JP3341363B2 - Humidity detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a humidity detector used for a copying machine, a cooler or the like.

[0002]

2. Description of the Related Art In general, a ceramic or organic polymer humidity sensor causes a polarization action when a DC voltage is continuously applied, and therefore it is necessary to apply an AC voltage. As a detection device using these humidity sensors, for example, Japanese Patent Application Laid-Open
There is an apparatus described in Japanese Patent Application Laid-Open No. 1-57841, which includes an oscillation circuit, a reference voltage generation circuit, a smoothing amplification circuit, and a humidity-voltage conversion circuit of a sensor unit. The oscillation circuit is a differential amplifier, an astable multivibrator consisting of a resistor and a capacitor, and the reference voltage generation circuit is a resistor that divides the DC voltage and a difference for converting the divided voltage into low impedance to generate a reference voltage. It consists of a dynamic amplifier. The sensor section is a series circuit of a humidity sensor and a resistor, and is connected to the output terminal of the oscillation circuit and the output terminal of the reference voltage generation circuit, and the output terminal of the sensor section is connected to the input terminal of the rectifying and smoothing circuit. A change in resistance value of the humidity sensor with respect to humidity is detected as a change in terminal voltage of the series resistor. The smoothing amplifier includes a differential amplifier, a diode, a resistor, and a capacitor, and has a configuration in which a non-inverting input of the differential amplifier is connected to a signal input terminal and a diode is connected to an output of the differential amplifier.

[0003] In the prior art, a single power source has been used to reduce the size and cost. In order to operate the humidity detector with a single power supply, when applying an AC voltage to the humidity sensor unit that converts humidity changes into voltage changes, a separate circuit is provided in the reference voltage generation circuit to divide the DC voltage with a resistor, etc. Is applied to the sensor section with positive and negative AC voltages based on the reference voltage. Then, a change in the resistance value of the humidity sensor with respect to humidity is detected as a change in the terminal voltage of the series resistor, and the detected voltage is converted into a desired DC voltage signal by a smoothing amplifier circuit.

[0004]

In recent electronic devices using a microcomputer or the like, further reduction in power consumption, simplification of the device, and cost reduction are strongly demanded. In a conventional humidity detecting device, an extra circuit for dividing a DC voltage is required to generate a reference voltage, and the circuit also consumes extra power. Therefore, there is a problem that it is difficult to simplify, reduce the cost and save power of the humidity detecting device.

An object of the present invention is to provide a humidity detecting device which operates with a single power supply and which has a simplified reference voltage generating circuit in order to reduce the cost and power consumption.

[0006]

In order to solve the above problems, a humidity-voltage conversion circuit for converting a humidity change into a voltage change, an oscillation circuit for applying an AC voltage to the humidity-voltage conversion circuit, and a reference voltage A generating circuit, and a rectifying and smoothing circuit for converting the output of the humidity-voltage conversion circuit into DC.
The oscillation circuit includes a first differential amplifier, a capacitor, and a resistor, one end of the resistor is connected to an output terminal of the first differential amplifier, and the reference voltage generation circuit includes a second differential amplifier. An output terminal of the operational amplifier is connected to an inverting input terminal of the second differential amplifier, and an output of the oscillation circuit is connected to one end of the humidity-voltage conversion circuit via a coupling capacitor. The other end of the voltage conversion circuit is connected to an output terminal of a second differential amplifier of the reference voltage generation circuit, and the input of the reference voltage generation circuit is connected to the second terminal via the resistor.
Connected to the non-inverting input terminal of the differential amplifier of
The output terminal of the voltage conversion circuit was connected to the input terminal of the rectifying / smoothing circuit.

[0007]

According to the present invention, a circuit component such as a resistor or a capacitor for determining an oscillation condition of an oscillation circuit is connected to a reference voltage generation circuit of a voltage follower having a high input impedance, and its output is further connected to a humidity-voltage conversion circuit. Connected to add the output of the oscillation circuit. As a result, an AC voltage centered on the reference voltage can be applied to the humidity-voltage conversion almost without being affected by the oscillation conditions of the oscillation circuit. Further, the reference voltage is smaller than the amplitude of the output voltage of the oscillation circuit, and is converted to DC by the rectifying and smoothing circuit together with the humidity detection voltage, so that it can be used as the reference voltage. Therefore, it is not necessary to divide the DC voltage by the resistor in the reference voltage generation circuit, and the reference voltage can be generated.

[0008]

【Example】

(Embodiment 1) FIG. 1 is a circuit diagram showing a humidity detecting apparatus according to an embodiment of the present invention. In FIG. 1, 1 is an oscillation circuit, 2 is a reference voltage generation circuit, and 3 is a humidity-
A voltage conversion circuit, 4 is a rectifying and smoothing circuit, 5 is a single DC power supply terminal, and 6 is an output terminal of the humidity detecting device.

The oscillation circuit 1 forms the same astable multivibrator as in the prior art. In the oscillation circuit 1, 7
Is a differential amplifier, 13 is a non-inverting input terminal, 14 is an output terminal, 15 is an inverting input terminal, 9 is a capacitor, 8, 10,.
11 and 12 are resistors. The reference voltage generating circuit 2 is a voltage follower, and includes a differential amplifier 17, and an output terminal of the differential amplifier 17 is connected to an inverting input terminal of the differential amplifier 17, and a non-inverting input terminal of the differential amplifier 17. The terminal is connected to the non-inverting input terminal 13 of the differential amplifier 7. 16 is a coupling capacitor. 19 of the humidity-voltage conversion circuit 3 is a humidity sensor (humidity sensing element), 20 is a thermistor (temperature-sensitive resistor), 18 and 21 are terminals for applying an AC voltage, and 22 is an output terminal of the humidity-voltage conversion circuit. It is connected to the input terminal of the rectifying / smoothing circuit 4. In this configuration, a humidity sensor using a porous ceramic impregnated with a polymer electrolyte was used as the humidity sensor. In addition, a ceramic humidity sensor may be used as the humidity sensor. Rectifying smoothing circuit 4
Is composed of an ideal diode and a smoothing circuit. In the rectifying and smoothing circuit 4, 23 is a differential amplifier, 24 is a diode, 25 is a capacitor, and 26 is a resistor.

Next, the operation will be described. The oscillation circuit 1 generates an AC waveform close to a rectangular wave. The operation will be described with reference to FIG. The waveform of the output terminal 14 of the oscillation circuit 1 is a rectangular wave of Vo shown in FIG. It is assumed that when the output terminal 14 is high at time t0, the voltage Vr of the non-inverting input terminal 13 of the differential amplifier 7 becomes the potential H in FIG. The capacitor 9 is charged via the resistor 8 and the terminal voltage of the inverting input terminal 15 is
Vc. At time t1, the value of Vc becomes H
, The output terminal 14 goes low. At that time,
It is assumed that the voltage Vr of the non-inverting input terminal 13 of the differential amplifier 7 has the potential L in FIG. The capacitor 9 is discharged via the resistor 8 and its terminal voltage falls as shown by Vc in FIG. When the value of Vc falls below the value of L at time t2, the output terminal 14 goes high again and this operation is repeated at a constant cycle. By the way, in the cycle, the values of the resistors 10, 11, and 12 are selected so that the voltage Vr becomes a predetermined divided value of H and L,
It is determined by selecting the time constant of the resistor 8 and the capacitor 9. The resistor 10 is provided to start oscillation with a single power supply.

What differs from the prior art is the method of generating the reference voltage. That is, the AC voltage Vr at the non-inverting input terminal 13 of the oscillation circuit 1 is not separately divided.
Is given to the reference voltage generation circuit 2 as a reference voltage. Since the input of the reference voltage generating circuit 2 is the non-inverting input terminal 13 of the differential amplifier 17, it has a high impedance. Therefore, even if connected to the non-inverting input terminal 13 of the oscillation circuit 1, the oscillation condition is hardly affected. Also, the differential amplifier 1
7 is connected to the inverting input terminal of the differential amplifier 17,
Further, it is connected to the terminal 21 and has a low impedance. Therefore, the voltage Vo at the output terminal 14 of the oscillation circuit 1 is applied to the humidity-voltage conversion circuit 3 via the coupling capacitor 16 with the AC voltage Vr at the non-inverting input terminal 13 as a center value.

When the resistance value of the humidity sensor 19 changes due to a change in the relative humidity, the humidity-voltage conversion circuit 3 detects the change as a change in the terminal voltage of the thermistor 20. The detected voltage is combined with the output of the reference voltage generation circuit 2. Then, it is applied to the input terminal of the rectifying / smoothing circuit 4.
The input of the rectifying and smoothing circuit 4 is a non-inverting input terminal of the differential amplifier 23 and has a high impedance. The output of the differential amplifier 23 is applied to an inverting input terminal of the differential amplifier 23 via a diode 24, and further, a DC voltage is smoothed by a capacitor 25 and a resistor 26 and output to the output terminal 6.

Here, the values of the resistors 10, 11, and 12 are selected such that the amplitude of the voltage Vr at the non-inverting input terminal 13 becomes smaller by one digit than the voltage Vo at the output terminal 14 of the oscillation circuit 1. Keep it. For example, assuming that the power supply voltage Vcc = 5 V, when the output Vo is high, about 3.7 V, and when the output Vo is low, about 0.7 V.
At V, the amplitude of the voltage Vr is about 0.4 Vpp. The amplitude of the voltage Vr is converted into a DC voltage together with the humidity detection voltage by the rectifying / smoothing circuit 4. However, since the amplitude of the voltage Vr is small, the use of the reference voltage does not adversely affect the humidity-output voltage characteristic of the humidity detection device. FIG. 3 shows an example of relative humidity and humidity-output voltage characteristics of the humidity detecting device. As shown in FIG. 3, a DC output voltage corresponding to the relative humidity can be obtained. In the configuration of this embodiment, the input terminal of the reference voltage is connected to the non-inverting input terminal 13 of the differential amplifier 7, but since the input of the reference voltage generating circuit 2 is the non-inverting input terminal of the differential amplifier 17, Impedance.
Therefore, it may be connected to a circuit component that determines oscillation conditions such as a resistor and a capacitor of the oscillation circuit 1 and hardly affects the oscillation condition.

(Embodiment 2) FIG. 4 shows a circuit configuration of a second embodiment. The difference from FIG. 1 is that in the oscillation circuit of FIG. 4, a variable resistor 27 is connected instead of the resistor 11 of FIG. 1, and the middle point of the variable resistor 27 is connected to an input terminal of a voltage follower 29, The output of 29 is the coupling capacitor 16
(Same as FIG. 1). Further, a variable resistor 28 is connected in place of the resistor 12 in FIG. 1, and a middle point of the variable resistor is connected to an input terminal of the reference voltage generating circuit 2 in FIG. Other configurations such as a humidity-voltage conversion circuit and a rectifying / smoothing circuit are the same as those in FIG. Also, 30
Is a differential amplifier.

The operation of the second embodiment uses the variable resistor 27 and the voltage follower 29, so that the input impedance of the voltage follower 29 is high, so that the output voltage on the multi-humidity side can be controlled without affecting the oscillation conditions of the oscillation circuit. Can be set. Further, by using the variable resistor 28 and the reference voltage generating circuit 2, the output voltage on the low humidity side can be set without affecting the oscillation condition of the oscillation circuit. Therefore, there is an advantage that no extra power is consumed.

[0016]

According to the present invention, a circuit component such as a resistor or a capacitor for determining the oscillation condition of the oscillation circuit is connected to a voltage follower having a high input impedance, and the output is further connected to a humidity-voltage conversion circuit. , The output of the oscillation circuit is added. As a result, an AC voltage centered on the reference voltage can be applied to the humidity-voltage conversion almost without being affected by the oscillation conditions of the oscillation circuit. Since a circuit for dividing the DC voltage is not separately required, the number of components can be reduced, the circuit configuration can be simplified, and the cost and size can be reduced. In addition, there is an excellent effect that power can be saved without consuming extra power for voltage division.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a humidity detecting device according to a first embodiment of the present invention.

FIG. 2 is a voltage waveform diagram of each part for explaining the operation of the oscillation circuit.

FIG. 3 is a diagram showing a humidity-output voltage characteristic of the humidity detection device according to the present invention.

FIG. 4 is a circuit diagram showing a humidity detecting device according to a second embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Oscillation circuit 2 Reference voltage generation circuit 3 Humidity-voltage conversion circuit 4 Rectifier smoothing circuit 5 DC power supply terminal 6 Output terminal 7 Differential amplifier 8 Resistor 9 Capacitor 10 Resistor 11 Resistor 12 Resistor 13 Non-inverting input terminal 14 Output Terminal 15 Inverting input terminal 16 Coupling capacitor 17 Differential amplifier 18, 21 Terminal 19 Humidity sensor 20 Thermistor 22 Output terminal

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Ohno 1006 Oaza Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-61-57841 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G01N 27/12 G01N 27/00-27/10 G01N 27/14-27/24

Claims (1)

(57) [Claims] 1. Humidity for converting a change in humidity into a change in voltage
A voltage conversion circuit, an oscillation circuit for applying an AC voltage to the humidity-voltage conversion circuit, a reference voltage generation circuit, and a rectifying and smoothing circuit for converting the output of the humidity-voltage conversion circuit to DC. The oscillation circuit includes a first differential amplifier, a capacitor, and a resistor, and one end of the resistor is connected to the first differential amplifier.
The reference voltage generating circuit is configured by connecting an output terminal of a second differential amplifier to an inverting input terminal of the second differential amplifier. Is connected to one end of the humidity-voltage conversion circuit via a coupling capacitor, and the other end of the humidity-voltage conversion circuit is connected to an output terminal of a second differential amplifier of the reference voltage generation circuit, An input of the generation circuit is connected to the resistor and a non-inverting input terminal of the second differential amplifier, and an output terminal of the humidity-voltage conversion circuit is connected to an input terminal of the rectifying and smoothing circuit. Humidity detector.