JPS61163874A - Head drive power supply protection circuit of line thermal printer - Google Patents

Abstract

PURPOSE:To protect against the heating of a power supply and other parts when any fault occurs by providing a means to charge/discharge a condenser and a means to stop the feeding of an output current from a power supply by a voltage at both ends of a condenser. CONSTITUTION:While a normal printing operation is carried out, a prearranged current is supplied to a thermal printing head 2 without the turning ON of a transistor Q2 regardless of a required degree of current for printing. When a voltage V3 has dropped below -Vs continuously due to an accident, the output voltage V5 of a comparator 3 reduces to a zero level and a transistor Q1 is turned OFF. Then a condenser C is charged resulting in the increase of a voltage V6 to (Vz+VBE) after the elapse of a prearranged time. Consequently, the transistor Q2 is switched ON and the voltage V1 of a power supply 1 is set at zero level. The voltage V1 of the power supply 1 continuous to be cut off unless the power supply is reset as a function. Therefore, an output current reaches a zero level after a prearranged time thus eliminating risks such as power supply breakdown or abnormal temperature rise.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a power supply protection circuit for driving a head of a line thermal printing device, and is a circuit that protects a power supply for driving a head mainly by a switching regulator used in a line thermal printing device such as a facsimile or a thermal printer. Regarding.

BACKGROUND OF THE INVENTION In recent years, electricity has been applied to predetermined heads of approximately 500 to 2,000 thermal printing heads arranged in parallel at the same time, and for each line, the predetermined heads heat an ink film to melt the ink and transfer it to paper. Various thermal printing devices such as line printers have been developed.

In such a line printer, the peak output current of the driving power supply is extremely large, approximately 30A to 50A, and the ll11[I current value of the power supply protection circuit must be greater than or equal to this peak output current value.

Problems to be Solved by the Invention However, if the control current value of the power supply protection circuit is set as described above, if the output current increases abnormally due to some kind of accident, several hundred W of power will be continuously supplied, and the power supply and other There was a problem in that the heat generated in the parts could lead to abnormal temperatures, creating a risk of burns or fire.

SUMMARY OF THE INVENTION An object of the present invention is to provide a power supply protection circuit that can prevent the power supply and other parts from generating heat in the event of an accident.

Means for Solving the Problems In FIG. 1, a resistor R1 is a means for detecting the output current of a power supply 1, and a voltage comparator 3. The transistor Q+ is a means for charging and discharging the capacitor C, and the transistor Q2 is a means for stopping the output current supply of the power supply 1 by the voltage across the capacitor C.

At the working resistance R+, the power supply output current that does not exceed the normal time value, or returns below the threshold after the head energization period for one line has passed even if it exceeds the normal time value, and exceeds the threshold for more than the abnormal head energization period, is detected, and the voltage comparator 3゜Transistor Q1 charges the capacitor at a predetermined time constant while the detected power supply output current exceeds the above threshold, and when the detected power supply output current becomes below the above threshold, rapidly discharges the capacitor,
At the transistor Q2, the supply of the power supply output current is stopped during a period when the voltage across the capacitor exceeds a certain value.

Example FIG. 1 shows a circuit diagram of an embodiment of the circuit of the present invention.

In the figure, 1 is a power supply that supplies current to a predetermined head of the thermal printing head 2. As shown in FIG. In this case, in a line printer, the power output current is generally stopped for a certain period of time (head cooling period) until the head starts printing one line of tangent lines. 3 is a voltage comparator, and current detection resistor R1
When the voltage V3 across the voltage ranges from zero to the threshold voltage -Vs, the output voltage Vs is the power supply voltage 10Vcc, and when the voltage V3 becomes lower than the threshold voltage -VS, the output voltage V5 becomes zero. .

The operation will be explained below with reference to FIG.

During the period from time t1 to t2, the current necessary for printing the first line flows, and the voltage across the resistor R+ (3) decreases as shown in Figure 2 (A>).In this case, the number of heads required for printing is relatively small. Therefore, the current does not flow that much, the voltage v3 does not go below -Vs, and the output voltage ■5 of the comparator 3 is as shown in the same figure (B).
As shown in the figure, the voltage remains at Vcc, and the transistor Q1 is turned on.

The period from time t2 to t3 is a head cooling period, and the resistance R
The voltage across the + terminal ■3 returns to zero.

Next, the current necessary for printing the second line flows during the period from time t1 to time t4, and the voltage (3) drops again.

In this case, since a relatively large number of heads are required for printing, a large amount of current flows, the voltage v3 becomes -V's or less, the output voltage 5 of the comparator 3 becomes zero, and the transistor Q1
is turned off. As a result, the capacitor C is charged via the resistor R3.

The terminal voltage (6) increases as shown in FIG. 4(C). However,
Since printing stops at time t4, the voltage ■3 is set to zero, and the output voltage ■5 of the comparator 3 is set to +Vcc, turning on the transistor Q1. As a result, the voltage v6 of the capacitor C becomes the voltage at which the transistor Q2 turns on, that is, the voltage at which the transistor Q2 turns on. (Vz
+VB E ) (Vz is the voltage across Zener diode D.

Vaε is the base-emitter voltage of transistor Q2)
Before this occurs, the voltage is rapidly discharged through the transistor Q1, and the voltage 6 becomes zero.

The above operation is repeated while normal printing is being performed, and the transistor Q2 is not turned on regardless of the magnitude of the current required for printing, and a predetermined current is supplied to the thermal printing head 2.

At time t5, due to some kind of accident, the voltage ■3 is continuously -
Suppose that it becomes below Vs. This allows comparator 3
The output voltage V5 becomes zero, the transistor QI is turned off, the capacitor C is charged, and the voltage 6 reaches the voltage (Vz +Vs E ) at time t6.

As a result, the transistor Q2 is turned on, and the voltage V+ of the power supply 1 is brought to zero as shown in FIG. Since the voltage 1 of the power supply 1 has a function of continuing to be cut off unless the power supply is reset, the output current becomes zero after time 6, and there is no risk of damage to the power supply or abnormal temperature rise. Note that the time required for printing (j+ ~ t2. t3 ~ t4,
...) is about 5 to 10n+s, and the time t5 to t
6 is about 20 to 4Qn+s.

Effects of the Invention The circuit of the present invention is a circuit that detects a power output current that does not exceed a normal time value, or returns below a threshold value after a head energization period for one line has passed even if it exceeds a normal time value, and that exceeds a threshold value for more than the head energization period in an abnormal state. , a circuit that charges a capacitor at a predetermined time constant while the detected power supply output current exceeds the above threshold, and rapidly discharges the capacitor when the detected power supply output current becomes less than the above threshold; Since the circuit is configured with a circuit that stops the supply of the power output current for a period when the power supply output current exceeds a certain value, for example, if a short-circuit accident of the power supply output occurs, the power supply output current supply will be stopped, causing burns or fires due to abnormal heat generation of the power supply, etc. It is possible to prevent the power supply from being destroyed, and even if the head drive signal system malfunctions, it can prevent the head from being continuously energized, thereby preventing the head from generating heat.

It has features such as being able to prevent damage.

[Brief explanation of the drawing]

1 and 2 are a circuit diagram of an embodiment of the circuit of the present invention and a signal waveform diagram for explaining its operation, respectively. DESCRIPTION OF SYMBOLS 1...Power source, 2...Thermal printing head, 3...Voltage comparator, R1...Resistor for current detection, C...Capacitor, Q+, Q2...Transistor.

Claims (1)

[Claims] In the head drive power supply protection circuit of a line thermal printing device that prints by repeating head energization and de-energization for each line printing, the normal threshold is not exceeded, or even if it is exceeded, the head power is turned on after one line's head energization period has elapsed. A circuit detects a power supply output current that returns below the threshold value and exceeds the threshold value for more than the head energization period in the event of an abnormality, and a capacitor at a predetermined time constant for a period during which the power supply output current detected by the circuit exceeds the threshold value. A circuit that charges the capacitor and rapidly discharges the capacitor when the power supply output current detected by the circuit becomes less than the threshold value, and a circuit that stops supplying the power supply output current for a period when the voltage across the capacitor exceeds a certain value. A power supply protection circuit for driving a head of a line thermal printing device, characterized by comprising a circuit.