JPH05296830A - Pyroelectric element - Google Patents

Abstract

PURPOSE:To provide an element which has a frequency characteristic suitable for fire detection by a method wherein the resistance value of a gate resistance is lowered. CONSTITUTION:An electric circuit is constituted of a pyroelectric body 1, a field-effect transistor(FET) 2 and the like. A gate electrode for the FET 2 is connected to one terminal of the pyroelectric body 1; a gate resistance 3 is connected in parallel across both ends of the pyroelectric body 1; the resistance value of the gate resistance 3 is set at 5 to 50GOMEGA When the resistance value of the gate resistance 3 is lowered, it is possible to obtain an element which is suitable for the frequency characteristic of infrared rays due to a flame. When the resistance value of the gate resistance is suppressed to 50OMEGAG or lower, a noise component due to a temperature change an be reduced, and a signalless state which is generated in a temperature cycle test or the like does not appear.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pyroelectric element for detecting infrared rays used in flame detectors for fire detection and intrusion detectors for crime prevention.

[0002]

2. Description of the Related Art A pyroelectric element is an infrared sensor using a pyroelectric body, and a flame detector for detecting infrared rays of a flame when a fire occurs, or an intrusion detector for detecting infrared rays of a human body used for crime prevention equipment. It is used as a detection element.

Conventionally, as shown in FIG. 2, a pyroelectric element has a casing made up of a metal can 10 having a filter 9 and a stem 11. The printed circuit board 12 is placed on the upper ends of the three pins 4, 5, 6 that form the drain terminal, the source terminal, and the ground terminal that penetrate the stem 11, and the electrical circuit section is configured.

In the electric circuit section of the pyroelectric element, a source electrode of a field effect transistor (hereinafter referred to as FET) for a preamplifier is usually connected to one electrode of a pyroelectric body. And
A gate resistor is provided in parallel with the pyroelectric body to control the frequency characteristic of sensitivity, and the cutoff frequency in the low frequency band is determined by the electrical time constant. For this gate resistance, one with a high resistance of 100 to 200 giga ohms is conventionally used,
Especially for the human body, the frequency characteristics of the element itself are widened.

[0005]

When the resistance value of the gate resistance is 100 giga ohms, the response frequency of the sensitivity of the pyroelectric element has a mountain-shaped characteristic with a peak at about 0.5 Hz.
The frequency characteristic due to the flame required for fire detection is about 8Hz
It is known that there is a peak at about 0.5Hz.
With a characteristic having a peak at, when detecting a flame, the output sensitivity has a peak outside the frequency band to be detected. However, the frequency characteristic of infrared rays from the human body when used for an intrusion detector has a peak at about 0.1 Hz.

Further, if the ambient temperature of the pyroelectric element is changed, the output may not be output. That is, when the resistance value of the gate resistance is large, the voltage applied to the pyroelectric body is small. For example, when the ambient temperature is decreased at a rate of 1.4 ° C./min, no signal occurs. A change in ambient temperature has various factors in a normal installation place such as a relation of air conditioning, and a no-signal state is very problematic.

[0007]

SUMMARY OF THE INVENTION The present invention aims at obtaining a pyroelectric element having a frequency characteristic suitable for fire detection, and if the resistance value of the gate resistance is lowered, the frequency response characteristic becomes high frequency side. Focusing on that the peak shifts to the high frequency side so as to reduce the sensitivity on the low frequency side without lowering the sensitivity, the electric circuit composed of the pyroelectric body and the FET etc. is configured in the housing, and the gate electrode of the FET is It is connected to one terminal of the electric body, the gate resistance is connected in parallel between both terminals of the pyroelectric body,
It is characterized in that the resistance value of the gate resistance is 5 giga ohms to 50 giga ohms.

Further, even if the ambient temperature fluctuates, an electric circuit including a pyroelectric body and an FET is formed in the housing for the purpose of obtaining a considerable output, and the gate electrode of the FET is one of the pyroelectric bodies. It is characterized in that it is connected to a terminal, a gate resistance is connected in parallel between both terminals of the pyroelectric body, and the resistance value of the gate resistance is 30 giga ohms or less.

[0009]

By lowering the resistance value of the gate resistance, a pyroelectric element adapted to the frequency characteristics of infrared rays due to the flame can be obtained. Further, if the resistance value of the gate resistance is suppressed to 50 giga ohms or less, the noise component due to the temperature change can be reduced, and a signalless state (a signal disappears when the temperature change of the element is steep) occurs during a temperature cycle test or the like. State, called pinch-off) disappears.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. 1 or 2. FIG. 1 is an embodiment of a circuit diagram of a pyroelectric element of the present invention, and FIG. 2 is an embodiment of a schematic sectional view thereof. However, since FIG. 2 is similar to the above-mentioned conventional example, description thereof will be omitted.

In FIG. 1, 1 is a pyroelectric body made of a material such as PVDF or PZT, 2 is a FET, and its gate electrode is connected to one electrode of the pyroelectric body 1. 3 is the gate resistance of the FET 2 connected in parallel to the pyroelectric body 1, 4 is the drain terminal connected to the drain electrode of the FET 2, and 5 is the FET 2
Is a source terminal connected to the source electrode, and 6 is a ground terminal connected to the electrode at the other end of the pyroelectric body 1. 7 is a FET
2 is provided between the drain electrode and the ground to stabilize the supply voltage, 8 is provided between the source electrode of the FET 2 and the ground, and is a capacitor for cutting induction noise on the high frequency side. No. 8 has a capacitance of about 10 pF, does not affect the experimental results described later, and may be omitted from the configuration. The electric circuit section including the FET 2 and the gate resistor 3 is mounted on the printed board 12 in the housing.

In such a pyroelectric element, the gate potential of the FET 2 changes in response to a change in infrared rays received by the pyroelectric body 1, and due to this potential change, a detection voltage is applied between the source terminal 5 and the ground terminal 6. Occurs. The detected voltage is connected to an electric circuit (not shown), amplified by an amplifier circuit, and then compared with a reference value by a discriminating circuit to output, for example, a fire signal.

FIG. 4 shows an apparatus configuration for examining the frequency response characteristic of the pyroelectric element. Reference numeral 13 is a light source for irradiating infrared rays, 14 is a rotation chopper capable of rotating at a constant cycle to create an arbitrary frequency, 15 is a pyroelectric element to be tested, and 16 is an AC coupling for electrically amplifying the output of the pyroelectric element 15. amplifier,
Reference numeral 17 is a bandpass filter for extracting only the limited output corresponding to the rotation frequency.

FIG. 3 shows the results of examining the frequency response characteristics of the pyroelectric element using the apparatus shown in FIG. In the figure, the horizontal axis represents the frequency (Hz) of the rotary chopper 14, and the vertical axis represents the output sensitivity (mV) of the pyroelectric element 15. Curve A is pyroelectric body 1
Is PVDF and the resistance value of the gate resistor 3 is 100 GΩ, and the curve B shows that the pyroelectric body 1 of the curve A is PZ.
This is the case of T. When the resistance value of the gate resistor 3 is 100 giga ohms, the same characteristics are exhibited regardless of the material of the pyroelectric body 1, and the peak shape is around 0.5 Hz. With this characteristic, it is possible to deal with both infrared rays from the flame and infrared rays from the human body.

Curve C is obtained by setting the resistance value of the gate resistor 3 of curve A to 10 gigaohms. In the case of the curve C, the sensitivity of the low frequency side is lowered without lowering the sensitivity of about 8 Hz, which is peculiar to the flame. Cutoff frequency,
That is, assuming that the peak value of the mountain shape is fc, fc has the following relationship.

Fc = 1 / (2π · c · Rg) where c is the capacitance of the pyroelectric body 1 and the FET 2
Of input resistances, Rg is the resistance value of the gate resistor 3, and π is the pi. According to this formula, the cutoff frequency fc increases as the resistance value Rg is decreased, and the resistance value Rg may be decreased to cut the low frequency range. Therefore, if the resistance value of the gate resistance 3 is set to 5 giga ohms, the peak is located at a portion of about 8 Hz, but the sensitivity itself is slightly lowered. Therefore, if the resistance value is further lowered, the sensitivity is lowered and the peak position is also deviated. These are problems with the electrical properties of the pyroelectric body 1 and are not related to the material of the pyroelectric body 1.

The magnitude of the resistance value of the gate resistor 3 also affects the temperature cycle test. The temperature cycle test is a test in which the temperature is normally changed from a high temperature to a low temperature periodically for a predetermined time, and a normal detection signal is output after several times. In fact, it does not affect the results of this test, but rather the signal when changing the temperature.

FIG. 6 shows an apparatus for performing a temperature cycle test. The mounting portion 22 of the element to be tested on the frame 21
Is provided on the element light receiving surface side of the frame 21.
An insulating cap 24 filled with 3 is attached. A signal line 25 for taking out a signal is soldered to the pin of the element protruding from the mounting portion 22. The entire device is installed in a temperature adjusting device (not shown).

FIG. 5 shows the noise level when the temperature cycle test of the pyroelectric element was conducted using the apparatus of FIG.
D is a resistance value of 5 giga ohms, E is 10 giga ohms, F is 30 giga ohms, G is 50 giga ohms, H
Is for 100 giga ohms, and 2 for each resistance value.
This is done for each device. I is the ambient temperature at that time, and is changed in the range of −50 ° C. to + 50 ° C., with a slope of about 3.7 ° C. for rising and about 1.4 ° C. for falling.

As is clear from the results, in the case of 50 G-ohms of G and 100 G-ohms of H, a complete pinch-off appears. On the other hand, in the case of 10 gigaohms of E and 30 gigaohms of F, 5 of D
It's not as stable as Gigaohm, but the signal keeps coming out.

This is because, in the circuit configuration of this embodiment, when the temperature is relatively rapidly decreased from the high temperature side to the low temperature side, electric charges are temporarily accumulated on the pyroelectric body 1 and the FET 2 is saturated. As a result, the signal disappears.
This suggests that this pyroelectric element has a state in which no signal is output when the fluctuation of the outside air is large.

That is, if a flame detector or an intrusion detector is made of an element that causes pinch-off, an accurate fire signal or crime prevention can be obtained when the ambient temperature changes, for example, when there is a sudden temperature change due to strong wind or moisture adhesion. Signal cannot be sent.

Therefore, reducing the resistance value of the gate resistor 3 is effective not only for frequency characteristics but also for pinch-off and signal stability.

[0024]

As described above, according to the present invention, the electric circuit is formed in the housing, the gate electrode of the field effect transistor is connected to one terminal of the pyroelectric body, and the gate is provided between both terminals of the pyroelectric body. A fire detection pyroelectric element characterized in that resistors are connected in parallel, and the resistance value of the gate resistance is 5 Giga ohms to 50 Giga ohms. A pyroelectric element adapted to the frequency characteristics of infrared rays can be obtained.

Further, if the resistance value is suppressed to 30 giga ohms or less, pinch-off does not appear even if the ambient temperature changes, and the signal stability is improved.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram briefly showing a circuit configuration of a pyroelectric element of the present invention.

FIG. 2 is a sectional view simply showing the structure of a pyroelectric element of the present invention.

FIG. 3 is a diagram showing frequency characteristics of a pyroelectric element in terms of sensitivity to response frequency.

FIG. 4 is an apparatus configuration diagram for measuring the frequency characteristic of FIG.

FIG. 5 is a diagram showing a noise level when a temperature cycle test of a pyroelectric element is performed.

6 is an apparatus configuration diagram for performing the temperature cycle test of FIG.

[Explanation of symbols]

 1 Pyroelectric body 2 Field effect transistor (FET) 3 Gate resistance

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H01L 37/02 9276-4M

Claims (2)

[Claims] 1. In a pyroelectric element used for fire detection for detecting a fire by detecting infrared rays of a flame, an electric circuit including a pyroelectric body and a field effect transistor is formed in a housing, A gate electrode is connected to one terminal of the pyroelectric body, a gate resistance is connected in parallel between both terminals of the pyroelectric body, and the resistance value of the gate resistance is 5 gigaohms to 50 gigaohms. Pyroelectric element. 2. In a pyroelectric element used for fire detection or intrusion detection by detecting flame or infrared rays from a human body, an electric circuit including a pyroelectric body and a field effect transistor is formed in a housing, and the field effect transistor is formed. Is connected to one terminal of the pyroelectric body, a gate resistance is connected in parallel between both terminals of the pyroelectric body, and the resistance value of the gate resistance is 30 gigaohms or less. Pyroelectric element.