Thermal stabilizer on the AD597 chip. Encyclopedia of radio electronics and electrical engineering

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The use of the AD597 thermocouple signal converter [1] manufactured by Analog Devices makes it possible to achieve high linearity of temperature measurement in a wide temperature range and apply digital temperature indication without additional correction. The offered thermostabilizer allows to measure temperature in the range of -200...+200 °С, to regulate it in the range of 0...+200 °С, to read the value from the 3,5-digit liquid crystal display.

Rice. 1 (click to enlarge)

The heat stabilizer (Fig. 1) contains a BK1 type K thermocouple (nickel-chromium / nickel-aluminum alloys (chromel / alumel)), its signal converter is a DA8 microcircuit integrating ADC DD2, a reference voltage source made on an op-amp DA6 and a transistor assembly DA4 [2], comparator on DA7 op amp, VS1 triac control circuit and stabilized power supply.

The required temperature value in the range of 0...+200 °C is set by a variable resistor R20. If necessary, this interval can be shifted to the region of negative temperature values ​​(you will need an additional source of reference voltage -1 ... -5 V, easily implemented on the basis of the one shown in the diagram or made on a specialized microcircuit) or expanded without violating the linearity of the resistor scale, which is important difference from heat stabilizers with a temperature sensor based on a thermistor.

A feature of the comparator used on the op-amp DA7 is the absence of a resistive hysteresis circuit, as a rule, high-resistance and requiring in the general case (to ensure the specified response thresholds - turn off the comparator at a given temperature value) an accurate selection of the circuit resistance. With a wide control interval, this, in turn, leads to the need to use a set of megohm constant and variable resistors.

At the moment the voltage at the inverting input of the op-amp DA7 reaches the threshold (when the temperature drops), a positive polarity voltage appears at its output and the transistor VT1 opens. As a result, if the switch contacts

SA1 are closed, relay K1 is activated. With its contacts K1.1, it short-circuits the resistor R4 in the circuit of the control electrode of the triac VS1, as a result of which it opens and turns on the heater EK1, and with the contacts K1.2 it starts the single vibrator DA1, made on the integral timer NE555N [3].

The outputs of the single vibrator and comparator are connected to the gate of the transistor VT1 according to the "diode OR" circuit. The duration of the pulse generated by the single vibrator tz = 1,1R5*C9 = 1 s. Thus, when the threshold of the comparator operation is reached, the relay K1 is switched on during the time tз, and further heating occurs until the comparator is stable. At the end of tz, if the comparator continues to be in an unstable state, the one-shot restarts at the first closing of contacts K1.2. This solution allows minimizing the bounce of the relay contacts when the comparator is in an unstable state.

The duration of the pulse t3, if necessary, you can choose a different one, thereby changing the lower and upper values ​​​​of the temperature of the heated body or object.

When switch SA1 is moved to the position shown in the diagram, relay K1 and triac VS1 are immediately turned off. In series with the heater, it is necessary to turn on the emergency thermal relay (its contacts are indicated in the diagram as KK1.1), set to the maximum allowable temperature, the sensitive element of which is located in the region of the highest temperature of the heated body or object.

To indicate the health of the power supply, yellow LEDs HL2, HL3 are used, which are also used to illuminate the HG1 liquid crystal display. The HL4 LED is a red glow indicator. The active state of the comma of the third digit of the display is provided by using the logic element DD1.1 [4].

The device uses fixed resistors MLT and trimmers SP5-3. Variable resistor R20 - wire of any design with a linear dependence of resistance on the angle of rotation of the engine, it is permissible to use an imported multi-turn with a revolution counter on the handle. Capacitors C1, C13, C15, C16-K73-17, C3, C4, C7-C10 - oxide K50-35 or imported, the rest - KM-6. Relay K1 - intermediate reed switch RPG-2-2202U3 (rated voltage - 12 V, power consumption - 0,3 W). Given the possible difficulties with its acquisition, I recommend making such a relay on your own based on two KEM-1 reed switches (ODO.360.037 TU).

Fig. 2

The coil of a home-made relay is wound with PEV-2 0,2 wire (2400 turns) on a frame (Fig. 2) made of fiberglass with a thickness of 1 ... 1,5 mm. After assembly, the joints of its parts are glued with an epoxy compound, the excess of which, after hardening, is removed with a needle file. coil after

it is not necessary to fill the windings with a compound, it is enough to protect it from the outside with varnished cloth. Reed switches are placed inside the frame. They must enter there freely, it is necessary to exclude the occurrence of mechanical stresses in their housings and shock loads during installation and operation. The finished relay is placed in a polystyrene case. It is allowed to use a heat-shrinkable tube of a suitable diameter, it should tightly cover the cheeks of the frame.

The elements of the device are mounted on three printed circuit boards. The first one contains all integrated circuits (the input terminals of unused elements of the DD1 microcircuit are connected to a common wire), with the exception of voltage stabilizers, the second one contains the power transformer T1, fuses FU1, FU2, integrated stabilizers DA2, DA3, DA5 and triac VS1 (with separate heat sinks each), relay K1 and neon lamp HL1, on the third - display and control elements. To simplify the installation and maintenance of the device during operation, the first board is connected to the third board using a bundle and a 26-pin connector: a two-row PBD-26 plug (with a pin spacing of 2,54 mm) is installed on the first one, and the mating part - a BLD-26 socket - mounted on the wiring harness coming from the third. A part of a built-in socket 1 A - 16 V with a ceramic base is used as a connector for connecting the load X250.

The temperature stabilizer is assembled in a modified plastic case for amateur radio structures of size G010 (95x135x45 mm) manufactured by Kemo Germany GmbH. The refinement consisted in increasing the size of 45 mm to 115 mm by inserting between the body halves of two plates measuring 70x135 mm from sheet organic glass 3 mm thick. The first two boards are installed in the provided places of the case halves, the third one - in its front part, and all the connectors - in the back. On fig. 3 shows the device from the front (with the top cover removed and the transparent front panel), and in fig. 4 - behind.

Fig. 3

Fig. 4

Setting up the device comes down to setting the frequency of the built-in clock generator of the DD2 chip (40 kHz) with a trimmer resistor R28 and the reference voltage Uobr = 1,000 V at its output 36 with a trimmer resistor R12.

Literature

1. Thermocouple Conditioner and Setpoint Controller AD596*/AD597*.-
2. Horowitz P., Hill W. The art of circuitry. In 2 volumes. 3rd ed. stereotypical. Translation from English. Edited by M.V. Galperin. - M.: Mir, 1986.
3. Kolombet E. A. Timers. - M.: Radio and communication, 1983.
4. Biryukov S. Application of ADC KR572PV5. - Radio, 1998, No. 8, p. 62-65.

Author: D. Molokov

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