JPH0823773B2 - Temperature detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a temperature detection circuit applied to, for example, an electric blanket or an electric carpet.

(Prior Art) Conventionally, a temperature detection circuit of this type is known as shown in FIG. That is, a temperature-sensitive resistance layer whose resistance value changes with temperature is interposed between the pair of conductors SAB and EAB,
One end of one conductor SAB is connected to the voltage dividing resistor R65, and a sensor wire SNW to which an AC voltage VAC is applied is provided between one ends of the conductors SAB and EAB via the voltage dividing resistor R65. The other ends of the conductors SAB and EAB are connected to each other via resistors R63 and R62, respectively. The connection point of the resistors R62 and R63 is connected to the non-inverting input terminal (+) of the operational amplifier OP1. The operational amplifier OP1 has an inverting input terminal (-) connected to one end of the other conductor EAB of the sensor wire SNW via a resistor R70, and has an output terminal inverting input terminal (-via a backflow prevention diode D10. ) Is connected to. A smoothing capacitor C6 is connected to the output terminal of the operational amplifier OP1 via the backflow prevention diode D10. The non-inverting input terminal (+) of the operational amplifier OP1 is connected to the + 12V terminal via the diode D21 in the forward direction and the diode D22 is connected to the -12V terminal in the reverse direction, and the power source of the operational amplifier OP1 is connected. Input terminals are connected to + 12V terminal and -12V terminal respectively.

In this circuit, the AC voltage VAC is divided by the impedance between the resistor R65 and the conductors SAB, EAB of the sensor wire SNW, further divided by the resistors R63, R62, and supplied to the operational amplifier OP1. Since the resistance value of the temperature-sensitive resistance layer of the sensor wire SNW changes depending on the temperature, the impedance between the conductors changes, and thus the voltage input to the operational amplifier OP1 changes depending on the temperature. . Therefore, the AC temperature detection voltage is input to the operational amplifier OP1.

This AC temperature detection voltage is peak-held by a peak-hold circuit composed of an operational amplifier OP1, a diode D10, a resistor R70, and a smoothing capacitor C6, and a DC temperature detection voltage VT is output. Thus sensor wire SNW
The direct current temperature detection voltage VT corresponding to the temperature detected at is output.

In this circuit, if the conductor EAB side is disconnected, the resistance
Since the voltage dividing circuit of R63 and R62 does not function, a large AC voltage is input to the operational amplifier OP1 regardless of the impedance change of the temperature sensitive resistance layer. However, after this voltage is held by the peak hold circuit, an abnormal high voltage is detected by the comparison circuit in the subsequent stage to detect disconnection and the fail-safe function is activated.

(Problems to be Solved by the Invention) By the way, since the temperature-sensitive resistance layer of the sensor wire contains an ionic substance, when a direct current component is applied to the sensor wire, the ionic substance forms an insulating layer due to one side. Therefore, it is necessary to apply only the AC component to the sensor wire. For this reason, it is necessary to drive the operational amplifier OP1 with a positive / negative power supply that is equal to or higher than the peak voltage of the input AC temperature detection voltage.
There is a problem that a power supply is required and the power supply circuit becomes complicated.

Therefore, the present invention is intended to provide a temperature detection circuit which can prevent application of a direct current component to a sensor wire even with a single power supply, and therefore can simplify the configuration of the power supply circuit.

[Configuration of the Invention] (Means for Solving the Problem) The present invention has a temperature-sensitive resistive layer interposed between a pair of conductors, the resistance layer having a resistance value that changes with temperature, and one end of one of the conductors. A voltage dividing resistor is connected, and a sensor wire to which an AC voltage is applied between one ends of a pair of conductors through this voltage dividing resistor, and an inverting input terminal via the first resistor, each other end of each conductor of the sensor wire. An operational amplifier connected to the non-inverting input terminal of the other conductor of the sensor wire via the ground, and a smoothing capacitor connected to the output terminal of the operational amplifier via a backflow prevention diode, A second resistor connected between the output terminal of the operational amplifier and the inverting input terminal via a backflow prevention diode, and the inverting input terminal of the operational amplifier and the ground are connected in parallel in opposite directions. A pair of clamp diodes, a temperature detection circuit to be achieved the above object comprises a.

(Operation) By providing such a means, a pair of clamp diodes opposite to each other are connected in parallel between the inverting input terminal of the operational amplifier and the ground. Even if a current flows through the resistor to the inverting input terminal side of the operational amplifier, this current also flows to the ground side through one of the clamp diodes, so the inverting input terminal becomes less than the forward voltage of the diode, and the DC component to the sensor wire is reduced. The application is virtually negligible.

(Example) Hereinafter, the Example of this invention is described with reference to drawings. In addition, this Example describes what applied this invention to the electric carpet.

FIG. 1 shows the entire circuit configuration of the electric carpet. CAR is a pair of heaters HA, HB, short-circuit detection lines GA, GB and a pair of conductors S attached to the heaters HA, HB, respectively.
The carpet body has a sensor wire SNW having a temperature-sensitive resistance layer interposed between AB and EAB.

This circuit is composed of a temperature detection circuit CC1, a temperature setting circuit CC2, a temperature comparison circuit CC3, a relay setting protection circuit CC4, a heating area switching / relay driving circuit CC5, a safety circuit CC6, and other power supply circuits.

The power supply circuit connects an AC power supply AC to a varistor VA via an operation switch SW1, a temperature fuse TF and a printed circuit board foil fuse PF in series, and a lamp PL and a resistor R20.
, And a smoothing capacitor C2 and a constant voltage diode ZD capacitor C3 are respectively connected via a rectifying diode D1 and a resistor R2 in series.

The safety circuit CC6 connects the AC power source AC to the operation switch S.
A series circuit of a resistor R6, a diode D6, a resistor R1, a resistor R8 and a three-terminal thyristor SCR is connected via a W1, a temperature fuse TF and a printed circuit board foil fuse PF in series. A resistor R7 is connected in parallel to the diode D6, and a diode D is provided in the series circuit of the resistors R1 and R8 and the thyristor SCR.
7 has the opposite polarity and is connected in parallel. An NPN transistor Q5 and a resistor R11 are connected between the gate and cathode of the thyristor SCR. A resistor R21 is connected between the base and emitter of the transistor Q5.

The heating area switching / relay drive circuit CC5 includes a smoothing capacitor C1 via the operation switch SW1, a temperature fuse TF and a printed circuit board foil fuse PF in series with the AC power source AC, and a rectifying diode D2 and a resistor R3 in series. Are connected. A coil of the first relay RYA, a resistor R4 and a series circuit of the NPN transistor Q1 and a coil of the second relay RYB, a resistor R5 and a resistor R5 are connected to the smoothing capacitor C1.
The series circuit of NPN transistor Q2 is connected. Each of the relays RYA and RYB has a diode D in the coil.
3 and D4 have opposite polarities and are connected in parallel. NPN transistor Q is used for the base and emitter of the transistor Q1.
3 and a resistor R13 are connected, and an NPN transistor Q4 and a resistor R17 are connected to the base and emitter of the transistor Q2.

Then, a switch SW2 for switching the heating interview and a series circuit of resistors R14 and R15 are connected between both ends of the constant voltage diode ZD, and a switch SW3 for switching the heating interview and a series circuit of resistors R18 and R19 are also connected. Then
The connection point between the resistors R14 and R15 is connected to the base of the transistor Q3, and the connection point between the resistors R18 and R19 is connected to the base of the transistor Q4.

Connect the connection point between the switch SW2 and the resistor R14 to the diode D1.
Connect to the anode of 3 and also switch SW3 and resistor R18
The connection point with is connected to the anode of the diode D14. The cathodes of the diodes D13 and D14 are connected to each other.

The heater HA is connected to the AC power supply AC with an operation switch SW1,
The temperature fuse TF and the contact of the first relay RYA are connected in series, and the heater HB is connected to the AC power source AC.
The operation switch SW1, the temperature fuse TF, and the contacts of the second relay RYB are connected in series. The connection point between the contact of the first relay RYA and the heater HA and the second
A series circuit of resistors R9 and R10 is connected between the contact point of the relay RYB and the connection point of the heater HB, and the connection point of the resistors R9 and R10 is connected to the gate of the thyristor SCR via the diode D5. It is connected.

One ends of the short circuit detection lines GA and GB are connected to each other and to a connection point between the resistors R1 and R8.

The temperature detection circuit CC1 constitutes the main part of the present application as shown in FIG. 2, and one end of the conductor SAB of the sensor wire SNW is connected to the anode of the rectifying diode D2 via the voltage dividing resistor R65. However, one end of the other conductor EAB is connected to the common line L. A series circuit of voltage dividing resistors 61 and R62 is connected between the other end of the conductor SAB and the other end of the conductor EAB.

The connection point between the voltage dividing resistors R61 and R62 is connected to the inverting input terminal (-) of the operational amplifier OP1 via the resistor R23 which is the first resistor. The non-inverting input terminal (+) of the operational amplifier OP1 is connected to the common line L. The diodes D8 and D9 are connected in parallel between the inverting input terminal (-) and the non-inverting input terminal (+) of the operational amplifier OP1 so as to be opposite to each other.

The output terminal of the operational amplifier OP1 is connected to the smoothing capacitor C6 via the backflow prevention diode D10 and further connected to its own inverting input terminal (-) via the resistor R24 which is the second resistor. And the operational amplifier
While converting AC voltage to DC voltage by OP1,
The peak voltage of the DC voltage is held by the smoothing capacitor C6. A resistor R25 is connected in parallel to the smoothing capacitor C6.

The temperature setting circuit CC2 has one end of a resistor R63 connected to the other end of the short circuit detection line GB and one end of a resistor R64 connected to the other end of the short circuit detection line GA.

The other end of the resistor R63 is connected to the resistor R44 and the capacitor.
It is connected to the other end of the AC power supply AC via C9, that is, to the negative electrode side of the smoothing capacitor C2.

Resistors R45, R46, R47, R48 across the capacitor C9.
, And R49 are connected in series to connect an NPN transistor Q8, and a resistor R50 is connected to a transistor Q9. A PNP transistor Q6 is connected in parallel to the resistor R45. A resistor R52 is connected between the emitter and the base of the transistor Q6. A switch SW4 is connected in parallel to the resistor R47, and a variable resistor VR for temperature adjustment is connected in parallel to the resistor R48.

The other end of the resistor R64 is connected to the AC power source AC via the resistor R53.
Is connected to the other end of. And a resistor R56 to the resistor R53,
A series circuit of R57 and R58 and a series circuit of resistors R59 and R60 are connected in parallel. The connection point of the resistors R57 and R58 is connected to the base of the transistor Q8,
The connection point of R59 and R60 is connected to the base of the transistor Q9.

An NPN transistor Q7 is connected in parallel to the series circuit of the resistors R57 and R58. A resistor R55 is connected to the base and emitter of the transistor Q7. Further, the base of the transistor Q7 via the resistor R54 the diode D13 of the heating area switching and relay drive circuit CC5 and
It is connected to the cathode of D14.

The temperature comparison circuit CC3 is provided with an operational amplifier OP2 that constitutes a comparator, and the smoothing capacitor C6 of the temperature detection circuit CC1.
The output from the operational amplifier OP2 is input to the non-inverting input terminal (+) of the operational amplifier OP2, and the voltage generated at the connection point between the resistors R46 and R47 of the temperature setting circuit CC2 is applied to the operational amplifier OP2.
Input to the inverting input terminal (-) of OP2.

To the output terminal of the operational amplifier OP2, a diode D11 is connected in the illustrated polarity via a resistor R26 and a capacitor C7 in series, and a parallel circuit of a capacitor C8 and a resistor R27 is connected via a diode D12. The positive terminal of the capacitor C8 is connected to the inverting input terminal (-) of the operational amplifier OP3, and is also connected to the base of the transistor Q5 of the safety circuit CC6 via the resistor R22.

A series circuit of resistors R28 and R29 is connected across the smoothing capacitor C2, and the connection point of the resistors R28 and R29 is connected to the non-inverting input terminal (+) of the operational amplifier OP3. A resistor R30 is connected between the non-inverting input terminal (+) and the output terminal of the operational amplifier OP3.

The relay contact protection circuit CC4 is provided with a counter CN, and the input terminal of the counter CN is connected to the output terminal of the operational amplifier CP3 of the temperature comparison circuit CC3. In addition, the relay contact protection circuit CC4 connects the smoothing capacitor C2 with resistors R39 and R4.
Connect a series circuit with 0 in parallel, and connect the resistor R40 to the resistor R3.
A series circuit of 5, R36, R37 and R38 is connected in parallel.
And a connection point between the resistors R37 and R38 via the resistor R34 connected to the output terminal to Q ₁ the counter CN, the resistor R36 and R
The connection point with 37 is connected to the output terminal Q ₂ of the counter CN through the resistor R33, and the connection point between the resistors R35 and R36 is connected with the resistor R3.
The output terminal Q ₃ of the counter CN is connected via 2 and the connection point between the resistors R 39 and R 35 is connected to the output terminal Q ₄ of the counter CN via the resistor R 31.

A non-inverting input terminal (+) of an operational amplifier OP4 that forms a waveform shaping circuit through a resistor R41 by connecting a capacitor C5 to the resistor R40 in parallel and connecting a resistor R39 and R40.
Connected to Further, the output terminal of the operational amplifier OP3 is connected to the inverting input terminal (-) of the operational amplifier OP4 via an integrating circuit composed of a resistor R43 and a capacitor C4.
A resistor R42 is connected between the output terminal of the operational amplifier OP4 and the non-inverting input terminal (+).

The output terminal of the operational amplifier OP4 is connected to the base of the transistor Q6 of the temperature setting circuit CC2 via a resistor R51, and the transistors Q1 and Q2 of the heating area switching and relay drive circuit CC5 are connected via resistors R12 and R16, respectively. Connected to the base of.

In the present embodiment having such a configuration, the switch SW2
If the operation switch SW1 is turned on with SW3 and SW3 in the open state and the temperature of the carpet body CAR is lower than the set temperature, the output from the output terminal of the operational amplifier OP4 of the relay contact protection circuit CC4 becomes high level. , Transistor
With Q3 and Q4 turned off and the transistors Q1 and Q2 turned on, the first and second relays RYA and RYB operate, respectively.
Then, energization of each heater HA, HB is started and the carpet body CAR is heated.

Here, the temperature detection circuit CC1 divides the AC input voltage VAC by the resistor R65 and the impedance RT of the sensor wire SNW to convert the sensor temperature into a voltage. This voltage is further divided by resistors R61 and R62 to obtain an AC temperature detection voltage V0.

Here, since the resistor R62 is connected to the outer conductor EAB of the sensor wire SNW, when the conductor EAB is broken, the voltage dividing circuit by the resistors R61 and R62 does not function, and therefore the detection voltage V0 is the normal temperature. It will be higher than the maximum voltage generated during control. This large voltage is the temperature comparison circuit CC
2 detects the disconnection fault by comparing and detecting.

Since the sensor wire SNW deteriorates with time due to a DC component, the temperature detection voltage V0 must not have a DC component.
Thus the non-inverting input terminal of the operational amplifier OP1 to obtain a temperature detection voltage VT _T of direct current (+) is connected to the common line (GND) L, if tentatively resistors R23 and R24 are the same position, -V0>
At VT, the operational amplifier OP1 functions as an inverting buffer,
VT = -V0. Here, the inverting input terminal (-) of the operational amplifier OP1 becomes an imaginary ground having a potential difference of zero with the common line L.

When -V0 <VT, the sum of the currents of the resistors R23 and R24 flows through the diode D9 to the inverting input terminal (-) of the operational amplifier OP1, and this forward voltage is applied to the inverting input terminal (-). Then, the output of the operational amplifier OP1 becomes low level, and the diode D10 is reverse biased. At this time, the voltage applied to the inverting input terminal (-) is small, so that the direct current component does not flow into the sensor wire SNW in combination with the above-mentioned image marginary ground.

From the above, a voltage equal to the negative peak voltage of the AC temperature detection voltage V0 is generated in the smoothing capacitor C6, and the discharge time constant (C6 × R25) is made sufficiently long with respect to the power supply cycle to detect the DC temperature. Obtain the voltage VT.

AC voltage VAV in this circuit, AC temperature detection voltage
If V0 and the DC temperature detection voltage VT are shown respectively, it becomes as shown in FIG. 3 (a), and at that time, the operational amplifier OP1
The output timing of is as shown in FIG.

In this circuit, there is a relationship of R62 << R23, RT << R61, and the voltages V0 and VT are respectively V0 = RTVAC / (R6
5 + RT) × R62 / (R61 + R62), VT = V0 Peak.

In the temperature setting circuit CC2, the half-wave power supply voltage from the short circuit detection line GB is divided by resistors R63 and R44 to smooth the capacitor.
It is smoothed by C9 and the disconnection detection voltage Vcut is obtained.

When the transistor Q9 is off, this becomes the temperature setting voltage Vset and is input to the operational amplifier OP2 of the temperature comparison circuit CC3. When the transistor Q8 is off and the transistor Q9 is on, the voltage is divided by the resistors R45, R46 and the resistors R47, R48, R49, R50 to obtain the temperature setting voltage VTset when half-powered. This is the temperature setting circuit Vset. It is input to the operational amplifier OP2 of CC3. Furthermore, when both the transistors Q8 and Q9 are on, the resistor R50 is short-circuited by the transistor Q8, so the voltage is divided by the resistors R45 and R46 and the resistors R47, R48, and R49. Temperature setting voltage VTset is obtained, which is the temperature setting voltage Vset
Is input to the operational amplifier OP2 of the temperature comparison circuit CC3.

ON / OFF control of transistors Q8 and Q9 is short-circuit detection line GA
The half-wave source voltage from is divided by resistors R64 and R53. When the switch SW2 is turned on and the half-surface energization signal is at the high level, the transistor Q7 is turned on, so that the transistor Q8 is always off.

In this way, the transistors Q8 and Q9 are turned on and off in synchronization with the power supply cycle, so that the temperature setting voltage Vset becomes a voltage generated by the disconnection detection voltage Vcut and the temperature setting voltage VTset in a time division manner.

Transistor Q6 is time-divided V of temperature setting voltage Vset
A transistor that gives a hysteresis voltage to give the differential of the sensor wire SNW to Tset.
When Q6 turns on, the resistor R45 is short-circuited and VTset rises by the hysteresis voltage, which gives a differential on / off temperature of the sensor wire.

However, when the transistors Q8 and Q9 are off, Vset voltage division is not performed, so that a constant Vcut is Vset regardless of whether the transistor Q6 is on or off.

The switch SW4 is a switch that raises the on / off temperature of the sensor wire SNW when the cover is used on the carpet to perform cover correction. When this switch SW4 is turned on, V
The set is lowered and correction is performed.

Further, in the temperature comparison circuit CC3, the temperature detection voltage VT converted into the direct current by the temperature detection circuit CC1 and the temperature setting voltage Vset from the temperature setting circuit CC2 are input to the operational amplifier OP2. The temperature setting voltage Vset is a voltage in which the voltages Vcut and VTset are generated in a time division manner.

Then, when the temperature detection voltage VT is between the voltage Vcut and the voltage VTset, the operational amplifier OP2 is turned on and off, the AC component is taken out by the capacitor C7, and then the diode D1.
It is converted to DC again by 2 and smoothed by the capacitor C8 and the resistor R27.

Here, the diode D11 is for charging the capacitor C7, and generates a DC component in the capacitor C8 through the diode D12 when the capacitor C7 is discharged. This is input to the waveform shaping circuit composed of the resistors R28, R29, R30 and the operational amplifier OP3 to obtain the comparison output VC. It is also supplied to the base of the transistor Q5 of the safety circuit CC6.

In the relay contact protection circuit CC4, the comparison output VC from the temperature comparison circuit CC3 is applied to the inverting input terminal (-) of the operational amplifier OP4 via the integrating circuit of the resistor R43 and the capacitor C4. Also on the comparison output VC, off they are counted by the counter CN, so that the output terminal Q ₁ to Q ₄ of 4 bits of the counter CN D / A conversion circuit according to ladder network (R31 to R38) is connected The output voltage V2 changes depending on the number of times the comparison output VC is turned on and off. Resistor R39
And R40 determine the minimum value and the maximum value of the output voltage V2, respectively, and these are applied to the non-inverting input terminal (+) of the operational amplifier OP4 via the resistors R41 and R42 for generating hysteresis.

The output voltage Vr of the operational amplifier OP4 changes depending on whether the threshold voltage of the operational amplifier OP4 is turned on or off by the counter CN and the D / A conversion circuit. Therefore, the voltage input to the inverting input terminal (-) is the threshold voltage. The relay RY changes as the time exceeds
A and RYB always turn on and off in different phases. Therefore, the occurrence of contact transition can be reliably prevented, and the life of the contact can be extended.

Further, in the heating area switching / relay drive circuit CC5, the transistors V1 and Q2 are controlled to be turned on / off by the output Vr from the relay contact protection circuit CC4, whereby the relays RYA and RYB are turned on / off to operate the heaters HA and HB. Energization control is performed.

In addition, the coil current when the relays RYA and RYB are turned off is discharged through the diodes D3 and D4, thereby increasing the opening speed and strengthening the welding due to the locking of the contacts.

The switches SW2 and SW3 are for switching the heating area. When the switches SW2 and SW3 are turned on, the transistors Q3 and Q4 are turned on, which turns off the transistors Q1 and Q2 and the relays RYA and RYB. Forbid to work. This can prevent unnecessary heating surfaces from being heated.

Furthermore, in the safety circuit CC6, an overheat prevention operation and a relay contact welding detection operation are performed.

a) Overheat prevention operation If the heater HA or HB becomes abnormally high temperature due to some abnormality, the temperature sensitive layer interposed between the heater and the short circuit detection line will melt and the temperature fuse TF, resistor R6, diode D6, resistor R
1, current flows through short-circuit detection line GA or GB, heater HA or HB, diode D7, resistor R1, short-circuit detection line GA
Alternatively, a current flows through the contacts of GB, heater HA or HB, relay RYA or RYB, and resistance R1 generates a large amount of heat. As a result, the temperature fuse TF is blown.

b) Relay contact welding detection operation When the contacts of relays RYA and RYB are on, resistance R9,
An AC half-wave is applied to the gate of the thyristor SCR via R10 and diode D5.
Since a high level voltage is applied to the base of the transistor Q5 prior to the comparison output VC from 3
Q5 turns on and bypasses the thyristor SCR gate current. Therefore, the thyristor SCR does not conduct.

However, if an accident occurs in which the relay contacts are welded, the sensor wire SNW becomes the off temperature and the voltage applied to the transistor Q5 becomes low level. Therefore, a gate current flows through the gate of the thyristor SCR and the thyristor SCR becomes conductive. Then, the current flows through the temperature fuse TF, the resistor R6, the diode D6, the resistor R1, and the thyristor SCR.
The thermal fuse TF is blown by the heat generation of 1.

As described above, in this embodiment, even if the operational amplifier OP1 of the temperature detection circuit CC1 is operated by a single power supply, the sensor wire SNW
Since it is possible to sufficiently prevent the DC component from flowing, the same function as that using the conventional two positive and negative power supplies can be obtained, which is excellent in practicality.

Therefore, the operational amplifier OP1 of the temperature detection circuit CC1 can be used with a single power source, and the direct current component can be cut so that the sensor wire SNW that cannot apply a direct current component can be used. Therefore, the cost can be reduced. Also, the mounting space for the power supply can be made small and the control unit can be made compact.

In addition, although the said embodiment applied the present invention to the electric carpet, it is not necessarily limited to this and various things, such as an electric blanket and an electric sheet, can be applied.

[Effects of the Invention] As described in detail above, according to the present invention, the operational amplifier can be used with a single power supply, and the direct current component can be cut so that the sensor wire to which the direct current component cannot be applied can be used. Since a single power source can be used, the structure can be simplified, and a temperature detection circuit that can reduce cost can be provided.

[Brief description of drawings]

1 to 3 show an embodiment of the present invention.
FIG. 2 is an overall circuit diagram of an electric carpet, FIG. 2 is a circuit diagram of a temperature detecting circuit which is a main part, FIG. 3 is a signal waveform diagram of each part in FIG. 2, and FIG. 4 is a circuit diagram showing a conventional example. . CC1 ... Temperature detection circuit, SNW ... Sensor wire, R65 ...
… Voltage-dividing resistor, OP1 …… Operational amplifier, D8, D9 …… Clamp diode, R23 …… First resistor, R24 …… Second resistor,
D10 ... Backflow prevention diode.

Claims (1)

[Claims] 1. A temperature-sensitive resistance layer whose resistance value changes according to temperature is interposed between a pair of conductors, and a voltage dividing resistor is connected to one end of one of the conductors, and the voltage dividing resistor is connected through the voltage dividing resistor. A sensor wire to which an AC voltage is applied between one ends of the pair of conductors, an inverting input terminal connected to each other end of each conductor of the sensor wire via a first resistor, and a non-inverting input terminal An operational amplifier connected to one end of the other conductor of the sensor wire via a ground; a smoothing capacitor connected to an output terminal of the operational amplifier via a backflow prevention diode; and an output terminal of the operational amplifier. A second resistor connected between the inverting input terminal and the inverting input terminal via the backflow prevention diode, and an inverting input terminal of the operational amplifier and the ground are connected in parallel in opposite directions. A temperature detecting circuit comprising: a pair of clamp diodes.