Heat stabilizer with a wide range. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers

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This device (unlike most others described in amateur radio literature) uses a thermocouple as a sensor. This significantly expands the scope of the proposed device. It is suitable not only for greenhouses and vegetable stores, but also for drying cabinets and even electric ovens.

The stabilizer maintains the temperature within the specified limits by turning the electric heater on and off. The maximum current of the switched load (heater) is 0,1 A at a voltage of 220 V, and with an additional triac switch - 80 A. The interval of controlled temperatures is 0 ... 500 ° С with a chromel-copel thermocouple or 0 ... 1200 ° C with chromel-alumel. The current temperature value is displayed on the LED digital display. Measurement error - no more than 1,5% of the upper limit of the interval. The accuracy of thermal stabilization largely depends on the thermal characteristics of the object (thermal chamber and objects in it) and the relative position of the thermocouple and the heater.

The schematic diagram of the device is shown in fig. 1. The voltage developed by the VK1 thermocouple and amplified by the op-amp DA1.4 is fed to the inputs of the op-amp DA1.1 - DA1.3, which serve as comparators. The thresholds for their operation are set by voltage dividers on resistors R1-R3, R7-R10. Resistor R2 sets the temperature threshold, below which the heater EK1 must be turned on. The temperature difference between switching on and off the heater is regulated by resistor R8. With the help of resistor R9, the comparator threshold is set at the op-amp DA1.3. When this threshold is exceeded, the comparator is triggered, the transistor VT1 opens, as a result, the HL1 LED lights up, signaling an unacceptable increase in temperature in the controlled area.

(click to enlarge)

Circuits VD2R14C2 and VD3R17C4 protect the trigger inputs DD1.1 from negative voltage at the outputs of the op-amp and interference. Depending on the state of the comparators DA1.1 and DA1.2, the output 5 of the trigger is set to a low or high logic level. The second trigger (DD1.2) is used to synchronize the moments of turning on and off the heater with the zero phase of the mains voltage, which significantly reduces the interference generated by the device. Pulses generated by optocoupler U1.2 from the voltage of the secondary winding of the power transformer T1 are fed to the input C of the DD1 trigger.

With the output 9 of the trigger DD1.2 is connected to the key input on the transistor VT2. The collector circuit of the transistor includes the HL2 LED (indicating that the heater is on) and the U2 optocoupler LED. Switch SA1 is used to force the heater to turn off.

The optocoupler thyristor U2 is located in the diagonal of the VD5 diode bridge and switches the load - the EK1 electric heater. Naturally, the current consumed by the heater should not exceed the values ​​​​allowed for the thyristor and the bridge. A more powerful heater can be connected according to the circuit shown in fig. 2.

Triac VS1 must be equipped with a heat sink.

The node for displaying the current temperature and its set value is assembled on the DA4 K572PV2 chip (foreign analogue - ILC7107), a detailed description of which can be found in [1]. The microcircuit is connected according to a typical circuit, seven-element LED indicators HG1-HG4 are connected to its outputs. If necessary, you can use a liquid crystal indicator by replacing the K572PV2 chip with K572PV5, as described, for example, in [2].

If the SB1 button is not pressed, the input 30 DA4 receives a voltage proportional to the current temperature from the output of the op-amp DA1.4. Otherwise, DA4 measures a voltage proportional to the heater switch-on temperature set by resistors R2 and R8.

The power unit consists of a transformer T1 with a rectifier on a diode bridge VD1 and two integrated voltage regulators - DA2 (+5 V) and DA3 (-5 V). The supply voltage of the collector circuits of transistors VT1, VT2 is not stabilized. The overall power of the transformer T1 is 5 ... 10 W, the secondary winding is 15 ... 20 V with a tap from the middle.

The device can be used fixed resistors MLT, tuning - SP5-2, variable (R2) - SPZ-45, capacitors K73-17 (C10, C12, C13), oxide - K50-35 or their foreign counterparts, the rest - ceramic, for example, KM-6. Optocoupler AOU115G can be replaced with ZOU1OZG Instead of LED indicators SA08-11HWA from Kingbright, others with a common anode are also suitable, for example, Paralight A-561SRD or KLTs402V - KLTs402E.

In the temperature range of 0 ... 1200 ° C, ready-made chromel-alumel sensitivity of 1 μV / C is used as a VK40,65 thermocouple. If the maximum temperature is not more than 500 ° C, chromel-copel (72,85 μV / ° C) is also suitable. In this embodiment, the value of the resistor R2 is reduced to 2,2 kOhm.

In the absence of ready-made thermocouples, they are made independently by spot-welding the ends of wire segments from the corresponding alloys and connecting ordinary copper wires up to several meters long to their opposite ends. It is not necessary to shield these wires, but they should not be laid near power circuits or wires that carry significant high-frequency and impulse currents. Some features of the device and the use of thermocouples can be read, for example, in [3].

Setting up the device consists in setting the correct readings of the LED indicator with a tuning resistor R6 at a minimum, and with a resistor R11 - at a maximum temperature. These adjustments are interdependent, so they must be repeated several times. In order to achieve the gain of the op-amp DA1.1 required for the chromel-copel thermocouple, it will be necessary to reduce the value of the resistor R13. In conclusion, resistor R8 sets the required temperature difference between switching on and off the heater, and resistor R9 - the threshold for switching on the emergency overheating alarm.

It is known that the EMF generated by a thermocouple is proportional not to absolute values, but to the temperature difference of its "hot" and "cold" junctions. To exclude the additional error caused by this, it is necessary to take care of the constancy of the temperature of the "cold" (non-working) junction of the thermocouple or compensation for its changes. One of the possible schemes of the compensation node is shown in Fig. 3.

The numbering of parts on it continues what was started in the previous figures. The temperature-sensitive microcircuit DA5 K1019EM1 [4] is located in the immediate vicinity of the "cold" junction and, if possible, in thermal contact with it. Part of the output voltage of the DD1 chip is added to the VK1 generated by the thermocouple. With an appropriate ratio of resistances of resistors R30 and R31, the voltage at the input of the op-amp DA1.4 will depend only on the temperature of the "hot" junction.

Literature

1. Anufriev L. Multimeter on BIS. - Radio, 1986, No. 4, p. 34-39.
2. Biryukov S. Application of ADC KR572PV5. - Radio, 1998, No. 8, p. 62-65.
3. Burkov V. Universal electronic thermometer. - Radio, 2000, No. 11, p. 34, 35.
4. Biryukov S. Thermal sensors K1019EM1, K1019EM1A. - Radio, 1996, No. 7, p. 59, 60.

Author: V.Tushnov

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