JPH0416366A - Thermal head-protecting circuit - Google Patents

Abstract

PURPOSE:To prevent fluctuation of linear density of recording data at the time of overheat of a thermal head by a method wherein electric current passing through exothermic resistive elements is controlled in conformity with outputs from a temperature detecting member incorporated in the thermal head. CONSTITUTION:A temperature detecting member 1 is composed of a thermistor Rsns having negative temperature characteristic, a fixed resistor R0 having no temperature characteristic, and a voltage follower amplifier IC1. Output voltage of the temperature detecting member 1 lowers as the temperature of a thermal head rises. A control member 2 is composed of a data register IC2 consisting of N bits (recording pixels) and transistors TR1 and TR2 in N groups for controlling of electric current in an exothermic resistive body 3. When a digital output QN is at the low level, electric current to an exothermic resistive element RTHN is cut off. When the QN is at the high level, the electric current to the exothermic resistive element RTHN is in positive characteristic against the output voltage of the temperature detecting member 1, and the electric current in the exothermic resistive element RTHN is restricted as the temperature of the thermal head rises, and thereby the thermal head is protected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thermal head protection circuit for thermally printing an image signal of a facsimile machine or the like.

[Conventional technology]

Conventionally, when the thermal head becomes overheated, this type of thermal head protection circuit protects the recording image signal by repeating an even numbered white mask and an odd numbered white mask for each line. , the configuration was such that the printing pixels were thinned out to protect the thermal head.

[Problem to be solved by the invention]

The conventional thermal protection circuit described above protects the printed pixels by repeating the even-number bit white mask and the odd-number bit white mask for each line of the recording image signal when the thermal head becomes overheated. Since the thermal head is thinned out to protect the thermal head, there is a drawback that the linear density of recorded image data changes rapidly before and after overheating is detected.

[Means to solve the problem]

The thermal head protection circuit of the present invention includes a register that stores image data for one line, a thermal head that has a plurality of heating resistive elements arranged in parallel to print the image data from this register, and a thermal head that controls the temperature of the thermal head and throat. It has a temperature detection section that measures the temperature and outputs it as temperature data, and a control section that controls the current applied to the heating resistor element according to the temperature data.

〔Example〕

Next, the present invention will be explained with reference to the drawings. FIG. 1 is a circuit diagram of an embodiment of the present invention.

The temperature detection section 1 of the thermal head includes a thermistor (Rsns) having negative temperature characteristics, a fixed resistor (RO) having no temperature characteristics, and a voltage follower amplifier (ICI).
As the temperature of the thermal head increases, the output voltage of the temperature detection section 1 also decreases. The control unit 2 includes an N-bit (number of recording pixels) data register (IC2),
N sets of transistors T for controlling the current of the heating resistor 3
It is composed of R1 and TR2. Recording control of the M-th bit is performed by transistors TRI (N) and TR2 (N).
, the digital output Q of the data register (IC2) (
Current control of the heat generating resistor element RTH(N) is performed according to the output voltage of the temperature detection section 1 and the output voltage of the temperature detection section 1. When Q(N) of data register (IC2) is L, heating resistor element RTH(
The current of N) is cut off. When Q(N) is H, the current value of the heating resistor element RT,(N) has a positive current characteristic with respect to the output voltage of the temperature detection section 1, and as the temperature of the thermal rad increases, the current value of the heating resistor element RT,(N) The thermal head is protected by limiting the current of N).

〔Effect of the invention〕

As explained above, the present invention prevents the linear density of recorded image data from changing when the thermal head or throat overheats by controlling the current passing through the heating resistor element according to the output of the temperature detection section built into the thermal head. , and can protect the thermal head from overheating.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention. 1...Temperature detection unit, 2...Control unit, 3...Heating resistor, RO...Fixed resistor, R5n5...Thermistor, ICI...Voltage follower amplifier, IC2
...Data register, TRI (0-N)...Current control transistor 1, TR2 (0-N)...Current control transistor 2, RTN (0-N)...Heating resistance element.

Claims (1)

[Claims] A register that stores one line of image data, a thermal head that prints the image data from this register and has a plurality of heating resistive elements arranged in parallel, and a temperature detection section that measures the temperature of this thermal head and outputs it as temperature data. A protection circuit for a thermal head, comprising: a control section that controls a current applied to the heating resistor element according to the temperature data.