Overheating sensor, 60°C 10 seconds. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Indicators, detectors, metal detectors

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The described device is a temperature-frequency discrete action converter. By structure, this is an autogenerator, the positive feedback loop of which contains an ultrasonic communication line with a sensitive element included in it. They serve as a polymer self-resetting fuse.

Due to the temperature dependence of not only the electrical, but also the acoustic properties of such a fuse, the oscillation frequency of the oscillator changes. The sensor is designed to work as part of devices for temperature tolerance control of objects located in explosive environments, but can also be used in other similar systems, for example, in emergency fire alarm devices. Since the connection of the sensitive element with the electronic unit is acoustic, the current flow in the measuring circuit and the possibility of sparking in the controlled area are excluded.

Main Specifications

Response temperature, °С, not more than…60
Response time, s, no more than ... 10
Output voltage range, V, not less than…5
Supply voltage, V…27±3
Consumption current, mA, no more than ... 100

The device consists of a sensitive element included in the break of the sound guide, forming an ultrasonic communication line between the emitting and receiving piezoelectric transducers, a power amplifier, a preamplifier and a feedback circuit connecting the output of the preamplifier to the input of the power amplifier. The radiating transducer excites acoustic waves in the sound guide, which pass through the sensitive element and reach the receiving transducer, which converts them into an electrical signal. This signal, amplified by the pre-amplifier, is fed through the feedback circuit to the input of the power amplifier.

As a result of positive feedback, self-oscillations occur in the system. The sensitive element of the sensor is made of a material whose acoustic impedance changes sharply at a certain temperature. As a result, an abrupt change in the oscillation frequency occurs, which serves as a signal of overheating. After the cause of overheating is eliminated, the temperature of the sensitive element decreases, the acoustic resistance of the communication line, and the oscillation frequency return to their original values ​​- the sensor is ready for operation again. The sensor circuit is shown in fig. one.

Fig1 (click to enlarge)

A power amplifier is made on transistors VT1-VT4. Its voltage gain is determined by the ratio of the resistances of resistors R6 and R4. An emitting piezoelectric transducer BQ1 is connected to the output of the amplifier; it is acoustically connected to the receiving piezoelectric transducer BM1 through a sound guide and a sensitive element VK1. Capacitors C1 and C4 are separating. Diodes VD1 and VD2 set the bias voltage of transistors VT3 and VT4. The power amplifier is powered by a 20 V voltage regulator on the DA1 chip. Capacitor C3 - filtering in the power circuit.

The pre-amplifier is assembled on the op-amp DA3. Since the power supply of the op-amp is unipolar, with the help of resistors R10, R11 and R13, a bias equal to half the supply voltage is applied to its non-inverting input. Capacitor C6 is a blocking capacitor in the bias circuit. Resistor R12 sets the operating mode of the op-amp. Resistors R14-R16 and capacitor C7 form a negative feedback circuit that sets the gain of the preamplifier. The output of this amplifier is connected to the input of the power amplifier through capacitor C9, which completes the positive feedback circuit. Capacitor C10 is separating. The pre-amplifier is powered by a 15 V voltage regulator on the DA2 chip. Capacitor C5 is a filter element in the power circuit. The BK1 sensing element is a Bourns MULTIFUSE polymer resettable fuse. In a cooled state, the structure of the polymer filling it resembles a crystal lattice. When heated, it changes, so when a certain temperature is reached, there is a jump not only in the electrical conductivity of the polymer, but also in its acoustic resistance.

Most of the sensor parts are located on a breadboard with plated holes, the mounting is done with thin insulated wires. The board is placed in a metal case on which piezoelectric transducers are installed. The sensitive element of the sensor is located outside and is connected to the piezoelectric transducers by a sound guide - a U-shaped elbow made of steel wire with a diameter of 0,8 mm and a length of 1 m. The opposite ends of the sound guide are soldered to the working surfaces of the piezoelectric transducers. The sensitive element is soldered into the gap of the sound guide at the place of its bend.

The sensor uses tantalum oxide capacitors K53-52, it is permissible to use others, for example K53-4. Ceramic capacitors - K10-176 (or KM-3 - KM-6). Fixed resistors C2-33 (possible replacement - C2-23, MLT, OMLT). Trimmer resistor - SPZ-39a (or SPZ-37, RP1-48). Diodes KD522B can be replaced with other silicon diodes, for example, from the KD503, KD521 series. KT503G transistors can be replaced by transistors of the same series or silicon devices of other series with similar parameters. KT814G and KT815G can be replaced by transistors of the same series or series KT816 and KT817, respectively. Instead of imported microcircuits L7815, L7820, domestic microcircuits KR142EN8V and KR142EN9A can be used, respectively.

Piezoacoustic transducers BQ1, BM1 - unpackaged three-output foreign-made (probable type FML-34.7T-2.9B1 -L). Resettable fuse MF-R025 can be replaced with a similar one from Raychemf/Tyco or Little Fuse. Setting up the sensor consists in setting the tuning resistor R16 to such a gain in the positive feedback loop, at which stable generation is observed, and the signal at the output of the power amplifier is sinusoidal with a slight two-sided limitation. By increasing the temperature of the sensitive element VK1, its value is fixed, at which an abrupt change in the oscillation frequency occurs. You should make sure that the frequency returns to its original value when the sensing element cools down. In the author's version of the sensor, the frequency of generated oscillations at a sensitive element temperature of +20 °C was equal to 12,9 kHz, and when the temperature reached +40 °C, it increased abruptly to 85,3 kHz.

Author: O. Ilyin, Kazan, Tatarstan; Publication: cxem.net

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