JP3039349B2 - Resistance change detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resistance change detecting circuit used in a water level control device and the like.

[0002]

FIG. 2 shows a water level control device such as a boiler which is a comparative example of the present invention. In FIG. 2, R11, R12, R13, R14 are resistors, IV11,
IV12 is an inverter, F11 is a bidirectional phototransistor, C11 is a capacitor, and Z11 is a surge absorber. O is an output terminal, E is a ground terminal, L is a terminal to which a water level electrode is connected, and AC 8 V is applied between the A terminal and the terminal E. The outline of the operation of this conventional circuit is as follows. When the water level electrode detects the presence of water, the voltage between the terminals L and E becomes low, the output of the inverter IV11 becomes H (high), the inverter IV12 becomes L (low), and the phototransistor F11
Does not conduct, and a voltage of about DC 5 V is output from the output terminal O. When the water level electrode detects the absence of water, the voltage between the terminals L and E increases, the output of the inverter IV11 becomes L, the inverter IV12 becomes H, the phototransistor F11 conducts, and the output of the output terminal O becomes 0V. . Thus, presence or absence of water is detected depending on whether the output of the output terminal O is 5V or 0V.

In the above comparative example, a situation in which the phototransistor F11 does not conduct due to some circuit failure, for example, disconnection of the resistor R11, LED of the phototransistor F11,
When the disconnection of the transistor side, the disconnection of the transistor side, and the disconnection of the resistor R14 occur, the phototransistor F11 becomes conductive even though the water level electrode does not detect the presence of water (safe state) (is in an unsafe state). However, there is a problem that a 5V water presence signal (safe state) is output.

[0004]

According to the present invention, there is provided a resistance change detecting means for changing a resistance value of an AC power supply when a resistance element and an object to be detected are on a safe side and on an unsafe side. Are connected in series, a continuous pulse is generated based on the voltage between both ends of the resistor when the resistance value change detecting means detects the safe side, and when the resistance value change detecting means detects the unsafe side, This object is achieved by providing a pulse generating means that does not generate a continuous pulse based on the voltage between both ends.

According to the above means, a continuous pulse is output from the pulse generation means when the resistance value change detection means detects the safe side, and a continuous pulse is output from the pulse generation means when the resistance value change detection means detects the unsafe side. No pulse is output. In the case of a failure that the AC voltage is not supplied to the pulse output means, in the case of a failure in which the voltage between both ends of the resistor becomes the voltage at the time of detection on the unsafe side, and in the case of a failure of the pulse generation means, Since a continuous pulse is not output, an output at the time of detection on the unsafe side is output from the pulse generation means in any case. Therefore, the safety can be maintained without outputting the signal of the safe side detection in the state where the unsafe side is detected at the time of the circuit failure.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the resistance change detecting circuit according to the present invention is a circuit for detecting a resistance between an AC power supply when a resistance element and a detection target are on a safe side and when it is on an unsafe side. A resistance value change detecting means whose value changes is connected in series, and a continuous pulse is generated based on a voltage between both ends of the resistor when the resistance value change detecting means detects a safe side, and the resistance value change detecting means Has pulse generating means that does not generate a continuous pulse based on the voltage between both ends when the unsafe side is detected.

This embodiment will be described below.
As the AC power supply, a power supply of, for example, 8 V AC generated by stepping down from a commercial power supply is generally used, but is not limited thereto.
The resistance element may be of any type, but the voltage between both ends of the resistance element when the resistance value change detecting means detects the safe side (for example, in the case of water level control of a boiler, there is water), that is, the resistance element and A continuous pulse can be generated by the pulse generation means by the potential between the resistance change detection means, and the pulse is generated by the pulse generation means by the potential when the resistance value change detection means detects an unsafe side (for example, no water in the case of boiler water level control). The resistance values of the present resistance element and the other elements are set so that a voltage change between both ends of the resistance value change detecting means is generated so as not to generate the following. The resistance change detecting means is a water level detecting electrode of a water level control device or a CdS cell as a flame detector. In short, the resistance value largely changes due to a change in the state of the detection target. When the flame is detected, the resistance value of the CdS cell is greatly reduced to about 1/100 or less as compared to when the flame is not detected.

The pulse generating means preferably has a unidirectional rectifier connected in parallel to the resistance element, and a voltage not less than a predetermined value is applied to the unidirectional rectifier connected to the unidirectional rectifier in a direction opposite to the rectifier. And a pulse generating circuit including a unidirectional conductive element that is turned on when turned on. The pulse generating means further preferably includes pulse shaping means for shaping the output pulse of the unidirectional conducting element. A rectifying diode is used as the unidirectional rectifying element. Any element may be used as long as it has a function of making the unidirectional conducting element nonconductive by bypassing the unidirectional conducting element during a half cycle of AC. As the unidirectional conducting element, a unidirectional phototransistor is used, but any element that conducts when an AC voltage is applied in the direction opposite to the rectifying direction of the unidirectional rectifying element, that is, in the current flowing direction opposite to the rectifying direction may be used. The pulse shaping means can be realized by combining, for example, a resistor and an inverter that inverts the input with a predetermined threshold value, but is not limited to this, and converts the pulse in which the rising and falling of the unidirectional conducting element is reduced into a square pulse. What is necessary is just what is molded. By adding this pulse shaping means, the recognition of the continuous pulse by the judgment means such as a microcomputer becomes easy.

A preferred embodiment of the boiler water level control device according to the present invention is a device for performing water level control based on a detection signal from a low water level abnormality detection electrode for detecting a low water level abnormality. The change detection means is at least the low water level abnormality detection electrode. By doing so, in the water level control of the boiler, it is possible to avoid the danger of outputting a signal on the safe side even though a low water level abnormality that causes abnormal overheating of the boiler and causes fatal damage to the boiler has occurred. Of course, it is needless to say that the water level control with higher safety can be performed by using the resistance value change detecting means of the present invention for the other water level detecting ends.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A specific embodiment of the present invention will be described below with reference to the drawings. The circuit shown in FIG. 1 is a resistance value change detection circuit having a fail-safe function used as a water level control device of a boiler. As the resistance value change detection means,
As shown in FIG. 3, a low water level abnormality detection electrode BL disposed in the water level control cylinder T, a middle water level detection electrode BM for detecting a higher water level than the low water level abnormality detection electrode BL, and a middle water level detection electrode B
And a high water level detection electrode BS for detecting a water level higher than M. Although the resistance value change detection circuit in FIG. 1 is for the low water level abnormality detection electrode BL, similar detection circuits are provided for the other electrodes. The low water level abnormality detection electrode BL is connected to the terminal L
And the water level control cylinder T is grounded to the same potential as the ground terminal E.

In FIG. 1, R1 and R3 are voltage-dividing resistors connected in series to each other, and are connected to an 8 V (volt) AC power supply (between terminal A and ground terminal E). P is a pulse generation circuit which is connected in parallel to the rectifier diode D1 connected to the resistor R1 and is connected in parallel when a predetermined voltage (for example, about 2 V) in a direction opposite to the rectification direction of the diode D1 is applied. A unidirectional phototransistor F1 and a resistor R4 connected to the collector of the phototransistor F1.
And a resistor R5 connected to the output terminal 1 of the phototransistor F1 and an inverter IV for inverting the input with a predetermined threshold.
1 and a pulse shaping circuit section. The inverter IV1 outputs L (low) when the input voltage is equal to or higher than the predetermined value V1, and outputs H (high) when the input voltage is equal to or lower than the predetermined value V2 (<V1). C1 is a capacitor having a surge voltage absorbing function together with the resistor R2.

The operation of the above embodiment will be described below.
Now, assuming that the detection end L detects the presence of water, L, E
The resistance between the terminals A is close to 0 and the potential of the detection end L is low,
When a positive half cycle of AC is applied, the phototransistor F1 conducts, and when a negative half cycle is applied, the diode D1 conducts.
The waveform at the output terminal 1 is as shown in FIG. This output waveform is shaped by the resistor R5 and the inverter IV1, and a continuous pulse having a rectangular shape as shown in FIG. This output signal is input to the judging means CP to judge the presence of water.

Conversely, if the detection end L detects that there is no water, the resistance between L and E is almost infinite and the potential of the detection end L is high, so that the phototransistor F1 does not conduct. Therefore, the waveform of the output terminal 1 of the phototransistor F1 becomes a DC voltage waveform as shown in FIG. 4, a continuous pulse is not output from the pulse generation circuit P, and 0 V is output from the output terminal O as shown in FIG. Is done.

Now, a case where a failure occurs in the circuit will be described.
If the resistor R3 is disconnected, no voltage is generated between both ends of the resistor R1 of the pulse generation circuit P, so that no pulse is output from the pulse generation circuit P, and the output without water, that is, 0V
Is fixed output. In this case, since there is no water output irrespective of the detection state of the resistance value change detecting means L, there is no water output even though there is no water, and the safety is maintained (fail safe is maintained). In the water level control of the boiler, the presence of water means a safe state, and the absence of water means an unsafe state.

Further, even when the phototransistor F1 is disconnected, the output is not output from the pulse generation circuit P in the same manner as when the resistor R3 is disconnected, so that the safety is maintained and the fail-safe control is performed.

[0016]

As described above, according to the first aspect of the present invention, a continuous pulse is not output in the event of a circuit failure in which no voltage is generated between both ends of the resistance element and a failure of the pulse generation means, and the unsafe side is not provided. Therefore, there is no output on the safe side even though the state of the detection target is the unsafe side at the time of the failure, and there is an effect that the safety can be ensured.
Further, according to the invention of claim 2, there is an effect that the pulse generating means can be configured simply and inexpensively. Further, according to the invention of claim 3, it is possible to provide a highly safe boiler water level control device.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram of one embodiment of the present invention.

FIG. 2 is an electric circuit diagram showing a comparative example of the present invention.

FIG. 3 is a diagram showing a schematic configuration of a water level control cylinder according to an embodiment of the present invention.

FIG. 4 is a signal waveform diagram of an output 1 when there is no water in FIG. 1 of the present invention.

FIG. 5 is a signal waveform diagram of an output 1 when water is present in FIG. 1 of the present invention.

FIG. 6 is a signal waveform diagram of an output terminal O when there is no water in FIG. 1 according to the present invention.

FIG. 7 is a signal waveform diagram of an output O when water is present in FIG. 1 of the present invention.

[Explanation of symbols]

BL Low water level abnormality detection electrode (resistance change detection means) R1 resistance (resistance element) P pulse generation circuit

Claims (3)

(57) [Claims] 1. An AC power supply, comprising: a resistance element and resistance change detecting means for changing a resistance value between when a detection target state is on a safe side and when it is on an unsafe side; A continuous pulse is generated based on the voltage between both ends of the resistor when the value change detection means detects the safe side, and a continuous pulse is generated based on the voltage between both ends when the resistance value change detection means detects the unsafe side. A resistance value change detection circuit comprising a pulse generation unit that does not generate a pulse. 2. A pulse generator comprising: a unidirectional rectifier connected in parallel to a resistor; and a pulse conduction means connected to the unidirectional rectifier when a voltage equal to or higher than a predetermined value is applied in the anti-rectification direction of the unidirectional rectifier. The resistance value change detection circuit according to claim 1, further comprising: 3. The resistance value change detection circuit according to claim 1, wherein the resistance value change detection means is a low water level abnormality detection electrode in the water level control device of the boiler.