JP2003103806A - Head drive circuit of impact printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the size of a power supply by reducing useless power consumption in a head drive circuit. SOLUTION: A print wire is driven by turning a transistor 33 connected with a head coil 59 on/off thereby supplying a drive current i to the coil 59. At the joint of the coil 59 and the transistor 33, input of a DC/DC converter 2 is connected and the output of the converter 2 is connected to the joint of a power supply 34 and the coil 59. The converter 2 clamps a high voltage being induced in the coil 59 when the transistor 33 is turned off at a constant level, e.g. 90 V, and then converts it into the voltage of the power supply 34, e.g. 35 V. Consequently, energy stored in the coil 59 is not consumed uselessly by the transistor 33 but returned back to the power supply 34 and reused. Input voltage of the converter 2 is held at the constant level by means of an initial charging circuit 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an impact dot printer, and more particularly to a head drive circuit for an impact dot printer and a power supply control technique for controlling a power supply for the head drive circuit.

[0002]

2. Description of the Related Art Impact dot printers are, for example,
Printing is performed by driving the printing wire by the magnetic attraction of the electromagnet. FIG. 6 shows an example of a wire impact print head in the print head unit of the impact dot printer having such a configuration.

In the example shown in FIG. 6, the wire impact printing head 51 has a plurality of wires 57 reciprocally attached by a wire lever 53 and a return spring 55.
When a drive current flows through the head coil 59, the wires 57 are driven by the magnetic attraction force of the electromagnet to cause the wire lever 53 to move.
Is attracted in the direction of the arrow shown in the figure,
When the wire 57 collides with the ink ribbon 61, the platen 6
Dots are formed on the printing paper 65 that moves by the rotation of 3.

FIG. 7 shows a basic structure of a drive circuit for the head coil 59 in the print head 51 described above. Although only one set of the head coil 59 and the head driver transistor 33 is shown in the illustrated example, there are actually a plurality of sets. The drive circuit (driver circuit) 30 for each head coil 59 includes a head driver transistor 33,
The head driving power source 34 and the Zener diode 35 are included. The control pulse 32 becomes H level for a predetermined energization time from the print control unit 31, the head driver transistor 33 becomes completely ON state (saturation region), and the voltage of the head driving power source 34 (for example, 35V) is applied to the head coil 59. The applied drive current I1 flows. When the control pulse 32 becomes L level, the head driver transistor 33 tries to turn off, and the head coil 5
9 generates an induced electromotive force, the induced voltage causes the Zener diode 35 to become conductive, and a base current flows through the head driver transistor 33. As a result, the head driver transistor 33 enters a linear operation region, and the head driver transistor 33 is turned on. The drive current I1 flows therethrough, the current value sharply decreases, and the head driver transistor 33 is turned off.

[0005]

However, the conventional head drive circuit has a problem in that the power supplied from the head drive power source is not effectively used when the head driver transistor is turned off. This problem will be described with reference to FIG. This figure is a diagram showing a simplified head drive circuit and showing the flow of its drive current together with the current waveform, the action of the Zener diode, and the like.

First, as shown in FIG. 8A, when the transistor is turned on, a driving current i flows from the power source Vp in the direction of the arrow to drive the head coil. At this time,
As shown in (a), the collector-emitter voltage (VCE) of the transistor is substantially zero.

Next, when the transistor tries to turn off and the induced electromotive force generated in the coil with the positive and negative polarities exceeds the Zener voltage, the Zener diode becomes conductive, and the dotted line in FIG. As shown by, the base current flows through the transistor through the Zener diode, the transistor enters the linear operation region, and the energy stored in the coil is discharged through the collector-emitter of the transistor. When the energy stored in the coil is released, the Zener diode becomes non-conductive again and the transistor is turned off.

Changes in the collector current i and the collector-emitter voltage (VCE) of the transistor in the above process are shown in FIGS. 8 (b) and 8 (c) with time, respectively. As a result, as shown in FIG. 8D, the transistor is turned OFF in the electric power received from the power source [see FIG. 8B].
A portion of the power P [current i × collector-emitter voltage (VCE)] at the time of F is consumed as heat generation in the transistor and becomes a loss.

As described above, in the conventional head drive circuit,
All the power supplied from the power supply when the transistor is turned off is lost and is not effectively used. In addition, the transistor generates a large amount of heat, and a cooling means such as a heat sink is also required, which inevitably requires a large power supply package.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a head drive circuit capable of efficiently driving a head by reducing power consumption in the head drive circuit, Moreover, it is intended to downsize the power supply.

[0011]

In order to solve the above problems, the head drive circuit of the present invention uses a DC / DC converter to drive the power when the head driver switching element (eg, transistor) in the head drive circuit is OFF. It is characterized in that it is configured to be returned to the power source for use.

That is, the head drive circuit of the present invention drives the print wire by applying the power supply voltage of the DC power supply for head drive to the head coil for a predetermined time by the ON / OFF control of the head driver switching element. In a head drive circuit of an impact printer for printing, a DC / DC converter for converting input DC power having an input voltage higher than the power supply voltage into output DC power having substantially the same output voltage as the power supply voltage, and head driver switching An input means for inputting an induced voltage generated in the head coil when the element is turned off to the DC / DC converter, and a return means for returning the DC power output from the DC / DC converter to the head drive power source are provided.

As a result, the energy stored in the head coil when the head driver switching element is turned off is returned to the head driving power source by the DC / DC converter and is effectively reused for driving the head coil.

In a preferred embodiment, the input means is in a conductive state when an induced voltage is generated in the head coil, and causes a current to flow in one direction from the head coil to the input end of the DC / DC converter. Element (eg diode)
Is. Further, the return means is a second rectifying element (for example, a diode) that allows a current to flow in one direction from the output end of the DC / DC converter to the head driving power source.

By providing such a backflow preventing rectifying element (for example, a diode), the power on the input side of the DC / DC converter flows back to the driver transistor or from the head driving power supply to the output side of the DC / DC converter. It is possible to prevent a problem in which electric power is reversely supplied.

In a preferred embodiment, an input voltage adjusting circuit for adjusting the input voltage of the DC / DC converter to a predetermined voltage higher than the voltage of the head driving power supply is provided. That is, if the voltage on the input side of the DC / DC converter is raised to a predetermined voltage higher than the voltage of the head driving power supply,
Only when the driver transistor is turned off, the high induction voltage generated in the head coil can guide the power from the head coil to the DC / DC converter and return it to the power supply.

In the preferred embodiment, the DC / DC converter has a capacitor on its input side for smoothing its input voltage. Then, the input voltage adjustment circuit is set to DC / DC before starting the printing operation and during the printing operation.
It has a charging circuit for charging the input side capacitor of the DC converter to the predetermined voltage.

In a preferred embodiment, the charging circuit is configured to always apply the predetermined voltage to the input side capacitor.

In another preferred embodiment, the head driver switching element and the head coil function as the charging circuit. That is, before starting the printing operation, the head driver switching element is repeatedly turned on / off at a frequency high enough not to drive the printing wire.
The OFF operation is performed, whereby the induced voltage generated in the head coil each time the driver transistor is turned OFF is repeatedly applied to the input side capacitor, and the input side capacitor is charged to a predetermined voltage. This charging operation is always performed before starting the printing operation.

As another embodiment, the charging operation using the head driver switching element and the head coil is performed in order to supplement not only the initial charging before the start of the printing operation but also the discharge amount of the input side capacitor during the printing operation. In addition, it can be configured to be performed at any time (for example, at the time of line feed) during the printing operation.

In a preferred embodiment, the charging circuit includes an initial charging circuit for charging the input side capacitor to the predetermined voltage before starting the printing operation, and a charging circuit for the input side capacitor during the printing operation. And an input voltage holding circuit that holds the charging voltage at the predetermined voltage. As the initial charging circuit, there is no special circuit, and the head driver switching element and the head coil are used as described above. On the other hand, the input voltage holding circuit is specially provided, which is a coil for charging and a coil for performing ON / OFF control such that the power supply voltage of the head driving DC power source is not applied to the charging coil. A driver switching element, and is configured to apply an induced voltage generated in the charging coil to the input side capacitor when the coil driver switching element is turned off. This input voltage holding circuit repeats the ON / OFF operation of the coil driver switching element at any time during the printing operation to generate the induced voltage generated in the charging coil every time the coil driver transistor is turned off. The charging voltage of the input side capacitor is maintained at a predetermined voltage by repeatedly applying it to the input side capacitor.

As another embodiment, by using the above-mentioned input voltage holding circuit, not only supplementary charging during printing operation but also
It can also be configured to perform the initial charging before the start of printing.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of a head drive circuit according to a first embodiment of the present invention.

As shown in FIG. 1, this head drive circuit has a head driver transistor 33 for driving the head coil 59, an input whose input voltage is 90V and whose output voltage is substantially equal to the voltage 35V of the driving power supply 34. Constant voltage D
An anode of the C / DC converter 2, an input constant voltage DC / DC converter 2, an initial voltage charging circuit 4 for boosting the input voltage of the DC / DC converter 2 to a voltage value of 90V, a head coil 59, and a collector 10 of a head driver transistor 33 are connected. A diode 6 as a first rectifying element whose cathode is connected to the input side of the input constant voltage DC / DC converter 2, and an anode thereof is connected to the output side of the input constant voltage DC / DC converter 2, and a head driving power source 34 And a diode 8 as a second rectifying element to which the cathode is connected. In the head drive circuit of the present embodiment, the Zener diode for connecting between the head coil 59 and the base of the head driver transistor 33 is not provided unlike the conventional example. The input constant voltage DC / DC converter 2 operates so that the input voltage is 90V and the output voltage is equal to a voltage obtained by adding the voltage drop of the diode 8 of the driving power supply 34, for example, 35V.

Before the print head starts the actual printing operation, the input constant voltage D is set by the initial voltage charging circuit 4.
Input voltage of C / DC converter 2 (shown in FIG. 2 described later,
The charging voltage of the smoothing capacitor 2a provided at the input end of the DC / DC converter 2) is set to a voltage value of 90V. Here, the head driver transistor 33 is turned on.
Then, a driving current flows from the head driving power source 34 to drive the head coil 59. Next, when the head driver transistor 33 is turned off, a high voltage is generated in the collector 10 of the head driver transistor 33 due to the induced electromotive force generated in the head coil 59 and is clamped at the input voltage 90V of the DC / DC converter 2. It Then, the drive current i is equal to the input constant voltage DC / DC converter 2
Is absorbed by the input constant voltage DC / DC converter 2 and returned to the head drive power source 34 from the output side of the input constant voltage DC / DC converter 2 via the diode 8 for reuse.

FIG. 2 shows a circuit example of the input constant voltage DC / DC converter 2 and the initial voltage charging circuit 4. Input constant voltage D
As shown in FIG. 2A, the C / DC converter 2 chops the voltage 90V of the input-side capacitor 2a by switching the chopper transistor 2b by the drive control circuit 2d to control the duty ratio,
This chopped voltage waveform is output to the capacitor 2c on the output side.
The output voltage is smoothed, stepped down to a constant voltage of 35V, and output.
In the free wheeling diode 2e, the transistor 2b is turned off.
At times, it is a diode for causing the energy stored in the DC reactor 2f to flow back to the output-side capacitor 2c. As shown in FIG. 2 (b), the initial voltage charging circuit 4 uses the transformer 4a to convert AC 100V from the commercial power source into AC 9V.
The voltage is stepped down to 0V, rectified by the diode 4b and smoothed by the capacitor 4c to generate DC 90V, and this DC 90V is applied to the input terminal of the input constant voltage DC / DC converter 2. Thereby, the input voltage of the input constant voltage DC / DC converter 2 is always constant at 90V.

The input constant voltage DC / DC converter 2
In addition to the chopper method using the constant voltage control amplifier as shown in FIG. 2A, various configurations such as a ringing choke converter can be considered. Also, the initial voltage charging circuit 4 is not limited to the configuration shown in FIG.

FIG. 3 is a simplified circuit diagram of the head drive circuit of this embodiment and a diagram showing the flow of its drive current together with current waveforms and the like. Referring to this figure, the first embodiment will be described. The operation of the head drive circuit of the embodiment will be described.

As shown by the alternate long and short dash line in FIG. 3A, it goes without saying that there is a set of the head coil 59 and the head driver transistor 33 for each of a large number of printing wires. The operation will be described focusing on the set of the head coil 59 and the head driver transistor 33 for the wire. First, when the head driver transistor 33 is turned on, a drive current i flows from the power supply Vp in the direction of the arrow to drive the head coil 59.

Next, the head driver transistor 33
Is turned off, an induced electromotive force is generated in the head coil 59 with the positive and negative polarities as shown in the figure, and the diode 6 becomes conductive due to the high induced voltage, so that the collector voltage of the head driver transistor 33 is as shown in FIG. Like D
The input voltage of the C / DC converter 2 is clamped to 90V, and the drive current i flows into the input side of the input constant voltage DC / DC converter 2 via the diode 6 as shown by the arrow in FIG. In this way, the electric power supplied to the head coils 59 of a large number of print wires at the time of its turn-off is absorbed by the input side of the input constant voltage DC / DC converter 2. The absorbed power is converted into DC power having a voltage substantially equal to the voltage Vp of the head driving power supply 34 by the input constant voltage DC / DC converter 2, and the diode is supplied from the output side of the input constant voltage DC / DC converter 2. It is returned to the head driving power source 34 via 8. Therefore, when the head driver transistor 33 is turned off, the head driver transistor 33 can instantly be completely turned off, and the current flowing through the head driver transistor 33 is substantially zero. As shown, there is substantially no power loss in the head driver transistor 33. That is, FIG. 3 (b)
As indicated by the finely shaded area, the electric power that was conventionally wasted when the head driver transistor 33 is turned off can be returned to the power supply 34 in the present embodiment and reused as head drive energy again. . Since the heat generated from the transistor 33 is also greatly reduced, a simple measure for cooling the transistor 33 is sufficient and the power supply package can be downsized.

In the above embodiment, the case where the head coil and the driver transistor are formed in one stage has been described, but the present invention is not limited to this. A configuration using two stages of driver transistors may be used. In the case of such a circuit configuration, the waveform of the drive current is different from the waveform shown in FIG. 3B, but the energy wasted in the conventional example when the transistor is turned off is returned to the head drive power source and reused. What can be done is the same as in the above-described embodiment.

Next, a second embodiment of the present invention will be described with reference to the drawings. FIG. 4 is a diagram showing the configuration of a head drive circuit according to the second embodiment of the present invention. This head drive circuit is different from the circuit of the first embodiment shown in FIG. 1 in that the initial voltage charging circuit 4 in FIG. 1 is removed and an input voltage holding circuit 21 is provided instead. The input voltage holding circuit 21 includes a charging coil 22, a coil driver transistor 23, and a diode 24, which are assembled in the same manner as the head coil 59, the head driver transistor 33, and the diode 6 which are print wire driving circuits. The current capacity is smaller than that of the print wire drive circuit. Then, before starting printing, the head coil 59 and the head driver transistor 33, which are the print wire drive circuit, are driven at high speed in small number of times to make the capacitor 2a (on the input side of the input constant voltage DC / DC converter 2) (Hereinafter, simply referred to as capacitor 2a)
Perform initial charging to. After printing is started, the input voltage holding circuit 21 is driven at any time to replenish the input side capacitor 2a to prevent the input voltage from being lowered due to the discharge of the input side capacitor 2a.

The operation of the second embodiment will be described below. The operation of the input constant voltage DC / DC converter 2 for absorbing the energy of the head coil 59 and returning it to the power supply 34 when the head driver transistor 33 is turned off is the same as that of the first embodiment. The description is omitted.

First, after the printer is turned on, the input constant voltage DC / DC converter 2 is initially charged at a predetermined time before the start of printing. At this time, the print wire drive circuit is operated repeatedly at high speed with a short pulse having a period such that the print wire of the head does not substantially operate. That is, an ON / OFF pulse having a frequency high enough not to operate the print wire is applied between the base and emitter of the head driver transistor 33. As a result, the head driver transistor 33 repeats the ON / OFF operation at high speed, and the head coil 59 accumulates the energy from the power source 34 to generate DC.
The operation of discharging and charging the input side capacitor 2a of the / DC converter 2 is repeated,
To 90V. Then, the normal printing operation is started.

Since the charging voltage of the capacitor 2a gradually decreases during the printing operation, the input voltage holding circuit is periodically or at any time, for example, every time one line is printed or 40 characters are printed. 21 is driven ON / FF a large number of times with a high-speed pulse similar to that at the time of initial charging to replenish and charge the capacitor 2a, and the charging voltage of the capacitor 2a is maintained at about 90V.

As described above with reference to FIG. 4, the input voltage holding circuit 21 includes the charging coil 22, the coil driver transistor 23 for driving the charging coil 22, and the induced electromotive force generated in the charging coil 22 when the transistor 23 is turned off. And a diode 24 that conducts with the current and allows a current to flow to the input end of the DC / DC converter 2. The supplementary charging operation of the input voltage holding circuit 21 is exactly the same as the initial charging operation of the print wire drive circuit described above. That is, every time the transistor 23 is switched by an ON / OFF pulse, the energy stored in the charging coil 22 at the time of ON is charged into the capacitor 2a through the diode 24 at the time of turn OFF. If this is repeated for a predetermined time, the insufficient voltage of the capacitor 2a is replenished and 90V
Held in. Since the charging current at the time of this supplemental charging may be smaller than the charging current at the time of initial charging, the current capacity of the voltage holding circuit 21 may be smaller than that of the print wire drive circuit.

The charging operation at the time of initial charging will be described in more detail with reference to the waveform chart of FIG. FIG. 5 (a),
(B) shows the operation waveform of the print wire drive circuit during the printing operation, (a) is the waveform of the current flowing through the head coil 59, (b) is the collector-emitter voltage of the head driver transistor 33. Shows the waveform of. On the other hand, FIG.
(C) and (d) are operation waveforms of the print wire drive circuit at the time of initial charging, (c) is a waveform of the initial charging current flowing through the head coil 59, and (b) is a collector / emitter of the head driver transistor 33. The waveform of the inter-voltage is shown.

In the printing operation, the head driver transistor 33 is driven by a pulse having a frequency of about 1 to 2 kHz as shown in FIG. 5B, and the head coil 59 receives a current as shown in FIG. 5A. Flows to drive the print wire. On the other hand, at the time of initial charging, the head driver transistor 33 is driven for about 100 ms with a pulse having a frequency of about 10 kHz (for example, an ON time width of 20 μs and an OFF time width of 80 μs) as shown in FIG. 5D. 1000
Turn on / off repeatedly. As a result, a minute pulse current having a short time width as shown in FIG. 5C repeatedly flows through the head coil 59, but the printing wire is not driven by such a minute current having a high frequency. The portion of these minute current pulses when the head driver transistor 33 is turned off (that is, the current in the hatched portion in FIG. 5C) is the diode 24.
Through the input constant voltage DC / DC converter 2 as a charging current to the input side capacitor 2a. The charging current due to such switching is 10 for 100 ms.
The charging voltage of the capacitor 2a reaches almost 90V by repeatedly flowing the current 00 times.

Replenishment charging by the input voltage holding circuit 21 during the printing operation can also be performed by turning on / off the coil driver transistor 23 with a pulse having the same frequency as the initial charging or with a pulse having a higher frequency. it can. For example, a charging inductance 2 of 3300 μH
2 and the coil driver transistor 23 is set to frequency 2
Replenishment charging is performed by driving each line feed of printing with a pulse having an ON period of 3 μs at 5 kHz.

As described above, the input voltage holding circuit 21
Requires a smaller current capacity than the print wire drive circuit since it is only replenished and charged. However, as a modification, the current capacity of the input voltage holding circuit 21 is made as large as that of the print wire drive circuit, and the input voltage holding circuit 21 is
It is also possible to perform initial charging before printing with. Alternatively, the initial charge may be performed by using the print wire drive circuit and the input voltage holding circuit 21 together.

Further, as another modified example, it is also possible that the input voltage holding circuit 21 is not provided and the supplementary charging is performed by the print wire driving circuit. For example, replenishment charging can be performed by driving the print wire drive circuit with a high-frequency pulse similar to the initial charge each time a line feed occurs (the number of times of driving may be much smaller than the initial charge).

Although two embodiments of the present invention have been described above, these embodiments are merely examples for explaining the present invention, and the present invention is not limited to these embodiments. Therefore, the present invention can be implemented in various forms other than the above embodiment.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a head drive circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram of an input constant voltage DC / DC converter and an initial voltage charging circuit of the same embodiment.

FIG. 3 is an operation explanatory view of the head drive circuit of the embodiment, FIG. 3A is a simplified circuit diagram of the head drive circuit,
(B) is a waveform diagram of the drive current flowing through the head coil,
(C) is a waveform diagram of the collector-emitter voltage of the head driver transistor, and (d) is a diagram showing power loss in the head driver transistor.

FIG. 4 is a diagram showing a configuration of a head drive circuit according to a second embodiment of the present invention.

5A and 5B are operation waveform diagrams of the same embodiment at the time of printing and at the time of initial charging, FIG. 5A is a waveform of a current flowing through a head coil during printing, and FIG. 5B is a collector-emitter voltage of a head driver transistor during printing. , (C) shows the waveform of the charging current flowing through the head coil during the initial charging, and (d) shows the waveform of the collector-emitter voltage of the head driver transistor during the initial charging.

FIG. 6 is a diagram illustrating an example of a wire impact print head in a print head unit of an impact dot printer.

FIG. 7 is an example of a configuration diagram of a conventional head drive circuit.

FIG. 8 is an operation explanatory diagram of a conventional head drive circuit,
(A) is a simplified circuit diagram of a conventional head drive circuit,
(B) is a waveform diagram of the drive current flowing through the head coil,
(C) is a waveform diagram of the base-emitter voltage of the head driver transistor, and (d) is a diagram showing power loss in the head driver transistor.

[Explanation of symbols]

2-input constant voltage DC / DC converter 2a, 2c capacitors 2b, 23 transistors 2d drive control circuit 2e Freewheeling diode 2f DC reactor 4 Initial voltage charging circuit 6, 8, 24 diodes 10 collector 21 Input voltage holding circuit 22 Charging coil 30 Head coil drive circuit (driver circuit) 31 Print control unit 32 control pulses 33 head driver transistor 34 Head drive power supply 35 Zener diode 36 diode 51 print head 53 wire lever 55 Return spring 57 Multiple wires 59 head coil 61 Ink ribbon 63 Platen 65 printing paper I1 drive current i Drive current

Claims (8)

[Claims] 1. A head drive for an impact printer that drives a print wire to print by applying a power supply voltage of a DC power supply for driving a head to a head coil for a predetermined time by ON / OFF control of a head driver switching element. In the circuit, a DC / DC converter for converting an input DC power having an input voltage higher than the power supply voltage into an output DC power having an output voltage substantially equal to the power supply voltage, and when the head driver switching element is turned off. Input means for inputting an induced voltage generated in the head coil to the DC / DC converter;
A head drive circuit comprising: a return means for returning the output DC power of the C / DC converter to the head drive power source. 2. The first rectifier, wherein the input means becomes conductive when the induced voltage is generated in the head coil, and causes a current to flow in one direction from the head coil to an input end of the DC / DC converter. 2. The head drive circuit according to claim 1, further comprising a second rectifying element, which has an element, and wherein the return means causes a current to flow in one direction from an output end of the DC / DC converter to the head drive power source. 3. An input voltage adjusting circuit for adjusting the input voltage of the DC / DC converter to a predetermined voltage higher than the voltage of the head driving power supply.
3. The head drive circuit according to any one of 1 and 2. 4. The DC / DC converter has an input-side capacitor for smoothing the input voltage, and the input voltage adjusting circuit inputs the input voltage before starting the printing operation and during the printing operation. The head drive circuit according to claim 3, further comprising a charging circuit that charges a side capacitor to the predetermined voltage. 5. The head drive circuit according to claim 4, wherein the charging circuit constantly applies the predetermined voltage to the input side capacitor. 6. The initial charging circuit, wherein the charging circuit charges the input side capacitor to the predetermined voltage before starting the printing operation, and the charging voltage of the input side capacitor during the printing operation. The head drive circuit according to claim 4, further comprising an input voltage holding circuit that holds a predetermined voltage. 7. The head driver switching element and the head coil also serve as the charging circuit, and at least before the printing operation is started, the head driver switching element is driven at a frequency high enough not to drive the print wire. By repeating the ON / OFF operation, the turn-OF of the head driver switching element is turned off.
5. The head drive circuit according to claim 4, wherein the induced voltage generated in the head coil is repeatedly applied to the input side capacitor every F hours to charge the input side capacitor to the predetermined voltage. 8. The charging circuit has a charging coil, and a coil driver switching element for performing ON / OFF control of applying and not applying a power supply voltage of the head driving DC power supply to the charging coil. , An input voltage holding circuit configured to apply an induced voltage generated in the charging coil to the input side capacitor when the coil driver switching element is turned off, and at least during a printing operation, By repeatedly turning on / off the coil driver switching element, an induced voltage generated in the charging coil is repeatedly applied to the input side capacitor every time the coil driver switching element is turned off, so that the input side capacitor The device according to claim 4, which is configured to hold a charging voltage at the predetermined voltage. Mounted head drive circuit.