Temperature Compensated Multiple Character Electronic Display

Abstract

A temperature compensated multiple character electronic display is described. Each character position includes a multielement matrix controlled by a transistor. The collectors of all transistors of all matrices are common. The emitters of all transistors of a single character matrix are common, and are selectively connectable to ground so that the characters can be sequentially enabled. The corresponding elements of all character matrices are connected to a common control line and are actuated by a single character generator. The power sequentially applied to each character is controlled separately to maintain a uniform display under varying operating temperatures by sampling the voltage of the base-emitter junction of a transistor of the enabled character as a measure of its temperature, and then applying power to the character during the following display cycle that is related to the sampled temperature.

Parent Case Text

This application is a continuation of application Ser. No. 788,249, filed Dec. 31, 1968, now abandoned.

Description

This invention relates generally to electronic display systems, and more particularly relates to multicharacter electronic displays of the thermal type.

In thermal type electronic displays, particularly those used to print on thermally sensitive paper, it has been found necessary to maintain a uniform temperature during successive display cycles in order to provide uniform printing density. Temperature compensation is particularly important in multicharacter displays wherein, for example, a large number of character matrices are sequentially energized to print an entire line on a page. In addition to variations in the ambient temperature, the rate at which each particular character matrix is energized, the type of characters being printed, and the ambient temperature to a lesser extent, all determine the temperature to which each matrix cools between print cycles. For example, the last few characters of a line may not be used as much as the first few characters when printing a page. If the same amount of power were to be applied to the printhead during a print cycle, the ultimate printing temperature of the matrix would be determined by the temperature at the beginning of the cycle. Of course, the beginning temperature increases sharply if the duty cycle of the character matrix increases.

In accordance with this invention, the temperature of each character is sampled prior to the print cycle for the particular character, and the power that is applied to the character matrix during the print cycle is adjusted in such a manner as to achieve a predetermined temperature during the print cycle. Another important advantage of the present invention is that the same circuitry used for selecting the character to be printed is used to select the temperature monitoring means at the character matrix.

The novel features believed characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as other objects and advantages thereof, may best be understood by reference to the following detailed description of an illustrative embodiment, when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a simplified plan view of a multicharacter electronic display in accordance with this invention;

FIG. 2 is an enlarged perspective view of a portion of the display device shown in FIG. 1;

FIG. 3 is a detailed circuit diagram of a temperature compensated electronic display in accordance with the present invention; and

FIG. 4 is a simplified block diagram of the circuit of FIG. 3.

Referring now to the drawings, and in particular to FIG. 1, three four-character electronic display devices in accordance with the present invention are each indicated generally by the reference numerals 10a, 10b and 10c. Each of the electronic display devices 10 includes four character matrices 12a-12d. Each matrix includes a 5 .times. 7 array of elements E.sub.1 -E.sub.35, each of which is air isolated around its periphery and which is bonded to a ceramic slice 14 by a thermal insulating epoxy layer 16. The ceramic slice 14 is mounted on a metal heat sink 17.

As will presently be described, a transistor T.sub.n with a series resistor R.sub.n in the collector branch is formed by a diffusion in the interior face of each of the elements E.sub.1 -E.sub.35 that is adjacent the epoxy layer 16, and thin film circuits disposed on the interior faces of the semiconductor device are used to interconnect the diffused devices into an integrated circuit. For example, element E.sub.1 in each of the characters includes a transistor T.sub.1 and a resistor R.sub.1 (see FIG. 3), element E.sub.18 includes a transistor T.sub.18 and a resistor R.sub.18 and element E.sub.35 in each character includes transistor T.sub.35 and resistor R.sub.35.

The collectors of all of the transistors T.sub.1 -T.sub.35 of a particular character are connected through the respective resistors to a common collector voltage supply line 18. All of the emitters of the transistors T.sub.1 -T.sub.35 of each of the characters 12a-12d are connected to separate common emitter supply lines 20a-20d. Each of the character matrices 12 may then be separately enabled by selectively connecting the appropriate emitter supply line 20a-20d to ground. Only one of the emitter circuits 20a-20d of the three heads 10 is selected by a character select switch 22 at any one time, so that only one matrix is enabled at a time.

The base contacts of the transistors are of the corresponding elements of all characters in the device are also common. For example, the bases of transistors T.sub.1 of characters 12a-12d of all display devices 10 in the system are connected to a common control line C.sub.1, the bases of all transistors T.sub.18 are connected to control line C.sub.18 and the bases of all transistors T.sub.35 are connected to a common control line C.sub.35. Of course, it will be understood that the bases of all transistors T.sub.2 -T.sub.34 (not illustrated) would be connected to corresponding control lines C.sub.2 -C.sub.34 (not illustrated). The control lines C.sub.1 -C.sub.35 extend to a character generator 24 which energizes those lines necessary to produce the desired character by turning selected elements in the enabled character matrix "on."

In the operation of such a system, the character select switch 22 would typically scan from the left-hand character matrix to the right-hand character matrix in sequence by connecting the common emitter line 20.sub.n of the successive character matrices to ground. Then during the period that a particular character is thus enabled, the character generator produces the positive voltage levels on the control lines C.sub.1 -C.sub.35 necessary to generate the desired character at the selected character position. The positive voltage on the control line C.sub.1, for example, would turn transistor T.sub.1 "on" thus causing element E.sub.1 to be heated by the power dissipated in resistor R.sub.1. Those elements on which control lines are at ground potential would remain turned "off." For example, if control line C.sub.35 remains at ground potential, transistor T.sub.35 of the enabled character, character 12a for example, remains "off" because the collector-base junctions of transistors T.sub.35 of all the other characters prevent current from flowing from the energized common collector supply voltage line 18 through the collector-base junction of the inactive transistors T.sub.35 to control line C.sub.35, and thus to the base of transistor T.sub.35 of the enabled character.

In accordance with the present invention, a constant current source 26 (see FIG. 4) is connected to control line C.sub.18 and supplies a constant current during a very short sample cycle that precedes the print cycle. The current passes along control line C.sub.18 and through the base-emitter diode of the transistor T.sub.18 of the character enabled by the character select switch 22. For example, if the character select switch 22 connects common emitter line 20a of device 10b to ground so as to enable character matrix 12a of device 10b, the current injected on control line C.sub.18 by the constant current source 26 will pass through the base-emitter diode of transistor T.sub.18 of matrix 12a of device 10b because the emitter circuits of transistor T.sub.18 of all other matrices are opened by switch 22. The voltage on control line C.sub.18 will then be related to the temperature of the base-emitter diode of transistor T.sub.18, and thus to the temperature of the enabled matrix.

When a short sample pulse is applied to input 34, the voltage of line C.sub.18 is amplified by amplifier 28 and the amplified voltage sampled and held by circuit 30. The stored voltage is then applied to the noninverting input of an operational amplifier 32 during a print cycle pulse applied to terminal 36 immediately following the sample pulse. During the print cycle pulse, a switch 38 turns a series regulator 40 "on" to apply a voltage to the common collector line 18 that has a magnitude in predetermined relation to the voltage stored by the sample and hold circuit 30 as a result of the 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 C.sub.1 -C.sub.35 to cause the desired character to be generated by the enabled character matrix 12a.

The circuit shown in the simplified block diagram of FIG. 4 is shown in greater detail in FIG. 3 wherein corresponding components are designated by the same reference characters. The constant current source 26 includes a transistor 42 which is turned "on" and "off" by a switch 44. When conducting, transistor 42 supplies a constant current as a result of the voltage divider connected to the base so that the voltage on line C.sub.18 is determined by the offset voltage of the base-emitter diode of the enabled transistor T.sub.18. This voltage decreases with an increase in temperature at a rate of about 0.02V/.degree. C.

The voltage on line C.sub.18 is applied to input 46 of the operational amplifier 28, the offset of which is set by variable resistor 48 in the conventional manner. Thus, the output of the amplifier 28 is proportional to the voltage on control line C.sub.18 and is applied to the base of transistor 52 through resistor 50, and to the base of transistor 54 through resistor 50 and diode 55. Transistors 52 and 54 form a complementary switching pair for charging and discharging storage capacitor 64 when turned "on." Diode 55 provides an offset voltage to eliminate the dead spot at the crossover voltage. Transistors 52 and 54 are turned "on" by the complement of the sample pulse derived from the inverter 56 and applied to the base of switching transistor 58. This turns transistors 58, 60 and 62 "off," enabling the complementary sampling transistors 52 and 54 so that the output voltage of the amplifier 28 will charge storage capacitor 64. The voltage on capacitor 64 is then stored during the subsequent print cycle after the transistors 52 and 54 are "off."

The voltage stored on capacitor 64 is applied to the noninverting input of amplifier 32. The output of amplifier 32 is passed through a Zener diode 66 and diode 68 and applied to the input base of a pair of transistors 70 and 72 of the series voltage regulator 40. When the print cycle pulse is applied to input 36, inverter 74 turns transistor 76 "off," thus enabling transistors 70 and 72 to be turned "on" by the output of amplifier 32. The voltage applied to the common collector supply line 18 is then maintained at a level in predetermined relationship to the voltage sample and stored on capacitor 64 as a result of the feedback network including resistors 78 and 80 to the noninverting input of amplifier 32.

Thus the voltage, and hence the power, applied to those elements of the enabled character matrix selected by the character generator 24 is controlled in accordance with the temperature of the center element E.sub.18 of the matrix immediately preceding the print cycle so that a print pulse of predetermined length will result in a uniform temperature regardless of the temperature of the matrix before the print cycle. This procedure is repeated for each character matrix immediately preceding the print cycle for the respective character matrix.

Although a preferred embodiment of the invention has been described in detail, it is to be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

What is claimed is:

1. In a multiple character electronic display device, the combination of:

a plurality of character matrices, each of said character matrices having

a plurality of thermally separated elements each including a heating means,

means for sensing the temperature of each of the character matrices individually, and

means responsive to said sensing means of a character matrix for selectively applying power to the heating means of the elements of the character matrix at a voltage level related to the temperature of said character matrix.

2. The combination defined in claim 1 wherein:

each character matrix includes at least one thermally separated element including a semiconductor diode junction in heat exchanging relationship with the heating means for sensing the temperature of the character matrix.

3. The combination defined in claim 2 wherein:

the diode junction is part of a transistor connected to control current through the resistive heating means, and

the temperature of the diode junction is selectively sensed by closing the circuit in which the diode is located.

4. The combination defined in claim 3 wherein:

the diode is the base-emitter junction of the transistor, and

the resistive heating means is connected in the collector circuit of the transistor.

5. The combination defined in claim 4 wherein:

the collectors of the transistors of the elements are completed through the resistive heating means of each character matrix to a common collector voltage supply line,

the bases of the transistors of the elements are common, and

the emitters of the transistors are separately connectable to an emitter supply voltage.

6. The combination defined in claim 1 wherein said power applying means includes:

circuit means for storing a voltage proportional to the temperature of a character matrix during a sample period, and

circuit means for applying power that is proportional to the stored voltage to the heating elements of said character matrix during a subsequent period.

7. In a multi-character electronic display device, the combination of:

a plurality of character matrices each comprising a plurality of thermally separated elements each including a heating element,

first switching means for controlling the current through each individual heating element,

temperature sensing means for each matrix for producing a signal representative of the temperature of the respective matrix,

voltage supply means connected to each character matrix including means for adjusting the supply voltage in response to a signal from a temperature sensing means,

second switching means for selectively enabling each of the character matrices for printing by connecting the voltage supply means to the respective character matrices and respective temperature means, and

circuit means for decoding electrical data representative of characters and producing outputs for operating the switching means of the enabled matrix in a manner to heat the elements in a geometric pattern corresponding to the character represented by the character data.

8. The combination of claim 7 wherein:

at least one of the first switching means of each matrix comprises a transistor in heat exchange relationship with the heating element it controls, the heating element being connected in the collector circuit, and wherein

the temperature of the respective matrix is sensed by passing a current through the base-emitter junction of the transistor and taking the offset voltage as a measure of the temperature of the matrix.

9. The combination of claim 8 wherein:

the second switching means are in the emitter circuits of the transistors used as temperature sensors.

10. In an electronic display, the combination of:

a plurality of matrices each comprised of a like number of semiconductor elements,

each semiconductor element including a transistor formed in the element with a resistance in the collector circuit for heating the element when current is passed through the transistor,

a separate control line common to the bases of the transistors of corresponding elements of the matrices,

switch means for selectively opening the emitter circuits of the transistors of each matrix to permit enabling of only one selected matrix at a time,

means for passing a current through one of the control lines and the base-emitter junction of the corresponding transistor of the enabled matrix to sense the temperature of the matrix, and

means for applying power that is related in magnitude to the magnitude of the sensed temperature to the enabled matrix.

11. The combination of claim 10 wherein:

the means for applying power is circuit means for adjusting the voltage applied across the resistances and corresponding transistors.

12. The combination of claim 11 wherein the means for applying power includes:

means for sampling the offset voltage of the base-emitter junction of the enabled transistor during a sample period and storing the voltage during a succeeding energizing period, and

means for regulating the voltage applied across the resistances and corresponding transistors in relation to the stored voltage during the energizing period.