JPS6264570A - Protective circuit for thermal head - Google Patents

Abstract

PURPOSE:To prevent a thermal head from being over-energized, by providing a time constant circuit for giving a time constant according to the duty on a thermal head to the trailing edge of an output signal from an OR circuiting for ORing printing signals, and energization time is limited by an output from a monostable multivibrator triggered by an output from the time constant circuit. CONSTITUTION:When the duty of a printing signal Pi exceeds a limit value due to the runaway of a printing-controlling circuit 7 or the like, formula (1) is established, wherein tON is energization time, tOFF is non-energization time, tON(L) is energization time in a period under limit energization, and tOFF(L) is non-energization time in the period under limit energization. For the trailing edge of a signal POR, the time constant RC of a time constant circuit 9 is so set that the period of time tOFF(L) is required for the signal POR to reach the input threshold level (a) of the monostable multivibrator 10. Therefore, due to the time constant RC, the signal POR is not lower than the input threshold level (a) of the multivibrator 10 even in the period tOFF. In this case, also, the period of time for which a voltage is impressed on a thermal head does not exceeds tON(L) due to the width of an output pulse S from the multivibrator 10.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement in a protection circuit for a thermal head that prevents overcurrent to the thermal head.

[Conventional technology]

In a thermal printer, when a control circuit goes out of control due to noise or the like, excessive current is applied to the thermal head, which may burn out the expensive thermal head or shorten its life. To prevent this, a thermal head protection circuit is provided.

Conventionally, as a protection circuit for a thermal head, for example, the seventh
There was something shown in the figure.

In Figure 7, 1 is the head heating element resistance, 2 is the inverter, and 3
4 is a print control circuit, 5 is a mono-multivibrator, and 6 is an energization limiter.

The number of head heating element resistors 1 from 11 to 1n corresponds to the number of dots in the thermal head.

Inverter 2 has head heating element resistance l from 21 to 2n.
There are an equal number of This inverter 2-2 is
The print signals P1n to P output by the print control circuit 4 are inverted and sent to the head heating element resistors 11 to ln.

n Print signals P1 to Pn are signals corresponding to characters to be printed.

The head application power source 3 is connected to the head heating element resistors 11 to 1 nK via the energization limiting means 6.

The print control 1 circuit 4 outputs the above-mentioned friend print signals P1 to F and the trigger signal T for the mono multivibrator 5. The print control circuit 4 also sends an operation signal K to a printer mechanism (not shown), which is a mechanism for feeding one sheet of paper while the carriage is running.

The mono multivibrator 5 is triggered by the trigger signal T and outputs a pulse signal with a constant time width.

The energization limiter R6 includes an inverter υ3, a resistor R, and a second
The output of the mono multivibrator 5 is inverted.

This output is applied to the base of transistor Q via resistor R3. The transistor Q, the power supply 3 for the head, and the head heating element resistance print signal P for the printer.
A trigger signal T is output at the same time as 1 to pn, and the mono multivibrator 5 is made to output a pulse signal with a constant time width.

This pulse signal drives the transistor Q for a certain period of time, thereby limiting the energization time of the head application power source 30.

[Problem that the invention seeks to solve]

However, in such a protection circuit, if the print signal P continues to turn ON due to runaway of the print control circuit, the trigger signal may also be synchronized, but depending on the runaway, the trigger signal may turn ON in a short period.

If the OFF operation is repeated, the thermal head may become constantly energized.

Further, if the trigger signal T and the print signal P are synchronized and repeatedly turn on and off in short cycles and with heavy duty, the thermal head cannot be protected.

For this reason, there has been a problem in that the thermal head can only be protected from abnormalities in power supply caused by a limited runaway mode.

The present invention aims at a rounding solution that eliminates the above-mentioned problems.

[Means for solving problems]

In a thermal head protection circuit that prevents over-current of the thermal head, the present invention includes an OR circuit that ORs print signals corresponding to characters to be printed, and a thermal head a time constant circuit that provides a time constant according to the duty of the mono multivibrator that is triggered by the output of the time constant circuit and outputs a pulse signal with a time width less than the energization limit time of the thermal head; energization time limiting means for limiting the energization time to the 4-multi head by the output of the multivibrator;
It has nine thermal head protection circuits.

〔Example〕

The present invention will be explained below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of a thermal head protection circuit according to the present invention. Components in FIG. 1 that are the same as those in FIG. 7 are given the same reference numerals.

In FIG. 1, 7 is a print control circuit, 8 is an OR circuit,
9 is a time constant circuit, and 10 is a mono multivibrator.

The print control circuit 7 is a circuit centered on a microprocessor, and outputs and controls print signals P1 to Pn and a drive signal for the printer mechanism.

The OR circuit 8 is composed of diodes D1 to D. The input side of the diodes D1 to Do is the print signal P1.
~Pn transmission lines, respectively, and their output sides are commonly connected to the time constant circuit 9. These diodes D
1 to DnK, the OR circuit 8 outputs the print signals P1 to Pn.
Take the logical sum and output.

The time constant circuit 9 consists of a resistor B and a capacitor C. One end of the resistor R and the capacitor C is connected to the signal transmission path between the OR circuit 8 and the monomultiviprator 100, and the other end is grounded. The time constant circuit 9 has a fixed time constant at the fall 9 of the output of the OR circuit 8. How to determine the time constant will be described later. The output of such a time constant circuit 9 is P. Let it be R.

The mono-multivibrator IO is triggered by the signal PoR and outputs a pulse signal S with a constant time width. This time width is determined by the resistance value of the resistor RM and the capacitance of the capacitor C1, and is the limit value t of the current-carrying time of the thermal head. N(LJK).

Next, the operation of such a thermal head will be explained.

First, a case will be described in which the thermal head is normally energized.

In this case, the actual energization time t. N and energizing duty D
are both below the limit value. Here 7, afif
s bow-aDf) @ limit value yu, limit energization.

t is the time during which the current is not energized in the cycle. P and ■ can be obtained from the following equation-6.

A time chart of each signal in a normal case is shown in FIG. 2. In FIG. (
c) Output signal of tf mono multivibrator 10.

(d) is a time chart of the voltage Vn applied to the thermal head, and (e) is a time chart of the current flow Hi to the thermal head. Right here! 'Hi NvI (/RH1)'
``Hi'' is the resistance value of the head heating element resistance 1i (representative of 11 to 1n).

FIG. 2(a) The energization time indicated by K is t. N, and the time when the power is not energized is t. 1. The print signal Pi is given a time constant at the time of falling by the time constant circuit 9 and becomes a signal as shown in FIG. 2(b). Triggered by R and time width t. N's pulse signal 8
Output. The transistor Q is driven by this signal 8, and the thermal head is energized for a limited time t. The head applied voltage is applied at the limit value of N and D. The head heating resistor 1i is turned on and off by turning on and off the drive signal Pi.
Follow off.

Next, abnormal case (■) will be explained. In this case, the print signal pt remains on for longer than the limit time due to the print control circuit running out of control. A time chart of the signals in this case is shown in FIG.

As shown in FIG. 3, even if the print signal PM continues to be in the ON state beyond the limit time 'ON plow, voltage is applied to the paper-making thermal head due to the time width of the output pulse of the 7-1 multivibrator 10. The time to call is by phone. Do not exceed NCLJ.

Next, abnormal case ■ will be explained. In this case, print f! IIJ i11 Due to runaway circuit, printing signal P
This is a case where the duty of l exceeds the limit value. A time chart of signals in this case is shown in FIG. In this case, it becomes .

Here, signal P. As shown in FIG. 5, the time constant RC of the time constant circuit 90 is set so that the falling edge of R takes as much time as the OFF state to reach the input current level of the mono-multivibrator IO.

Therefore, in the case of ■, the time constant RCK and the signal P
does not fall below the input Casrec cold level even at tOFF time.
This rounding gives the same result as ■. In this case as well, the time during which the voltage is applied to the paper thermal head depends on the time width of the output pulse S of the mono-multivibrator IO. Do not exceed N plow.

In the embodiment, a case will be described in which the energization control means 6 including the inverter U8, the resistor R8, and the transistor q is configured to limit the energization time of the thermal head. ! As shown in IK,
The output S of the mono-multivibrator IO and the print signal pt may be input to a Nant gate NK, and the head heating element resistor RH1 may be energized by the output of this Nant gate N.

Furthermore, the time width of the output pulse of the mono-multivibrator IO may be less than or equal to the energization limit time of the thermal head.

〔effect〕

According to the thermal head protection circuit according to the present invention, the following first-order effects can be obtained.

That is, if the print signal Pi continues to be in the ON state beyond the energization limit time, the mono multivibrator 10 limits the entire energization time by the time width of the output pulse.

Also, if the duty of the print signal exceeds the limit value,
The print signal is smoothed by the time constant circuit 9, similar to the case described above, and the mono-multivibrator 10 limits the energization time from the output pulse time of 1 mK.

For this reason, it is possible to widely prevent overcurrent in the head heating element resistance caused by runaway of the print control circuit, etc., and it is possible to effectively protect the thermal head, which is a core component of a thermal printer.

In addition, the drive signal itself can cause the mono multivibrator lO
The thermal head can be protected from burnout regardless of whether it is rolled or runaway.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of a protection circuit for a thermal head according to the present invention, FIGS. 2 to 5 are diagrams explaining the operation of the circuit in FIG. 1, and FIG. FIG. 7 is a block diagram of a conventional example of a protection circuit for a thermal head. 1.11-1n...Head heating element resistance, 6...Electrification time limiting means, 7...Print control circuit, 8...OR circuit, D1-D,...diode, 9... -Time constant circuit, 10...Printing control circuit. 5-1. Agent Patent attorney Nobuaki Ozawa 1(・1
1,71 Figure 1 Figure 6 Figure 7

Claims (1)

[Claims] A thermal head protection circuit for preventing over-current of the thermal head, comprising: an OR circuit that ORs print signals corresponding to characters to be printed; and a falling edge of the output signal of the OR circuit. a time constant circuit that provides a time constant according to the duty of the thermal head; a mono-multivibrator that is triggered by the output of the time-constant circuit and outputs a pulse signal with a time width less than the energization limit time of the thermal head; A protection circuit for a thermal head, comprising: energization time limiting means for limiting the energization time to the thermal head by the output of a vibrator.