GB1284939A - Electronic thermal devices for producing thermal patterns representing characters - Google Patents

Abstract

1284939 Thermal type printing; selective printing TEXAS INSTRUMENTS Inc 10 Oct 1969 [31 Dec 1968] 49923/69 Heading H2H [Also in Division B6] In a printing apparatus for printing on thermally sensitive paper wherein a plurality of print heads 10a, 10b, 10c (Fig. 1) each head comprising four character matrices 12a-12d and each matrix a plurality of printing elements E1-E35 (Fig. 2 not shown) which are selectively heated to form a desired character, the energizing voltage and hence power fed to each element is controlled during a printing cycle to control its temperature in dependence upon the temperature of an element sensed by a respective controlling transistor T18 (Fig. 4) during a preceding sampling cycle. Each element, for instance element E18 of matrix 12a, is heated by current through a respective resistor e.g. R18 (Fig. 4) when a respective controlling transistor T18 is turned ON by a signal from a character generator 24 on the appropriate line C18. Each resistor and transistor is diffusion formed on the inner surface of a respective element and the elements are mounted on a common ceramic substrate below which is a common heat sink Fig. 2 (not shown). All the emitters of transistors T1-T35 of a matrix are connected to a supply line 20a-20d exclusive to that matrix. Each matrix may then be separately enabled by a character select switch 22. Temperature control.-The power applied to those elements of the enabled matrix selected by the character generator is controlled in accordance with the temp. of the centre element E18 thereof, immediately preceding the print cycle, so that a 'print' pulse of predetermined length will result in a uniform temp. regardless of the temp. of the matrix before the print cycle. The sampling procedure is effected for each character immediately preceding the print cycle thereof. During the short sampling cycle a constant current from a source 26 is applied to the control line C18 and through the base-emitter diode of T18 of the enabled matrix e.g. 12a. The voltage on the control line C18 will then be related to the temp. of base-emitter diode and thus to the temp. of the enabled matrix. When a 'sample' pulse is applied to an input 34 the voltage on line C18 is amplified at 28 and stored by a capacitor in a unit 30. During a print cycle pulse applied to input 36 the stored voltage, which is significant of sampled temperature, is applied to the non- inverting input of an amplifier 32 and a switch 38 turns ON a series regulator 40 to apply a voltage to line C18. This voltage magnitude has a predetermined relationship to the voltage stored, by virtue of a feedback loop 41 to the inverting input of amplifier 32. The character generator 24 is also activated during the print cycle to produce a positive voltage on the appropriate control lines C1 to C35 to cause a desired character to be generated by the enabled matrix 12a. Fig. 3 (not shown), illustrates in more detail the circuit of Fig. 4.