JPS5964374A - Apparatus for driving thermal head - Google Patents

Abstract

PURPOSE:To prevent the damage of a thermal head, by providing a detecting circuit for detecting the power source voltage state of a logical circuit for controlling the heat generation of the thermal head and a cut-off circuit for cutting off power source voltage of the basis of the output therefrom. CONSTITUTION:The state of the second power source voltage V1 of a logical circuit 6 is detected by a detecting circuit 10 which is, in turn, operated in such a way that a transistor T1 is turned ON so as to be delayed by the charging time of a condenser C1 at the rising time of the voltage V1 and directly turned OFF at the falling time thereof. The delay time at the rising time of the voltage V1 includes a time required before the logical circuit 6 is normally operated. A cut-off circuit 20 cuts off the output of the detecting circuit 10, that is, first power source voltage VH being the power source voltage for a thermal head on the basis of the output P of the transistor T1. When power source voltage is normal, a transistor T2 is under an OFF state by the output P of the detecting circuit 10 and a switching transistor 7 is operated corresponding to a heat generating signal HP while the thermal head 5 performs usual heat generating operation.

Description

[Detailed description of the invention] Technical details〉 The present invention relates to a thermal head driving device.

<Seibin Technology> Figure 1 shows a conventional device of this type. The power supply circuit (1) is
When the power switch 12) is turned on, the commercial power supply (3) is turned on.
The voltage from VH is stepped down, rectified, and smoothed to obtain the first power supply voltage VH.
and a nest 2 power supply voltage VL are generated, VH is applied to the length head 15) via a transistor chopper +4), and VL is applied to the logic circuit (6).

The thermal head (5) has a configuration in which a large number of series bodies each consisting of a heat generating resistor (5a) and a selection transistor (5b) are connected in parallel to a power supply line. The logic circuit (6) sends selection signals S1, S2, etc. to the thermal head (5) according to the data to be recorded, and at the same time sends a heat generation signal HD to the four switching transistors that control the operation of the transistor chopper (4). give. By applying these signals, the thermal head (5) generates heat in a predetermined pattern using the first power supply voltage VH as the heat generating power source. There is a risk that the thermal head (5) may be burnt out when the power supply voltage is unstable, such as when the power is restored. That is, for example, suppose that the 1'4 source voltage VH is close to normal due to instability, and the 2nd source voltage VL is in an abnormal state.
At this time, the operation of the logic circuit +61 is in an abnormal state, and the selection signals S1, S2, . . . and the heat generation signal HD are both output, and there is a possibility that the thermal head (5) is driven to generate heat. The time required for one heat generation under normal conditions is usually 1 to 2 milliseconds, but in abnormal conditions it often reaches 10 milliseconds.1. Such long-term heat generation immediately burns out the thermal head 15).

Conventionally, in order to eliminate the shortcomings that occur when power is turned off, when starting power supply to the power supply circuit (1), the first power supply voltage VH is raised after the second power supply voltage VL reaches a sufficiently normal value, and when the power is cut off, On the contrary, it has been proposed to drop VH without dropping VL before dropping VL, but such a power supply circuit structure is inevitably complicated.

<Disclosure of the Invention 2 The present invention avoids damage to the thermal head caused by the instability of the power supply voltage by simply adding a simple circuit without changing the power supply circuit itself. , a device that drives and controls the heat generation of a thermal head using the output of a logic circuit, which includes a detection circuit that detects a power supply voltage state of the logic circuit that causes an abnormal operation of the logic circuit; Figure 2 shows the main parts of this embodiment, which consists of a thermal head (5), a transistor chopper (4), and its switching transistor (power supply). are the same as in FIG. 1, and the first power supply voltage VH to the thermal head 15) and selection signal (S
1), (S2)... and the heat generation signal HD are also supplied from the power supply circuit (6) and the logic circuit (6) in FIG. 1, respectively.

The detection circuit (10), which is a feature of the present invention, is the above logic circuit (
6) Detecting the state of the second power supply voltage VL of the circuit that causes the abnormal operation. To explain more specifically, the detection circuit u
O) uses the second power supply voltage VL as a voltage source, and when VL rises, a charging current flows to the capacitor (C1) through the resistor (R1) and the diode (D2).

Therefore, no current flows through the Zener diode (1') s ) and the resistor (R2) during the trench where the potential of the capacitor→no (C+) is greater than that of the Zener diode (D5). Therefore, transistor (T+) l'i: is off. As charging of the capacitor (C1) progresses, current begins to flow through the Zener diode (I) 3), causing a voltage drop across the resistor (R2). This voltage is 0.6
When the voltage exceeds V, the transistor (T1) is turned on.

When vL falls, the current flowing through the resistor (R1), Zener diode (D5), and resistor (R2) decreases, turning off the transistor (T1). When rising, the capacitor (C1
), the transistor (T1) is turned on with a delay of charging time, and when VL falls, the transistor (T1) is turned on immediately.
It works to turn off. The delay time when VL rises above sufficiently includes the time required for the logic circuit (6) to operate normally. Furthermore, the diode (D+) discharges the charge of the capacitor (C1) by 2 when the power is turned off. Furthermore, the cutoff circuit (20), which is a feature of the present invention, determines the first power supply voltage VH, which is the power supply voltage for the thermal head, based on the output of the detection circuit (10), that is, the output P of the transistor (T1). Cut off. More specifically, when the power supply voltage is normal, the transistor (T2) is turned off by the output P of the detection circuit uO), the switching transistor (7) operates in response to the heat generation signal HD, and the thermal head (5) is normally turned off. After generating heat, assume that the power supply voltage is in an unstable state when power supply to the power supply circuit (1) (Fig. 1) starts, and at this time, vj1
If the power supply voltage VH rises to around the normal value before the second power supply voltage VL, at this time the initial detection circuit (10)
Therefore, the base current flows to the control transistor (T2) through the resistor (R4) using VH as the power source, turning on the single-core transistor ('1' 2 ) and turning on the switching transistor (power Therefore, even if the thermal head drive signal is output due to unstable operation of the logic circuit 16), the heat generating power to the thermal head (5) is cut off. Thereafter, the output P of the detection circuit UO+ is generated and the switching transistor (the logic circuit Wr+6 at the time when the power reaches the operable window) is operating sufficiently normally and there is no problem.

As another unstable state of the power supply voltage, suppose that the power to the power supply circuit (1) is turned off. At this time, the output P of the detection circuit (+13110) immediately disappears, so even if the first power supply voltage VH Even if the voltage is longer than the second power supply voltage Vl-, the resistor (R4) and transistor (T2
), the switching transistor (7) is forced into the OFF state, and the power supply to the thermal head (5) is cut off.

Effects> According to the present invention, it is possible to prevent burnout of the thermal head caused by instability of the power supply voltage with a simple configuration without making any changes to the power supply circuit itself.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional example, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. (5)...Thermal head, (6)...Logic circuit,
(10)...Detection circuit, (20)...Blocking circuit.

Claims (1)

[Claims] (1) A detection circuit that detects a power supply voltage state of the logic circuit that causes an abnormal operation of the logic circuit by transmitting the heat generation of the I-maru head to an IJE operation control device using the output of the logic circuit; A thermal head driving device characterized by being provided with a cutoff circuit that cuts off the power supply voltage for the thermal head based on the output.