Typewriting machines – Typing by other than type-face or type-die – Character formation by impact
Reexamination Certificate
2001-09-18
2004-05-11
Nguyen, Anthony H. (Department: 2854)
Typewriting machines
Typing by other than type-face or type-die
Character formation by impact
C400S124160
Reexamination Certificate
active
06733195
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an impact dot printer, and more specifically, relates to a circuit for driving a head of an impact dot printer and to a power control technique for controlling a power source for a head drive circuit.
To perform printing, an impact dot printer drives a print wire by using, for example, the magnetic attractive force of an electromagnet.
FIG. 13
is a diagram showing an example wire impact print head for the print head of the thus arranged impact dot printer.
In the example in
FIG. 13
, a wire impact print head
51
has a plurality of wires
57
that are attached, by wire levers
53
and return springs
55
, so that they reciprocate. When a drive current flows through a head coil
59
, a wire lever
53
is attracted by the magnetic attractive force produced by the electromagnet in the direction indicated by an arrow in
FIG. 13
, and a wire
57
strikes an ink ribbon
61
and forms dots on a printing sheet
65
moved in consonance with the rotation of a platen
63
.
FIG. 14
is a diagram illustrating the fundamental structure of the circuit of the print head
51
for driving the head coil
59
. In this example, only one head coil
59
and head driving transistor
33
set is shown, but in actuality, a plurality of these sets are provided. A drive circuit (driver)
30
for each head coil
59
is constituted by one of the head driving transistors
33
, a head drive power source
34
and a Zener diode
35
. During a predetermined conductive period, a control pulse
32
is maintained at level H by a print controller
31
, and a pertinent head driving transistor
33
is maintained in the ON state (in the saturated region). Then, a voltage (e.g., 35V) supplied by the head drive power source
34
is applied to the head coil
59
, and a drive current i
1
flows through it. Thereafter, when the control pulse
32
falls to level L, the head coil
59
generates an inductive electromotive force to render off the head driving transistor
33
. For this, the Zener diode
35
is rendered conductive at the induced voltage, and a base current flows to the head driving transistor
33
, while the head driving transistor
33
enters a linear operating region. Subsequently, the drive current i
1
flows through the head driving transistor
33
and the current value is drastically reduced, and as a result, the head driving transistor
33
is rendered off.
However, in the related head drive circuit, when the head driving transistor is turned off, the power supplied by the head drive power source is not effectively employed. This problem will be described while referring to
FIGS. 15A
to
15
D. In these drawings are presented a diagram showing a simplified head drive circuit, and other diagrams showing the flow of the drive current, as well as its current waveform and the operation of the Zener diode.
First, as is shown in
FIG. 15A
, when the transistor is rendered on, a drive current i is supplied by a power source Vp in the direction indicated by the arrow, and a head coil is driven. At this time, the collector-emitter voltage (V
CE
) of the transistor is substantially zero.
To render off the transistor, when the inductive electromotive force that is generated at the coil at the polarities shown in
FIG. 15A
exceeds the Zener voltage, the Zener diode is rendered conductive, and a base current flows via the Zener diode to the transistor, as is indicated by a broken line in FIG.
15
A. Then, the charge on the transistor falls in the linear operation mode, and the energy accumulated in the coil is discharged through the collector and the emitter of the transistor. When the discharge of the energy has been completed, the Zener diode is again rendered non-conductive and the transistor is rendered off.
FIGS. 15B and 15C
are graphs showing the changes produced by this process in the collector current i and the collector-emitter voltage (V
CE
) of the transistor as time elapses. As a result, as is shown in
FIG. 15D
, of the power (see
FIG. 15B
) supplied by the power source, power P (=i·V
CE
), which is required to render off the transistor, is consumed for heat generation at the transistor as thermal loss represented by Q in the figure.
As is described above, in the related head drive circuit, the power supplied by the power source to render off the transistor is lost and is not effectively employed. Furthermore, since a great deal of heat is generated by the transistor, a cooling member, such as a heat sink, is also required, and accordingly, the size of the package of a power source is enlarged.
SUMMARY OF THE INVENTION
To resolve these shortcomings, it is one objective of the present invention to provide a head drive circuit that not only drives the head efficiently, but also reduces the consumption of power, and to produce a compact power source.
To achieve the above objective, according to the present invention, there is provided a head drive circuit for an impact dot printer which performs printing by driving a print wire, comprising:
a DC power source for supplying a power source voltage;
a head coil;
a switching element which is on/off controlled to apply the power source voltage to the head coil for a predetermined time period;
a voltage regulator for converting an input voltage having a value higher than the power source voltage into an output voltage having a value as substantially same as the power source voltage;
a voltage introducer for inputting an inductive voltage, generated in the head coil when the switching element is turned off, into the voltage regulator as the input voltage; and
a voltage returner for feeding back the output voltage of the voltage regulator to the DC power source.
Namely, the head drive circuit is so configured that the voltage regulator returns to the power source the power that accumulates when the switching element (e.g., a transistor) is rendered off.
With this arrangement, the energy that accumulates in the head coil when the switching element is turned off is returned to the power source by the voltage regulator, and is effectively utilized for driving the head coil.
A DC/DC converter or a voltage dropper may be adopted as the voltage regulator.
Preferably, the voltage introducer includes a first rectifier which is rendered conductive when the inductive voltage is generated in the head coil to unidirectionally supply the inductive voltage into the voltage regulator as the input voltage, and the voltage returner includes a second rectifier for unidirectionally supplying the output voltage from the voltage regulator to the DC power source. For example, diodes may be adopted as the rectifiers.
Since the rectifiers (e.g., diodes) required for the prevention of a crosscurrent are provided, the backflow of power, from the input end of the voltage regulator to the switching element, or the inverted supply of power, from the power source to the output end of the voltage regulator, can be prevented.
Preferably, the head drive circuit further comprises an input voltage adjuster for adjusting the input voltage of the voltage regulator so as to have a predetermined value higher than the power source voltage. Specifically, so long as the input voltage of the voltage regulator is raised to a predetermined voltage that only when the switching element is rendered off is higher than the voltage provided by the power source, the power from the head coil can be led to the voltage regulator and can thereafter be returned to the power source by a high induction voltage that is generated at the head coil.
Preferably, the voltage regulator includes an input condenser for smoothing the input voltage thereof. The voltage adjuster includes a charger for charging the input condenser so as to have the predetermined value of input voltage before and while the printing is performed.
Preferably, the charger always applies the predetermined value of voltage to the input condenser.
Alternatively, the switching element is turned on/off repeatedly at a frequency too high to drive the print wire to apply the inductive voltag
Shirotori Hiroshi
Watamura Hisashi
Nguyen Anthony H.
Sughrue & Mion, PLLC
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