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Temperature Compensation in Thermal Print Element

IP.com Disclosure Number: IPCOM000084067D
Original Publication Date: 1975-Sep-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 3 page(s) / 46K

Publishing Venue

IBM

Related People

Matsubara, S: AUTHOR [+2]

Abstract

Temperature variation of heating elements for a thermal printing head is automatically compensated for by controlling the heating current, depending on the time length of the pause duration before printing takes place. The heating element is energized by a higher current when the duration of the pause is longer, because a longer pause causes the heating element to be cooled more than a shorter pause.

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Temperature Compensation in Thermal Print Element

Temperature variation of heating elements for a thermal printing head is automatically compensated for by controlling the heating current, depending on the time length of the pause duration before printing takes place. The heating element is energized by a higher current when the duration of the pause is longer, because a longer pause causes the heating element to be cooled more than a shorter pause.

Fig. 1 shows the temperature rise of the heating element when energized with constant-current pulses A. As shown in B, the temperature rises exponentially, resulting in exponentially changing the density C for printing.

The arrangement shown in Fig. 2 compensates for changing density. Thermal print head TH comprises a plurality of heating elements r1, r2, ... rn, which are connected to respective transistor switches Tr1, Tr2, ... Trn. Each switch is activated by a print pulse applied to the base electrode, in accordance with the matrix heating elements r1, r2, ... rn.

Junctions between heating elements and switches are connected through respective diodes D1, D2, ... Dn, to a time constant circuit which comprises resistor R1 and capacitor C1. Connected to capacitor C1 is a discharging circuit comprising resistor R2 and transistor switch TrO, which is switched on by a blank signal applied to the base electrode when printing is in a pause.

In Fig. 3A, longer blank signal B1 causes TrO to be conductive for a longer time so that C1 is discharged by a larger amount. When B1 terminates, TrO is turned off to allow C1 to be charged through D1, D2, ... Dn and r1, r2, ... rn. Charging takes place during the interval between successive print pulses in an exponential manner, as shown in Fig. 3C. These charging currents flowing through r1, r2, ... rn provide an additional rise to r1, r2, ... rn to those provided by print pulses, so that the heating elements are energized with extra power at the beginning and then such extra power decreases exponentially.

When the pause is shorter as shown in B2, Fig. 3A, TrO discharges C1 by a smaller...