Abstract:
The present invention provides a method and apparatus that selectively connects a voltage source to a plurality of groups of resistive printing elements, arranged substantially in a single row, in a thermal print head. The power available at printing elements selected to print is greater than that available at printing elements not selected to print.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     This invention relates to a thermal print head for a thermal printer. More particularly, this invention relates to a diode-less thermal print head for use in a thermal printer. 
     B. Discussion of the Related Technology 
     Thermal print heads (TPHs) are used in a wide array of electronic printing applications. In general, to print, a thermally sensitive print medium, such as thermal film or paper, passes between a thermal print head and a printer platen on a thermal printer. The individual resistive printing elements, or nibs, arranged on the thermal print head are selectively controlled to conduct current, or not. Nibs selected to conduct current heat up rapidly, causing the thermal print medium in close proximity to the nib to darken. Thermal printing is accomplished in this manner. Generally, the higher the nib density, the higher the printing quality and definition. 
     The first TPHs utilized discrete wiring for the individual connections for each nib. As the number of nibs increase, this approach becomes unwieldy because of the number of wires involved. An electronic solution is to place a shift register inside the TPH. The data corresponding to the individual nibs is serially entered into the shift register, requiring only data and clock lines. Associated with each bit in the shift register is an electronic current sinking switch on the output tap of the shift register to connect a particular nib to ground, such that if the stored bit is a `one` the switch is on and the nib is connected to ground, and if the stored bit is a `zero` the switch is off and current does not flow through the nib. 
     Each shift register switch controls one terminal of an individual nib. The other nib terminal is accessed through a power bus common to all the nibs. After all the data is loaded into the shift register to select which nibs are to be turned on, the power bus common to all nibs is powered momentarily to provide power to the nibs connected to the bus, which causes the selected nibs, which have been grounded by the shift register switch, to heat up. 
     As nib density is increased further, a two phase nib control circuit is commonly used. In the two phase method, the nibs are divided into two groups, A and B. The nibs are typically placed in a row as follows: 
     ABABABABABABABABABAB . . . 
     FIG. 1 illustrates the two phase approach for a 16 nib TPH. The shift register/switch combination is indicated by the switches S1 through S8. The shift register is shared between the two nib group phases. In operation, data for Phase A nibs is loaded into the shift register and the Phase A common bus is powered by a voltage source, V A , to heat the selected Phase A nibs. Data for Phase B nibs is then loaded into the shift register and the Phase B common bus is powered by a voltage source, V B , to heat the selected Phase B nibs. Using this approach, diodes I1a-I8b are used to prevent cross-talk between the two nib phases. The diodes, however, are problematic since one diode is required for each nib and the diodes are typically discrete and are not incorporated into the shift register/switch integrated circuit. For a TPH having N nibs, the diodes add 2N connections, thereby increasing the cost of the TPH. 
     Thus, a need exists for a thermal print head having increased nib density to perform high contrast printing for graphic arts and other applications, without being limited in cost and design by use of discrete diodes. 
     SUMMARY OF THE INVENTION 
     The present invention is for a method of selectively heating individual resistive printing elements, or nibs, substantially arranged in a single row on a thermal print head (TPH) without the use of diodes. By being able to selectively control the flow of current to a particular nib, without cross-talk and without using diodes, the nib density can be increased and the cost of the TPH reduced. 
     Two voltage sources are selectively connected to a first and a second group, or phase, of nibs in a TPH. The nibs from the two groups are arranged to form the pattern ABABABABABABA . . . . The magnitude of the voltage from the first voltage source is about three times that of the second voltage source. Thus, the power available to nibs selectively connected to the first voltage source is about nine times that available to nibs selectively connected to the second voltage source. Because of this 9 to 1 power ratio, resistive heating of the nibs connected to the higher voltage source will occur, and will be minimal for nibs connected to the lower voltage source. 
     In this manner, selective nib heating occurs, causing printing on a thermal print medium, without the use of diodes to prevent cross-talk between the two groups, or phases, of nibs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a TPH, with diodes, of the prior art; 
     FIG. 2 is a schematic diagram of a TPH having no diodes; 
     FIG. 3 is a schematic diagram of a TPH of the present invention, having two power sources and two phases; and 
     FIG. 4 is a schematic diagram of a TPH of the present invention having three phases. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As discussed above and as illustrated in FIG. 1, the use of diodes in the prior art in a two phase TPH adds significantly to the cost. Diodes have been traditionally used to prevent cross-talk between the two nib phases, or groups, of thermal printing elements, or nibs N1-N16. FIG. 2 illustrates the problem. Assume current sinking switch S1, which is typically the output tap of a shift register (not shown) and is thereby programmable, is ON and all other current sinking switches, S2-S8, which are also programmable, are OFF. When Phase A nibs, N1, N3, N5, N7, N9, N11, N13 and N15, are powered with Va volts, the Phase A nib N1 associated with switch S1 in nib set I is powered with Va 2  /R watts of power. If Phase B is left disconnected, the Phase B nib N2 associated with switch S1 will be powered through the remaining seven nib sets II-VIII. Thus, the Phase B nib N2 associated with switch S1 will be powered by  Va/(R+2R/7)! 2  ×R Watts. 
     In general, for a TPH having N nibs, the OFF nib in the selected nib set will be powered by  Va/(R+2R/(N/2)-1))! 2  ×R Watts, or, as N gets large, by Va 2  /R watts, which is the same power as the ON nib. The nibs in the unselected nib sets will be powered by  Va/(R+2R/((N/2)-1))/((N/2)-1)! 2  (R) watts, which, as N gets large, becomes insignificant. If Phase B is grounded rather than remaining disconnected, the Phase B nib associated with S1 will receive no power. The remaining unselected nibs will be powered by  Va/(2R)! 2  ×R watts, or, Va 2  /(4R) watts. This method establishes a 4 to 1 power ratio between selected and unselected nibs, a ratio that might be insufficient for high contrast printing. 
     Referring to FIG. 2, if Phase B is maintained at some other voltage, e.g. Vx, while Phase A is powered by Va, the power distribution can be altered. The selected nibs in all nib sets I-VIII of Phase A continue to be powered by Va 2  /R watts. The Phase B nibs associated with selected Phase A nibs will be powered by Vx 2  /R watts. The remaining unselected nibs in both Phases will be powered by  (Va-Vx)/(2R)! 2  ×R, or, (Va-Vx) 2  /(4R) watts. Thus, Vx can be used to lower the power dissipation in the unselected nibs at the expense of increasing the power dissipation in the Phase B nibs associated with selected Phase A nibs. The preferred overall power ratio is obtained when the two spurious responses are equal, that is, when Vx 2  /R=(VaVx) 2  /(4R), or, Vx=Va/3. The use of Vx=Va/3 results in each unselected nib being powered by Va 2  /9R Watts. This 9 to 1 power ratio is sufficient for high contrast printing. This method is incorporated in the present invention. 
     FIG. 3 illustrates the preferred method of powering the diode-less TPH. Voltage V is the nominal TPH voltage, which is preferably the sum of Va+Vb, where Va=2Vb, or stated differently, Va=2V/3 and Vb=V/3. Power switches PS1-PS4 are preferably controlled as follows by a manner known in the art: 
     
         ______________________________________                                Phase A                                      Phase B______________________________________Condition    PS1     PS2     PS3   PS4   Voltage                                      Voltageno power off     off     off   off   0     0Power Phase A    on      off     on    off   V     V/3Power Phase B    off     on      off   on    V/3   V______________________________________ 
    
     An expression can be derived for the currents IA and IB as follows: Let m be the fraction of the switches S1-S8 that are in the ON state. For example, if 2 of the 8 switches S1-S8 are ON, then m=0.25. When Phase A nibs are powered, then IA=m (V/(2R/N))+(1-m)(2V/3)/(4R/N) and IB=IA+m ((V/3)/(2R/N))-(1-m) (2V/3)/(4R/N). 
     All power switches PS1-PS4 are preferably capable of conducting current in the ON state, from left to right. Power switches PS3 and PS4 are preferably capable of conducting current right to left, as well. All power switches PS1-PS4 are preferably capable of blocking voltage in the OFF state, from left to right. Preferably, power switches PS3 and PS4 are capable of blocking voltage right to left, as well. 
     The Va (2V/3) power supply source preferably provides a worst case current for m=1 of IAmax=V/(2R/N) amps and a worst case power of PAmax=(2V 2  /3)/(2R/N) watts. The Vb (V/3) power supply source preferably provides a worst case current for m=1 of IBmax=IAmax+(V/3)/(2R/N) amps and a worst case power of PBmax=(4V/9)/(2R/N). 
     The supply voltages Va and Vb may be taken from separate terminals on a single power supply, or may be supplied from separate power supplies. 
     In other embodiments, a TPH might have three or more phases, or groups, of nibs. Referring to FIG. 4, a TPH having three groups of nibs, Phases A, B and C, is depicted. In this three-phase example, the nibs, N1-N15, would be arranged in the TPH to form the pattern ABCABCABCA . . . . To power selected nibs in Phase A, V volts is applied to Phase A and Vx volts applied to Phases B and C. This can be generalized to more groups of nibs, such that the voltage V is applied to the selected phase and voltage Vx is applied to the unselected phases. 
     In FIG. 4, the selected nibs in Phase A dissipate V/R watts, as in the previous embodiments. The nibs in Phases B and C, associated with selected nibs in Phase A, dissipate Vx 2  /R watts, as in the previous embodiments. The unselected nibs in Phase A dissipate  (V-Vx)/(R+(R/(N-1)))! 2  ×R watts, where N is the number of Phases, or groups, of nibs. The unselected nibs in Phases B and C dissipate  (V-Vx)/NR! 2  watts. 
     Since the power dissipation in the unselected nibs in Phases B and C of FIG. 4 is less than that in the unselected nibs in Phase A, the power dissipation in Phases B and C nibs can be ignored. Equating the remaining two spurious dissipations, gives: Vx 2  /R= (V-Vx)/(R+(R/(N-1)))! 2  ×R watts, or, Vx=V/(2+(1/(N-1))). Table 1 shows for various values of N, the resulting values for Vx and the power ratio between selected and unselected nibs. The worst case result is that for a large N, or Vx=V/2, a 4 to 1 power ratio results between selected and unselected nibs. 
     
                       TABLE 1______________________________________N            Vx            Power Ratio______________________________________2            V/3           9/13            V/(2.5)       6.25/14            V/(2.33 . . . )                      5.44 . . . /15            V/(2.25)      5.0625/16            V/(2.2)       484/1∞      V/2           4/1______________________________________ 
    
     The foregoing disclosure and description of the invention are illustrative and explanatory of the preferred embodiments, and changes in the components, circuit elements, or connections may be made without departing from the spirit of the invention.