JPS58150873A - Disconnection detecting circuit - Google Patents

Abstract

PURPOSE:To detect disconnection of signal source element in a measuring device such as thermocouple protecting it from influence of spike noises, offset and the like by employing an AC signal with a zero crossing as the starting point. CONSTITUTION:An AC voltage from an AC power source AC is turned to a square waveform turning ON or OFF at the zero crossing with a comparator C1 and then, applied to a clock terminal of a D type flip flop F to which a detection input signal is applied to a data terminal. A switch S is closed with a flip flop F and the AC voltage is applied to a series circuit of the power source AC, a resistance R, the switch S, a TC such as thermocouple to detect disconnection of signal source element in the TC sush as the thermocouple. This AC voltage with the zero crossing as the starting point prevents the generation of spike noises and AC components can be cancelled by processing the output of the TC such as thermocouple with an integral type A/D converter. This enables a better detection of disconnection of the signal source elements in the measuring device such as thermocouple protecting it from influence of spike noises and offset.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a disconnection detection circuit for detecting a disconnection accident in a signal source element such as a thermocouple in a measuring instrument using a thermocouple or the like.

A conventional circuit of this type has a thermocouple T, as shown in FIG.
The electromotive force generated at C is amplified by an amplifier l to obtain a signal output proportional to the temperature from a signal output terminal 2, and a DC voltage V is applied to both ends of the thermocouple Tc for detecting disconnection. By using the thermocouple TC itself as a voltage dividing resistor, a burnout output is obtained from a burnout output terminal 4 through a comparator 3 in response to a voltage change caused by a disconnection of the thermocouple.

However, the conventional circuit described above has a problem in that an offset occurs in the signal output, and another problem in that it is not possible to connect another meter in parallel to the thermocouple. In order to eliminate these problems, a switch 5 is provided in the middle of the wiring for applying the DC voltage V, and
It is possible to not perform input detection when detecting a disconnection, but in this case, spike noise will be added to the signal output, and if the instruments are connected in parallel, this spike noise will affect the other instrument. There is a problem.

The present invention has been made in view of the above points, and it overcomes the above-mentioned conventional problems, and enables parallel connection of instruments without the risk of generating offset or spike noise. The object of the present invention is to provide a disconnection detection circuit. That is, the present invention is characterized in that an alternating current signal for disconnection detection is intermittently applied to both ends of a signal source element such as a thermocouple, starting from a zero cross, in response to an external disconnection detection input signal. be.

Next, a first embodiment of the circuit according to the present invention will be described with reference to FIG.

In the figure, TC is a thermocouple as a signal source element,
Both ends of this thermocouple TC are connected to an amplifier 101, and the output side of this amplifier 101 is used as a signal output terminal 102. Further, the negative side of the thermocouple TC is grounded, and the positive side is connected to the AC voltage generator AC for disconnection detection via the voltage dividing resistor R of switch s4, and the voltage Vac of this AC voltage generator AC. is applied to a zero-cross detection comparator C1, and the output of the comparator C1 is applied to a switch operation circuit 105 formed by a D flip-flop F. The Q output terminal of this switch operation circuit 105 is connected to the opening/closing drive terminal of the switch S, and the D input terminal is used as the detection operation signal input terminal 103. The middle part is connected to the comparator C3 for disconnection determination, and this comparator C
The second output terminal is a burnout output terminal 104. Note that the voltage dividing resistor R and the resistance value Ra of the thermocouple TC are set to R>>R@, and the above-mentioned comparator C2 for disconnection determination is set to a voltage expressed by (R8/(R+R8)l It is set so that when the voltage is at a high level, the output is inverted and an output is obtained from the burnout output terminal.

The operation of the disconnection detection circuit configured as described above will be explained below.

Normally, the switch S is open, so the thermocouple T
signal output terminal 1° without any influence on C.
A normal measurement signal is obtained from 02. On the other hand, the AC voltage from the AC voltage generator AC is applied to the clock input side of the switch operation circuit 105 by the comparator C1 as a square wave signal that turns on and off at the zero cross point of the AC voltage. In a normal state where the thermocouple TC is not disconnected, when a signal is applied to the detection operation signal input terminal 103, the switch is operated for a time corresponding to the detection operation signal starting from the rise of the pulse wave from the humperator C0. A switch S input drive signal is applied from circuit 105. In this way, an alternating current with a sine curve from the alternating current voltage generator AC flows through the voltage dividing resistor R, switch S, and thermocouple TC in series, but the voltage waveform at this time always starts at the zero cross. It becomes. Therefore, there is no risk of spike noise occurring.
Even if you always take out the signal output, the signal output terminal 1
The signal output from 02 is converted to A-D by an integral type A-D converter.
If conversion is performed, the alternating current component will be canceled and there will be no possibility of offset.

In addition, the comparator C2 for determining disconnection includes a thermocouple TC.
The voltage divided by the voltage dividing resistor R is applied, but since the output inversion voltage of the comparator C7 for disconnection determination is set to a higher level than the above normal voltage, burnout occurs at this time. There is no disconnection detection output from the output terminal 104, which means that it can be recognized that the thermocouple TC is normal.

On the other hand, when a signal is applied to the detection operation signal input terminal 103 in a state where the thermocouple T=C is disconnected, the same operation as described above is performed, and an AC voltage is added to the voltage dividing resistor R and the thermocouple TC. However, because the thermocouple TC is disconnected, a voltage higher than normal is applied to the comparator C2 for determining burnout, and as a result, the burnout output terminal 104 of the comparator C8 for determining disconnection An out signal will be output. Note that even when the switch S is in the OFF state during normal operation, there is an output from the burnout output terminal 104, but when a signal is being applied to the detection operation signal input terminal 103, there is no output from the burnout output terminal 104. Since it is sufficient to detect a disconnection from the presence or absence of an output, the output when the switch S is in the OFF state does not cause any trouble. Furthermore, if it is desired to eliminate the output when the switch S is in the OFF state, a separate switch interlocked with the switch S may be provided between the power supply AC and the voltage dividing resistor 8.

Next, a second embodiment of the present invention will be described with reference to FIGS. 3 to 5. In this second embodiment, as in the first embodiment described above, both ends of a thermocouple TC as a signal source element are connected to an amplifier 101, and the output side of this amplifier 101 is used as a signal output terminal 102. However, the above thermocouple T
A CR filter 106 is provided between C and the amplifier 101. Further, the negative side of the thermocouple TC is grounded, and the positive side is connected to an AC voltage generator AC for disconnection detection via a switch S and a voltage dividing resistor R, and the voltage of this AC voltage generator AC is It is applied to the positive input side of the first zero-cross detection comparator C3 and the negative input side of the second zero-cross detection comparator C2, and the negative input side of the first zero-cross detection comparator C1 and the second zero cross Detection comparator C
The positive input sides of 1 are set to the same potential. Each output of each of the above-mentioned combiner ICs CI + CI is applied to a switch operation circuit 105 consisting of a first D flip-flop F1, a second D flip-flop F, and an OR circuit 107, and is applied to a switch operation circuit 105 for first zero-cross detection. The output side of the comparator C3 is connected to the clock input side CP of the first D flip-flop F1, and the output side of the second zero-cross detection comparator C1 fil GJ 2nd c7) D y I
It is connected to the clock input side CP of the Jypflop F. Furthermore, the Q of the first D flip-flop F,
The output side is connected to one input side of the OR circuit 107, and the reset terminal R of the second D flip-flop F2.
The Q output side of the second D flip-flop 70 F is connected to the other input side of the OR circuit 107, and is also connected to the first D flip-flop F and the glue set terminal RE. ing. The D input sides of each of the flip-flops F1 and F2 are connected together to serve as a detection operation signal input terminal 103, and the output side of the OR circuit 107 is connected to the opening/closing drive terminal of the switch S, and the voltage dividing resistor The middle of the wiring connecting R and the thermocouple TC is connected to the positive input side of the first comparator C6 for determining disconnection, and is also connected to the negative input side of the second comparator C6 for determining disconnection. Comparator C6 for each of the above discriminations
17) Each output side has a first and a second D flip-flop.
are connected to the set terminals of the hold circuits H, H2 of the first. Second hold circuit HI +
The reset terminal of H2 is the detection operation signal input terminal 103.
It is connected to the. Furthermore, the above 1. Each Q output side of the second hold circuits H, H2 is connected to each input side of an AND circuit 108, and the output side of the AND circuit 108 is used as a burnout output terminal 104.

Next, the operation of the second embodiment having the above-described configuration will be explained based on the waveform explanatory diagrams of FIGS. 4 to 6.

Since the switch S is open during normal measurement operation, a normal measurement signal can be obtained from the signal output terminal 102 without any influence on the thermocouple TC. on the other hand,
The alternating current voltage from the alternating current voltage generator AC has a sine curve as shown by the zero point waveform, and the zero crossing position of this sine curve is the first. It is detected by the second zero cross detection comparator C8, C, and the output of each of these comparators C, C4 becomes a square wave pulse that coincides with the zero cross timing of the above sine curve, as shown in the waveform diagram ■ and ■. ing.

Note that the above 1. Second zero cross detection humpator C
Since the input sides of 8 and C are connected inversely to each other, ■
The waveforms of , ■ are inverted from each other, and this ■
, ■ are applied to each D flip-flop F, , F of the switch operation circuit 105, so when an external detection operation signal is applied to the detection operation signal input terminal 103, the switch operation signal ■ is applied to each D flip-flop F, , F of the switch operation circuit 105. The timing of the start of this switch operation signal ■ coincides with the timing of the rise of either pulse signal ■ or ■ immediately after the input of the detection operation signal ■, and the timing of the end is as follows. Similarly, the timing coincides with the rise of either pulse signal (2) or (2). Therefore, the AC voltage applied to the thermocouple TC has a waveform that starts and ends at the zero cross as shown in the waveform diagram, so there is no risk of generating spike noise, and the filter 106
As a result, the alternating current component is canceled, and the applied voltage for detecting disconnection does not cause disturbance.

At this time, if the thermocouple TC is in a normal state with no disconnection, the voltage divided by the thermocouple TC is applied to each disconnection detection comparator C, C6 on both the positive and negative sides. Therefore, each comparator C5+
The output of C6 is not inverted, and there is no output from the burnout output terminal 104, which makes it possible to recognize that the thermocouple TC is normal. In addition, if noise is added in this normal state as shown in FIG. 5, and the second disconnection detection comparator C6 malfunctions, the signal ■ is added from the second hold circuit H8 to the AND circuit However, unless the other first disconnection detection comparator Cs is in operation, the AND circuit 108 will not output either, so misidentification due to noise can be avoided.

On the other hand, when the thermocouple TC is disconnected, each disconnection detection comparator C,
, C6 is applied with a voltage exceeding the set voltage, so the output of each disconnection detection comparator C, , C, is inverted in turn,
A set signal is applied to each hold circuit H, , H2.

Therefore, as shown in FIG. 6, signals ■ and ■ are output from each hold circuit H, H, and an AND signal ■ of the signals ■ and ■ is output from the AND circuit 108, causing a disconnection to the thermocouple TC. This makes it possible to recognize that an accident has occurred.

As explained above, according to the present invention, an alternating current voltage is used as a voltage for detecting a disconnection, and this alternating current voltage is intermittently applied to a detection element such as a thermocouple in response to an input signal for detecting a disconnection from the outside. Since the AC voltage applied to the AC voltage is applied starting from the AC zero cross, there is no risk of offset in the signal output, and even though the voltage for disconnection detection is intermittent, spike noise is eliminated. This has the effect that there is no risk of this occurring. Furthermore, burnout output can be obtained instantaneously after applying the disconnection detection input signal. For example, when commercial 50Hz is used as the voltage for disconnection detection, one cycle of 20 m s
It has the advantage of being able to detect at a high speed within ee, and also has the advantage of being able to be provided at low cost due to its simple circuit configuration. Further, according to the present invention, there is an effect that even if another meter is connected in parallel to the signal source element, the voltage for detecting disconnection will not be disturbed.

Further, in the present invention, using a comparator for detecting disconnection,
By detecting the abnormal voltage that occurs when a wire is disconnected on both the positive and negative sides of AC, and by using an AND circuit or the like to detect a disconnection from the logical product of both, malfunctions can be prevented even if a pulse-like disturbance is applied. This has the effect of improving reliability without the risk of damage.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional wire breakage detection circuit, FIG. 2 is a circuit diagram showing a wire breakage detection circuit according to a first embodiment of the present invention, and FIG. 3 is a circuit diagram showing a wire breakage detection circuit according to a first embodiment of the present invention. FIGS. 4, 5, and 6 showing the disconnection detection circuit are waveform explanatory diagrams for explaining the operation of the circuit according to the second embodiment. TC...Thermocouple as a signal source element, C1, C6...
・Comparator for disconnection detection, lO8...AND circuit. Patent applicant Chino Seisakusho Co., Ltd. Representative Patent attorney Norimitsu Nishimura Figure 3R Figure 4

Claims (2)

[Claims] (1) A disconnection detection circuit characterized in that an AC signal for disconnection detection is intermittently applied to both ends of a signal source element such as a thermocouple, starting from the zero cross of the AC signal in response to an external disconnection detection input signal. . (2) As a means of detecting a disconnection in the signal source element described above, a comparator for determining a disconnection is used to detect the abnormal voltage that occurs when the disconnection occurs on both the positive side and the negative side of AC, and from the logical product of both. A disconnection detection circuit according to claim 1, which detects disconnection.