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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Temperature stabilizers in household appliances. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers The published article is devoted to the choice and practice of implementing electronic machines designed to maintain the required temperature in various household devices. The author's recommendations may be useful to many radio amateurs - designers. The scope of temperature stabilizers in devices used in the household is quite wide. These are, for example, vegetable storages, aquariums, small-sized incubators, bee heat treatment chambers, greenhouses and much more. An extensive literature is devoted to the design of thermal stabilizers for various purposes, the description of their work. Nevertheless, this topic, in my opinion, remains relevant, especially for those who decide to build such devices on their own. Taking into account certain difficulties associated with the acquisition of a number of parts, and various operating conditions of stabilizers, I would like to dwell on some general issues before describing specific designs. First of all, when starting to design a heat stabilizer, it is necessary to determine the power of the heater that provides the required temperature in a given volume. This is a separate, sometimes complex task that requires thermal engineering calculations. For approximate calculations, you can use simple formulas. So, for example, to protect products from freezing in your vegetable store at an outdoor temperature of up to -30 ° C in a box made of boards or chipboard 20 mm thick, with a foam layer 25 ... 30 mm thick, the required heater power should be , as indicated in [1]: P = V2/3, where P is the heater power, expressed in watts; V is the internal volume of the box in liters. For a loggia, frame greenhouse coated with glass or polyethylene, the required total power of the heater is determined by the following formula [2]: P \u1,23d XNUMX Sp Kt (tin - tnap), where P - heater power in watts; Sp is the total area of the cooling surface (walls, floor, ceiling) in m2; Kt - heat transfer coefficient in W/m2 °С; tin and tout are the internal and external temperatures in degrees, respectively. The value of the Kt coefficient can be from Kt = 3,3 (for double glazing) to Kt = 7,5 (for a single-layer polyethylene film). Any temperature stabilizer includes a sensitive element - a temperature sensor and a sensor signal amplifier; signal comparator or comparator; an electronic key that performs the functions of an actuating device; power supply and heating element. As a temperature sensor, thermistors of the KMT, MMT, ST series are usually used, the temperature coefficient of resistance (TCS) of which is negative - 2 ... 7% / deg. - and varies depending on the temperature, and the tolerance for the thermistor resistance value is 10 ... 30%. In amateur thermal stabilizers, thermistors are most often used because of the large TCR. However, their significant non-linearity and large tolerances require individual adjustment of the designed heat stabilizers, graduation of scales, making it difficult to replace in case of repair. The calculation of the parameters of a bridge with a semiconductor thermistor, with increased requirements for accuracy, is described, for example, in [3, 4]. The best metrological characteristics have temperature sensors of the TSM series - copper. Their TCR is positive, but it is only 0,3% / deg. = = 1/293°, and the linearity of the characteristic is ensured in a wide temperature range. They belong to devices of a high accuracy class (0,1 ... 0,5%) and can work even in aggressive environments. The disadvantage of TCM is a relatively large length (about 300 mm) and high cost.
Less known as a temperature sensor is a silicon diode, the negative conversion factor of which is 2 mV / deg. [5, 6]. Almost any low power silicon diode will provide linear temperature to voltage conversion. Any of the thermal converters listed here is usually included in one of the arms of the resistive bridge, the power supply of which is stabilized. The output signal of the bridge is fed to the input of the comparator or, if necessary, pre-amplified. To compare signals, it is most convenient to use a comparator, which is an operational amplifier (op-amp) with positive feedback. The comparison function can be performed by any op-amp of the K140, K553 series or specially designed comparators of the K554 series. The most preferred comparator is K554SAZ, which provides an output current of up to 50 mA, which allows you to directly turn on the electromagnetic relay of the actuator without an additional amplifier. The choice of one or another type of relay is determined by two factors - the value of the operating current and the allowable voltage and current of its switching contacts. At a mains voltage of 220 V, the relay contacts must reliably switch the heater current. The most common low-power relays are RES8, REN18 [7]. The windings of the REN20 and MKU-48 relays (passport 4.509.146) are designed to operate directly from an AC voltage of 220 V with a permissible contact current of 5 A, which in practice allows them to be used in most cases. With a parallel connection of two groups of contacts, these relays provide the inclusion of heaters with a total power of up to 2,2 kW. In addition to an electromagnetic relay, an element of an actuator that includes a heater can be a trinistor or triac. These devices make it possible to switch the current of heaters up to 80 A. The absence of contacts makes their use preferable. True, the design of the heat stabilizer itself becomes more complex than with an electromagnetic relay in the executive link. The thermal stabilizer power supply is, as a rule, a transformer that lowers the mains voltage to 13 ... 16 V, with one or two rectifiers and the simplest rectified voltage stabilizers. The power of the network transformer usually does not exceed 10 ... 15 watts. You can use unified transformers of the TPP series with the required set of secondary windings [8]. As a heat source, especially in terms of electrical safety, it is best to use tubular electric heaters - heating elements; suitable, of course, and conventional incandescent lamps, designed for mains voltage. Today, there are many circuit solutions for the construction of thermal stabilizers, in which the listed elements are combined in various combinations. For orientation in the choice of a designed temperature stabilizer, you can use the table proposed here, which shows the main technical data of some thermal stabilizers published earlier in Radio. At the same time, I propose for repetition a widely used thermal stabilizer (Fig. 1), in which a silicon diode or a copper resistor serves as a temperature sensor. Another difference of this version of the electronic machine is the absence of transistors in it and the presence of a microammeter for measuring temperature.
Like most of the thermal stabilizers listed in the table, it consists of four nodes: a sensitive element, a comparator, an actuator and a power supply unit. The temperature sensor, the function of which is performed by the VD1 diode, is included in the measuring bridge with resistors R1 - R4 in its other three arms. The signal from the output of the bridge is supplied (through resistors R5 and R6) to both inputs of the operational amplifier DA1 covered by negative feedback (circuit R8R9), and from its output to the inverting input of the comparator DA2. The required temperature in a closed volume is set by a variable resistor R12, equipped with an appropriate scale. The function of the actuator is performed by the electromagnetic relay K1. Actuating on the output signal of the comparator, the contacts K1.1 of the relay turn on the HL1 LED, which signals the heating is on, and the contacts K1.2 - the heater (Rn). The power supply is formed by a transformer T1, a rectifier bridge VD6, smoothing filters C5R17 and C6R18. Zener diodes VD4 and VD5 provide the microcircuits of the device with a bipolar voltage of ±10 V. For visual control of the air temperature in the heated volume, a microammeter RA1 was introduced into the device for a current of full deflection of the needle of 100 μA (M4248), the scale of which is calibrated in degrees. If the electronic part of the device is outside the heated volume, then the diode sensor (VD1) is connected to the resistive bridge with a shielded wire. When indicated in Fig. 1 microchips, resistor values and other details, the device provides temperature stabilization in the range of 0...20°C. To stabilize the temperature within +36 ... +45 ° C, necessary, for example, for an incubator, the nominal resistance of the resistor R13 should be 2 kOhm. All fixed resistors used in the heat stabilizer are MLT, and variables are SP5-2 (R4, R9 and R14), PPZ-40 or PPB (R12). Capacitors C3 - C6 - oxide K50-6, K50-16 or K50-29, the rest - KM-5 or KM-6. We will replace the KTs407A diode bridge with the KTs402 assembly with any letter index. Zener diode VD2 - for a stabilization voltage of 8 ... 8,5 V, and VD4 and VD5 - for 9,5 ... 10,5 V. Relay K1 - REN18 (passport РХ4.564.509) or MKU-48 (passport 4.500.232). Temperature sensor VD1 - any silicon. Better, however, in a metal case, for example, the D207 or D226 series with any letter index, since such a diode has less thermal inertia. The power of the mains transformer T1 of the power supply is approximately 5 watts. Its secondary winding must provide an alternating voltage of 2x12 V at a load current of 80 ... 100 mA. The heat stabilizer is mounted in a housing with dimensions of 170x90x60 mm. Most of its parts are placed on a printed circuit board with dimensions of 100x85 mm (Fig. 2), made of one-sided foil fiberglass. Transformer T1 and relay K1 are mounted separately, and microammeter RA1, variable resistor R12 and LEDs HL1 and HL2 are placed on the front panel of the housing.
It is better to set up the device in the following sequence. Place the VD1 diode in an environment with a temperature corresponding to the lower control limit (0 ° C), and balance the bridge with resistor R4. In this case, the readings of the microammeter should be zero. Then increase the temperature of the diode to the maximum value (20 ° C) and resistor R9 to achieve the maximum deviation of the microammeter needle up to 100 μA. Next, you need to adjust the operation of the comparator DA2. To do this, the engine of the resistor R12 is set to the uppermost position according to the diagram, and the diode VD1 is heated to a maximum temperature (20 ° C). The trimmer resistor R14 is used to switch the comparator to another state, operate the relay K1 and light up the HL2 LED. In this case, the division on the scale of the resistor R12 will correspond to a temperature of 20 ° C. Then, without changing the resistance of the resistor R14, the scale of the resistor R12 is calibrated at several points, achieving the operation of the comparator at different temperatures of the VD1 sensor diode. If a copper thermistor is used as a temperature sensor, the TKE of which is positive, it is included in the measuring bridge in place of the resistors R3 and R4, and these resistors in place of the VD1 diode. The procedure for adjusting the lower and upper limits of the temperature range remains the same. If the electronic part of the temperature stabilizer is outside the heated volume, the VD2 zener diode should be installed thermally compensated, for example, the D818 or KS191 series, to improve the accuracy of the device. Literature
Author: Yu.Andreev, St. Petersburg
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