Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to a drive circuit for a matrix
connected thermal printing array used in a thermal facsimile printing
system, and more particularly to one which inhibits the thermal resistance
elements in leakage paths of the matrix from reaching prlnting temperatures
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without utilizing isolating diodes for each element.
Background of the Invention
A thermal printing head is an array of contiguous thermal
resistance elements. By selectively passing an electric current through
certain of the elements while a heat sensitive paper is progressively
lQ advanced over the top of the head, facsimile printing can be achieved.
One arrangement for driving the head is to irldividually
access each thermal resistance element. This is generally accomplished
by utilizing a current sinking transistor for each resistive element.
Because of the large number of interconnecting leads required, the
transistor elements and the accompanying decoding logic must be mounted on
the thermal head, resulting in a relatively costly and complex structure.
An alternate arrangement is to connect the array as a
matrix of rows and columns. Printing is then achieved by simultaneously
applying a voltage between say one of the columns and selected ones of
2~ the rows, and thereafter sequentially repeating the operation until all
columns have been accessed to print one complete line. The heat sensitive
paper is then advanced relative to the head after which the operation is
repeated to print the following line.
This matrix arrangement contains a large num~er of
paralleled leakage paths which under certain operating conditions can
result in sufficient voltage drop across certain of the unaccessed
elements to cause spurious printing. One arrangement which circumvents
this utilizes a diode connected in series with each thermal printing
element which blocks the applied d-c voltage from passing through the
reverse leakage paths. ~ith large arrays such as those containing over
1,000 elements, the mounting and connection of these diodes to the
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thermal elements requires a large number of bonds which increase costs
and reduce reliability.
In an alternate arrangement the balance of the rows are '
connected to one intermediate source and the columns to a second
intermediate source; e.g. voltage sources of one-third and two-thirds that
applied to the selected elements. Under these conditions~ the power in any
one of the balance of the elements in the leakage paths is one-ninth that
applied to the selected elements; well below that required to raise these -
elements to their printing threshold temperature, so that no spurious
1~ printing results. This is similar to the arrangement described in
United States Patent No. 3,938,136 ~ntitled "Method and Device for Driving
a Matrix Type Liquid Crystal Display Element" issued February 10, 1976 and
invented by Hideaki Kawakami, ~hich is utilized to reduce crosstalk in
a liquid crystal display. While this arrangement minimizes the power
applied to each individual leakage element in the thermal array, the overall
power may be relatively high since each of these leakage elements has the
same voltage applied thereto. This can be of particular concern in a large
matrix array as it substantially increases the power requirements of the
drive circuitry. ~ ~ -
~0 Statement of the Invention ;
The present invention is based on the realization that a
substantial reduction in power is achieved by applying an intermediate
voltage to the unaccessed points in one coordinate of the array, while
allowing the unaccessed points in the other coordinate to float. ~ith
this arrangement the balance of the thermal resistance elements in the
leakage paths do not reach their printing temperature threshold. Thus,
in accordance with the present invention there is provided a drive circuit
for a thermal printing array having a plurality of thermal resistance
elements connected in a matrix of rows and columns. The drive circuit
3Q comprises a control circuit for connecting a first source of voltage
between a single column and selected ones of the rows to heat selected
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thermal resistance elements connected directly therebetween to printing
temperatures. The control circuit concurrently connects the balance of
the rows to a second source of voltage wh;ch is intermediate that of said
first source, while ~he balance of the columns float. This maintains the
balance of the thermal resistance elements in the array below printing
temperatures. Thus, when the intermediate voltage is one-half that
applied to the selected elements, the power in any one of the leakage
path elements is no greater than one-quarter that applied to the selected
elements. In practice, the control circuit sequentially repeats the operation
1~ until all columns have been accessed after which the operation is repeated.
In addition to utilizing less power, the drive circuitry for such an array
is simpler than that required when both the rows and columns are connected
to intermediate voltage sources.
Brief Description of the Drawing
An example embodiment of the lnventlon will now be described
with reference to the single figure of drawings which illustrates a block ;
and schematic circuit diagram of a drive circuit for a matrix connected
thermal printing array.
Description of the Preferred ~mbodiment
2~ Referring to the single figure, the thermal printing array T
comprises a plurality of thermal resistance elements Al, A2---Nn which are
connected in a matrix of rows 1, 2, 3---n and columns A, B, C---N. The
elements are generally realized in thin or thick film technology. While
each of the elements Al, A2---Nn is shown as being separate and distinct, -
they may be constructed as part of a contiguous bar of elements as
illustrated in applicant's copending application Serial No. 239,106
entitled: "Thermal Printing De~ice" invented by D.R. Baraff et al,
filed NoYember 6, 1975.
~ach of the columns A, B, C---N is connected through a
~0 selector switch KA, KB, KC---KN respectively, to a source of voltage V.
HoweYer, each of the rows 1, 2, 3---n is connected through a selector
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~ L~3;~3~ 3switch Kl, K2, K3---Kn respectively, to either a source of voltage V/2 or
ground. All of the selector switches are under control of a logic control
circuit K. For simplicity, the switches are illustrated as being
mechanically actuated. However, in a practical embodiment, semiconductor
gating circuitry would normally be utilized to provide rapid and reliable
control of the voltages applied to the thermal printing alrray T.
In operation the voltage source V is first applied between
a single column A and selected ones of the rows 1, 2, 3---n to raise the
temperature of the elements Al, A2, A3---An at the selected junctions
1~ thereof to printing temperatures while heat sensitive paper (not shown~
is held in contact therewith. The operation is sequentiall~ repeated
for columns B through N with selected combinations of rows, all under
control of the logic control circuitry K~ While each of the selected
elements in one column is accessed by grounding the associated rows, the
balance of the rows are connected to the intermediate voltage source V/2,
in order to prevent spurious printing by other elements in the leakage
paths. Once all columns have been accessed, the heat sensitive paper
(not shown~ is advanced and the operation repeated to print the following
line.
2~ In the following example, a voltage sufficient to raise thethermal resistance elements Al, A2---Nn to printing temperatures is
designated V; the other voltage level V/2 is designated with respect
to this voltage V. Assume that a source of voltage V is to be initially
applied to selected printing elements Al and A3 in column A. Switch KA ~;
is connected to source V while all other switches in the columns KB, KC---KN
remain open under control of the logic control circuitry K. Goncurrently
switches Kl and K3 are connected to ground while the balance of the
switches K2---Kn are connected to a source of voltage V/2. The power
applied to the selected elements Al and A3 in column A is equal to V2/R,
3~ where R is the resistance of each element. If the remainder of the rows
and columns were both allowed to float~ the paralleling effect of the
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thermal elements in the other columns e, C---N could cause the remainder
of the elements in column A to rise to printing temperatures. This effect
is particularly pronounced when the majority of the elements in column A
are directly accessed and only a few receive power through the leakage
paths. However, because the balance of the rows K2---Kn are tied to a
voltage source V/2, and the balance of the columns KB, KC---KN are
permitted to float, the maximum power dissipated in each unaccessed element
is limited to V2l4R. This is below that required to raise the temperature
af these thermal resistance elements to printing temperatures and hence
only those elements Al and A3 which are directly connected between voltage
source V and ground are raised to printing temperatures.
For a square matrix of Y-Y rows and columns, in which x
selected elements in a single column are accessed at any one time, the
total power P applied to the array is:
P ~ Xp ~ p ~ ~ pX(Y-X4yy~
where: p = VR the power applied to each of the selected elements x.
In a prior art structure of the type described in the
above-mentioned patent to Kawakami where intermediate voltages of V/3
and 2V/3 are applied to both the columns and rows respectively, the total
power P applied to the array is:
P = xp ~ ~ p (2)
The saving in power of the present invention over this
prior arrangement will be evident from the following comparison of several
examples of a 40 x 40 matrix array when applied to equations (l) and ~-
(2~ above.
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Y = 40 p - V2/R
TOTAL POWER = P
x PRIOR ART PRESENT INVENTION
1 178.6p 20.2p ~ ~
195.5p 122.~p ~ ~ -
39 212.4p 48.7p -
It will be evident that the intermediate voltage applied to
all the unaccessed rows need not necessarily be one-half that applied to
the accessed columns. The main criteria is that the intermediate voltage `1~ be such that none of the elements in the leakage paths rise to printing -
temperatures.
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In a typical non-limiting example, a thermal printing bar
of the type illustrated in the above-mentioned application to D.R.Baraff et al
has a densiky of about 80 elements per centimeter. Thick film technology is
utilized in the constructlon of the bar with each element having a resistance
of approximately 1.5 Kohms. Satisfactory printing temperatures, for a heat .
sensitive paper having a printing threshold of 120C and a normal printing - ~ -
temperature of 180C, were obtained with the application of voltages ;~V = 38 volts and V/2 = 19 volts for a period of one millisecond to the
elements, without any resultant smearing. All unaccessed columns were
allowed to float. The application of this intermediate voltage tends to
preheat the thermal printing bar thus reducing the time reqwired to raise
the elements to printing temperatures. This secondary effect permits an
increase in the attainable writing speed of the array. ~- ~In the foregoing detailed description, a sin~le column is ~ ;
accessed in conjunction with a plurality of rows at any one time. It will
be evident that this designation is purely arbitrary and that the -~
arrangement could be reversed with a single row being accessed in
conjunction with a plurality of columns. With ~his latter arrangement, the
balance of the columns as opposed to the rows would be connec~ed to the
intermediate source of voltage.
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