JPS59156766A - Driving apparatus of thermal head - Google Patents

Abstract

PURPOSE:To prevent positively abnormal temperature rise of a thermal element comprising a thermal head by installing a means to watch heating time of the thermal element and stopping compulsorily head driving when the heating time exceeds the predetermined value. CONSTITUTION:Normally, by each element signal driver transistors Tr1-Trn of each driving member 10 are driven, and thermal elements R1-Rn of a thermal head 12, are heated individually. A comparater CMP compares an electric voltage value basing upon an electric current detecting resistance Rs and a reference electrical potential Vref. generating a comparative output Sgi. A counter circuit 20, while the output Sgi counts a clock signal SCLK on an H-level, campares the result with a set value of setting unit 22b by a comparator 22a. If no abnormal temperature rise occurs in elements R1-Rn, as the output Sgi drops down to an L-level, the SSTP is not generated and supply of electric power is continued. Consequently, at the time of abnormal temperature rise, the H-level continues with generation of the output SSTP stopping supply of power from the power source.

Description

TECHNICAL FIELD The present invention relates to a thermal head driving device, and more particularly to a thermal head driving device that drives a plurality of thermal elements according to printing information to perform thermal printing.

BACKGROUND ART Generally, when driving a low-resistance load such as a heating element (thermal element) constituting a thermal head, a method is used in which the load is heated for a certain period of time. In this type of heating method, in order to suppress variations in density, in the case of continuous heating, it is recommended to gradually shorten the heating time, or attach a temperature sensor to the thermal head and adjust the heating time according to temperature changes. It is being done.

In this case, an accident may occur where the drive system (-) malfunctions for some reason and the drive circuit continues to heat up the elements of the head. , resulting in a dangerous situation where other parts of the unit may be damaged.

For this reason, monitoring using a temperature sensor has been proposed, but in this case, the temperature sensor itself needs to be incorporated into the thermal head, and the additional circuitry becomes complicated.

Purpose The present invention has been made in view of the above points, and is to provide a thermal head which is capable of preventing abnormal heating accidents of thermal elements constituting a thermal head with a simple circuit configuration and at a low cost. The object of the present invention is to provide a head driving device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a block diagram showing the thermal head drive circuit of this embodiment. In the figure, 17N is a drive unit that drives elements of a thermal head (1-1), which will be described later, in response to an element drive signal (hereinafter abbreviated as "Nirementi") inputted from 10 frames. In 10 to this horse memo sound, ■1
~In is an input terminal for an element signal given as a time signal, A1~An& is an overnight type buffer amplifier, and Tr+~Trn are driver transistors. A thermal head 12 includes a plurality of thermal elements R+ to Rn, one end of each element being connected to the collectors of the transistors Tr+ to TYn, respectively. 14 is a power supply section that supplies driving power to the thermal head 12. In this power supply section 14, T
rp is a transistor that functions as a power supply switch, and controls the power supply by using the power supply voltage Vop applied to the power supply input terminal lop as a control input and a common signal to the common input terminal IcM. Ap is an inverter type Balanor amplifier connected between the common input terminal IC and the transistor Trp.

16 is a comparator CMP composed of an operational amplifier.
The first comparison section consists of: R8 is the common connection point P of the thermal element and the transistor Tr of the power supply section 14.
Vref is a reference potential obtained by dividing the power supply voltage Vop by a resistor.

A detection resistor type is connected to a non-inverting input terminal of the comparator CMP, and a reference potential V ref is connected to an inverting input terminal.

Reference numeral 18 denotes a gate that passes a clock pulse input from an oscillation circuit (not shown) via an input terminal IOLK using the comparison output of the comparator CMP as a gate condition. The input from comparator CMP to gate 18 is Sgi
, the output of the gate 18 is Sgo.

20 is a counter circuit that performs a counting operation as an output or input of this gate 18, and a comparator C
The counter operation is configured to be cleared in synchronization with the fall of the MP output.

22 is a second comparison section, for example, a 4-bit digital IC.
The comparator 22a is composed of a comparator 22a, and a setter 22b, for example, a 4-bit digital IC, which provides a comparison input to one side of the comparator 22a.

The setting device 22b sets the maximum pulse count value as a set value. In this case, the maximum pulse count value is defined as the value obtained by adding 1 to the maximum value of the heat time counted using the same signal as the clock signal inputted via the input terminal IOLK. The comparator 22a includes a counter circuit 20 that is input to one comparison input terminal A1 to A4.
The count output of the counter circuit 20 is compared with the output of the setter 22b inputted to the other comparison input terminals B1 to B4, and if the condition A2B is satisfied, that is, the output of the counter circuit 20 is
2b or higher, a command is generated from the output terminal OUT to stop the power supply by the power supply circuit 14.

This operation will be explained using the timing chart of FIG. In Fig. 2, the signals Sgi, SOT, KI Sg
.

is the 1- signal shown in FIG. signal i. (i(+,・
...1cn) is a transistor T provided in the drive unit 10
These are the collector signals of r1 to Trn, that is, the supply current of the thermal element. tA is the time point at which heating of all thermal head elements starts. In addition, tll should normally return to a high level as shown by the dotted line in the figure when the heat is over, but an abnormality has occurred and the heat is tied. to
is the point in time when the first counter circuit 20 is cleared.

The energization drives the driver transistors Trr - Trn f of the drive section 10 using each element signal, and heats each of the thermal elements R1 to Rn of the thermal head 12. Comparator CMP compares the voltage value based on current detection resistor R8 and reference potential Vref, and generates a predetermined comparison output Sgi. The counter circuit 20 counts the clock signal 5CLK while the comparison output Sgi is at H level. The comparator 22a compares this count value with the set value of the setter 22b.

If there is no abnormal heat generation in the thermal elements RI to Rf, the comparison output Sgi described above falls to the L level after a predetermined time, so the comparison output S_STP of the comparator 22a is not generated and power supply continues as it is.

Next, the operation during abnormal heat generation will be explained. As described above, the counter circuit 20 performs a counting operation based on the output signal Sgoi of the gate 18. This count output can obtain states such as 1- as shown in C1, C2, C3, and C4 from its lower bits. These count outputs C1 to C4 and the output of the setter 22'b are connected to the comparator 2.
Comparison is made in 2a.

In this case, the value of the reference potential Vref to the comparator CMP is set such that the comparison output Sgo of the comparator CMP is outputted even if at least 1-9--mal elements (R1-Rn) of the thermal radar 12 are heated. It is set.

Therefore, it is assumed that an abnormality occurs at the time 1B and that the abnormality is detected by the current detection resistor R9 at the time tD.

'If it is normal, the comparison output Sgi of the comparator CMP should have completely fallen at t'I3, for example, but since an abnormality has occurred, the heating element continues to be heated, so it remains at the H level.

If this state continues, the counter contents of the counter 20 increase sequentially, and for example, when the maximum pulse count value set by the setter 22b is exceeded at time tp, the ratio of the comparator 22a (biting force 5STP is generated from the output end). do.

Therefore, the generation of this comparison output is detected, and the level of the common input terminal IOM is changed in the direction of turning off the transistor Trp, or the power supply voltage Vp is cut off using an appropriate circuit means (2).

As a result, it is possible to prevent an overheating or abnormal heating accident in which at least one of the thermal elements R+ to Rn of the thermal head 12 continues to be heated for a predetermined period of time or more.

Effects As described above, the present invention provides means for monitoring the heating time of the thermal element, and forcibly stops the head drive when the heating time exceeds a predetermined value.
Abnormal heat generation of the thermal element constituting the thermal head can be reliably prevented. Further, in order to realize this, it is only necessary to add two simple circuit means to a conventional circuit, and it can be implemented at an extremely low cost.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a circuit configuration of an embodiment of a thermal head driving device according to the present invention, and FIG. 2 is a timing chart showing the operation of each part in the circuit shown in FIG. 1. DESCRIPTION OF SYMBOLS 10... Drive part 12... Thermal head 14... Power supply part 16... First comparison part 18... Gate 20... Counter circuit 22... Second comparison part 22a...・Comparator 22b...
・Setting device R1 to Rn...Thermal element Rs...
・Current detection resistor Vref・Par reference potential CMP・・
·comparator

Claims (2)

[Claims] (1) In a thermal head drive device that performs thermal printing by heating a plurality of thermal elements constituting an f-mal head in accordance with a drive signal, a means for monitoring the heating time of the thermal head is provided, and this monitoring means allows the thermal head to 1. A driving device for a thermal head, characterized in that head driving is stopped when a heating time exceeds a predetermined time. (2) The monitoring means includes a first comparator that compares the current value supplied to each thermal element with a predetermined value, a counter circuit that counts a predetermined clock signal using the output of the first comparator as a gate condition, and A patent claim comprising a second comparator that compares the output of the counter circuit with a set value based on the maximum value of the heat time, and stops power supply to the thermal element based on the output of the second comparator. A driving device for a thermal head according to item 1.