P are provided as separate circuits, the fault detection power supply C may be utilized for printing in addition to fault detection. Even if such a dual-purpose power supply is employed, the voltage drop in the fault detection circuit 42 during printing remains roughly constant regardless of the number of heating elements R₁, R₂, R₃, ..., R n , so there is no destruction or the like of the photocoupler 47 and printing is not affected. Also, while in the example that has been shown, for the heating circuit fault detection operation the pulse of the strobe signal STR is set to a narrow enough width that printing does not take place, the pulses of the strobe signal STR may also be set to a width that is the same for both printing and fault detection, with the fault detection power supply C being set to such a value that printing will not take place. Thus, as described in the foregoing, the present invention comprises providing between the print head possessing the heating circuits and the power supply circuit of the print head a required number of diodes connected in series to form a constant voltage circuit and a photocoupler arranged so that the emitter of the photocoupler is driven by the difference in the potentials arising between the two ends of the diodes, and a control circuit which applies a sequential electrical condition to the heating circuits of the print head and checks the output of the photocoupler collector to thereby perform detection of heating circuit faults. Therefore, with the present invention there is no need to provide an amplification circuit having a high amplification factor and fine adjustment is also unnecessary, so the circuitry and operation are extremely simple and it is therefore possible to manufacture it at very low cost, and as in addition it is also possible for the detection operation to be performed at high speed, it has high commercial utility.

Claims: 1. A thermal print head heating circuit fault detection device comprising a power supply circuit for supplying electrical power to a thermal print head having numerous heating circuits, a requisite number of diodes connected in series between the power supply circuit and the print head, a photocoupler the emitter of which is driven by the difference between the potentials produced at the two ends of the diodes, and a control circuit which applies a sequential electrical condition to the heating circuits of the print head and checks the output of the collector of the photocoupler to thereby perform detection of heating circuit faults.

Inventors: ITO, TOYOZO
Applicants: KABUSHIKI KAISHA SATO
Application year: 1986
Main classification: B41J2
IPC codes: B41J2, B41J29