JP2784274B2 - Power protection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply protection device for supplying a voltage to a load such as a thermal head.

[0002]

2. Description of the Related Art For example, in a thermal head, as shown in FIG. 3, several hundred resistors 1 as heating elements for converting electric energy into heat energy are arranged in parallel, and each resistor 1 has a switching element as a switching element. Transistors 2 are connected in series, and a predetermined power supply voltage is applied to both ends of each resistor 1, and ON / OFF of each transistor 2 is individually switched and controlled with timing by a latch circuit 3 to perform printing. is there. In the figure, reference numeral 4 denotes a power supply voltage input terminal.

In this device, the electric power applied to one resistor 1 is usually 0.5 to 1.0 W (watt). For example, when 1 W of power is applied to 500 resistors 1, 50
0 W of power is consumed. At this time, when the power supply voltage is 24 V, a current of about 20 A (ampere) flows, and a considerably large power supply is required.

However, it is rare that the thermal head is used while all the resistors 1 constituting the thermal head are energized. In particular, in a printer for printing characters, the average use efficiency of one resistor is about 10%. In some cases, such as the printing of ruled lines, the usage efficiency may increase to nearly 100%, but this is an extremely short time. Therefore, a relatively small power source is used as the power source of the printer, which is designed so that the average capacity is set to 10% of the usage efficiency and a large power supply with the usage efficiency of 100% can be handled in a short time.

[0005] In this case, however, if the state of use efficiency of 100% occurs for a long time and exceeds the capability of the power supply, the power supply voltage drops. Nevertheless, continued use may cause a failure of the power supply, and in the worst case, it may lead to dangerous conditions such as smoking and ignition.

Therefore, conventionally, a power supply protection device has been considered which protects the power supply by preventing the power supply to the heating element (resistor 1) for a long time. FIG. 5 is a circuit configuration diagram of a conventional power supply protection device of this type. A processor 5 constituting a control unit of the printer outputs an ON signal or an OFF signal of a transistor to each latch circuit 3 of the thermal head 8 via a head driver 7 connected via a bus line 6, A head strobe signal A of negative logic is output as a control signal for controlling power supply to the head 8. The head strobe signal A is connected to one input terminal of a negative logic two-input AND circuit (hereinafter abbreviated as NAND gate) 9 and a one-shot multi-input circuit.
The signal is input to a trigger terminal T of a vibrator (hereinafter abbreviated) 10. The vibrator 10 outputs a one-shot pulse determined by the value of the charging resistor 11 and the capacitance of the capacitor 12 at the fall of the head strobe signal A supplied to the trigger terminal T. The output from the negative logic output terminal Q0 The pulse signal B is N
The signal is input to the other input terminal of the AND gate 9. NAN
The output signal C of the D gate 9 is applied to the base of the PNP transistor 14 via the resistor 13. A power supply voltage generated from a head voltage generation circuit 15 which is a power supply for the thermal head 8 is applied to an emitter of the transistor 14, and a collector is connected to a power supply voltage input terminal 4 of the thermal head 8. Here, the transistor 14
Constitutes switching means for switching whether or not the power supply voltage generated by the head voltage generation circuit 15 is supplied to the thermal head 8.

The processor 5 lowers the head strobe signal A at the timing of printing by the thermal head 8 (time points t1, t3, t5 in FIG. 4).
t7). Then, the output pulse B of the vibrator 10
Changes to the low level, and the output signal C of the NAND gate 9 changes to the low level accordingly. As a result, the transistor 14 is turned on, and the power supply voltage of, for example, +24 V generated from the head voltage generation circuit 15 is supplied to the thermal head 8.

Usually, the head strobe signal A returns to a high level after a lapse of a predetermined time T0. On the other hand, the pulse width T1 of the output pulse B of the vibrator 10 is determined by the charging time of the capacitor 12. The pulse width T1 of the output pulse B is set to be slightly longer than the low level time T0 of the head strobe signal A. Therefore, the output signal C of the NAND gate 9
Normally changes to a high level in response to the rise of the head strobe signal A (time points t2, t4, t in FIG. 4).
6). As a result, the transistor 14 is turned off, and the power supply voltage is not supplied to the thermal head 8.

However, there is a case where the head strobe signal A does not rise even after the predetermined time T0 has elapsed due to runaway of the processor 5 or the like. In such a case, the output signal C of the NAND gate 9 becomes the output pulse B of the vibrator 10.
Changes to a high level in response to the rising edge of the signal (at time t8 in FIG. 4). As a result, the transistor 14 is turned off, and the power supply voltage is not supplied to the thermal head 8.

As described above, by using the conventional power supply protection device, the ON time of the transistor 14 can be limited within a predetermined time, and the supply of the power supply voltage to the thermal head 8 for a long time can be prevented. .

[0011]

However, in the conventional power supply protection device described above, the head strobe signal becomes abnormal due to runaway of the processor 5 or the like, and the continuous power supply time to the thermal head 8 becomes longer, thereby reducing the power supply capability. It is effective when the power supply voltage exceeds the power supply capacity. However, it is impossible to cope with the case where the use efficiency of the thermal head 8 is high for a long time and the power supply capacity is exceeded for a long time even though the head strobe signal is normal and the power supply voltage is lowered. Was.

Therefore, according to the present invention, a circuit for supplying a power supply voltage to a load through a switching means that is controlled to be switched on and off by a control signal from a control unit maintains a state in which the load usage efficiency is high for a long time. It is an object of the present invention to provide a power supply protection device that can reduce power consumption when the power supply voltage exceeds the capability of the power supply and the power supply voltage can be reduced, thereby preventing the power supply from going down and improving reliability.

[0013]

According to the present invention, there is provided a power supply protection device for a circuit for supplying a power supply voltage to a load via a switching means which is switched on and off by a control signal from a control unit. Voltage detecting means for detecting, voltage comparing means for comparing the power supply voltage detected by the detecting means with a preset reference voltage, and switching means when the power supply voltage becomes smaller than the reference voltage as a result of the comparison by the comparing means. And on-time limiting means for shortening the on-time of the control signal compared with the control signal.

[0014]

According to the present invention having such a configuration, when the use efficiency of the load is high for a long time and exceeds the capability of the power supply, and when the power supply voltage decreases and becomes lower than the reference voltage, the ON time of the switching means is reduced. Becomes shorter than the on-time by the control signal. As a result, the supply time of the power supply voltage to the load is shortened, so that the energy consumption of the power supply is reduced and the power down is prevented.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention to which a power supply protection device for a thermal head in a thermal printer is applied will be described with reference to the drawings.

FIG. 1 is a circuit diagram of a power supply protection device according to this embodiment. The same parts as those in FIG. 5 are denoted by the same reference numerals, detailed description thereof will be omitted, and only different parts will be described. In FIG. 1, reference numeral 16 denotes a resistor 16a and a resistor 1
6b interposed between the output terminal of the head voltage generation circuit 15 and the ground level. A power supply voltage generated by the head voltage generation circuit 15 is divided by the resistors 16a and 16b. A voltage detecting means for detecting based on the pressure ratio is configured. Reference numeral 17 denotes a comparator which connects the non-inverting input terminal (+) to each of the resistors 16a and 16
b, and the inverting input terminal (-) is connected to the output terminal of a reference voltage generating circuit 18 for generating a preset reference voltage. That is, the comparator 17 is connected to the head voltage generating circuit 15 detected by the voltage dividing circuit 16.
And a voltage comparing means for comparing the power supply voltage of the reference voltage with a preset reference voltage of the reference voltage generating circuit 18. The output terminal of the comparator 17 is connected to the base of a PNP transistor 20 via a resistor 19. The emitter of the transistor 20 is connected to a +5 V DC power supply terminal, and the collector is connected via a resistor 21 to the vibrator 10.
Is connected to the connection point between the charging resistor 11 and the capacitor 12. Here, the transistor 20, the charging resistors 11 and 12,
As a result of comparison by the comparator 17, the capacitor 12, the vibrator 10, and the NAND gate 9 change the power supply voltage from the head voltage generation circuit 15 to the reference voltage generation circuit 18.
A limit value changing means for shortening the ON time limit value of the transistor 14 as the switching means when the voltage falls below the reference voltage from the control circuit.

In the apparatus of this embodiment having such a configuration, the head strobe signal A falls at the timing of printing by the thermal head under the control of the processor 5 (time points t1, t3, t5, t8, t10, and t10 in FIG. 2). t1
2). Then, the output pulse B of the vibrator 10 changes to low level, and the output signal C of the NAND gate 9 changes to low level accordingly. As a result, the transistor 14 is turned on, and the power supply voltage of, for example, +24 V generated from the head voltage generation circuit 15 is supplied to the thermal head 8.

Normally, the head strobe signal A returns to a high level after a lapse of a predetermined time T0. On the other hand, the pulse width of the output pulse B of the vibrator 10 is determined by the charging time of the capacitor 12. Here, when the power supply voltage v1 of the head voltage generation circuit 15 detected by the voltage dividing circuit 16 exceeds the reference voltage v2 of the reference voltage generation circuit 18, the output signal D of the comparator 17 is at a high level, and the transistor 20 is turned off. ing. Therefore, the capacitor 1 is charged by the charging current i1 flowing through the charging resistor 11.
2 is charged. The pulse width at this time is T1 (> T0)
become. On the other hand, when the power supply voltage v1 of the head voltage generation circuit 15 falls below the reference voltage v2 of the reference voltage generation circuit 18 (time t7 in FIG. 2), the output signal D of the comparator 17 is output.
Is inverted to a low level. As a result, the transistor 20 is turned on, and a parallel circuit of the charging resistors 11 and 21 is formed. Therefore, the charging resistor 11 and the charging resistor 2
The capacitor 12 is charged by the charging current i1 + i2 flowing through the capacitor 12 respectively. The pulse width at this time is T2
(<T0 <T1).

Therefore, when the power supply voltage v1 of the head voltage generation circuit 15 exceeds the reference voltage v2 of the reference voltage generation circuit 18, the output signal C of the NAND gate 9 changes to a high level in accordance with the rise of the head strobe signal A. (Time points t2, t4, t6 in FIG. 4). As a result, the transistor 14 is turned off, and the power supply voltage v1 is not supplied to the thermal head 8.

On the other hand, when the power supply voltage v1 of the head voltage generation circuit 15 falls below the reference voltage v2 of the reference voltage generation circuit 18, the output signal C of the NAND gate 9 changes in response to the rise of the output pulse B of the vibrator 10. It changes to the high level (time points t9 and t11 in FIG. 4). This allows
The transistor 14 is turned off, and the power supply voltage v1 is not supplied to the thermal head 8.

When the power supply voltage v1 of the head voltage generation circuit 15 is higher than the reference voltage v2 of the reference voltage generation circuit 18, even if the head strobe signal A elapses a predetermined time T0 due to runaway of the processor 5 or the like. If it does not rise, output signal C of NAND gate 9 is output as in the prior art.
Changes to a high level in response to the rise of the output pulse B of the vibrator 10. Thereby, the transistor 1
4 is turned off, and the power supply voltage v1 is no longer supplied to the thermal head 8.

As described above, according to this embodiment, when the power supply voltage v1 of the head voltage generation circuit 15 decreases and falls below the reference voltage v2 of the reference voltage generation circuit 18, the charging time of the capacitor 12 in the vibrator 10 is reduced. Is accelerated, the pulse width of the output pulse B is reduced, and the on-time of the transistor 14 is shortened. Therefore, the energy consumption of the head voltage generation circuit 15 is reduced, and the head voltage generation circuit 15 is hard to be down. Therefore, the head strobe signal A is effective not only when the head strobe signal A becomes abnormal due to runaway of the processor 5 and the continuous power supply time to the thermal head 8 becomes long but exceeds the power supply capability. Even when the thermal head 8 is used normally, the state in which the usage efficiency is high continues for a long time and exceeds the capability of the head voltage generating circuit 15, and the energy consumption is reduced even when the generated voltage v1 of the head voltage generating circuit 15 decreases. It is possible to prevent the power supply from being reduced by reducing the power consumption, and it is possible to respond effectively. As a result, the reliability of the printer can be improved.

In the above embodiment, the thermal head 8 is shown as the load to which the power supply voltage is supplied. However, the present invention is not limited to this. For example, a fluorescent display tube having a plurality of fluorescent elements arranged, an EL display, (Electroluminescence) The power supply protection device of the present invention can also be applied to an edge emitter array head, which is a line head in which a plurality of elements are arranged and arrayed. Further, in the above-described embodiment, an example has been described in which the power supply protection device of the present invention is configured by hardware, but it is also possible to configure the power protection device by software. In addition, it goes without saying that various modifications can be made without departing from the spirit of the present invention.

[0024]

As described above in detail, according to the present invention, a circuit for supplying a power supply voltage to a load through a switching means that is controlled to be switched on and off by a control signal from a control unit is provided. In the case where the state of high use efficiency continues for a long time and exceeds the capacity of the power supply and the power supply voltage is lowered, the power consumption can be reduced, the power supply can be prevented from being reduced, and the power supply protection device that can improve the reliability can be provided. Further, according to the second aspect of the present invention, the above effect can be obtained by also using the pulse generating means for generating the pulse signal for limiting the ON time of the switching means so as not to be continued for a predetermined time or more.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a circuit configuration according to an embodiment of the present invention.

FIG. 2 is a main signal waveform diagram in the embodiment.

FIG. 3 is a configuration diagram of a general thermal head.

FIG. 4 is a main signal waveform diagram in a conventional device.

FIG. 5 is a block diagram showing a circuit configuration of a conventional device.

[Explanation of symbols]

5 Processor, 7 Head driver, 8 Thermal head (load), 9 NAND gate, 10 One-shot multivibrator, 11, 21 Charge resistance, 1
Reference numeral 2 denotes a capacitor, 14 denotes a PNP transistor (switching means), 15 denotes a head voltage generating circuit, 16 denotes a voltage dividing circuit, 17 denotes a comparator, and 18 denotes a reference voltage generating circuit.
20 ... PNP type transistor.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B41J 2/35-2/38 H02H 9/02

Claims (2)

(57) [Claims] 1. A power supply protection device for a circuit for supplying a power supply voltage to a load via a switching means that is switched on and off by a control signal from a control unit, comprising: a voltage detection means for detecting the power supply voltage; Voltage comparison means for comparing the power supply voltage detected by the detection means with a preset reference voltage; and, as a result of the comparison by the comparison means, when the power supply voltage becomes smaller than the reference voltage, the on-time of the switching means is reduced. A power supply protection device, comprising: an on-time limiting unit that shortens the ON time limit than the control signal. 2. A pulse having a pulse width longer than a normal on-time of the control signal to a circuit for supplying a power supply voltage to a load via a switching means that is switched on and off by a control signal from a control unit. A power supply protection device comprising a pulse generation means for generating a signal, wherein an on-time of the switching means is limited so as not to be continuous for more than a pulse width of the pulse signal to protect a power supply. Detecting means, voltage comparing means for comparing a power supply voltage detected by the detecting means with a preset reference voltage, and as a result of the comparison by the comparing means, when the power supply voltage becomes smaller than the reference voltage, the pulse generation Limit value changing means for shortening the pulse width of the pulse signal generated by the means from the normal on-time of the control signal. Power protection apparatus according to claim and.