Heat stabilizer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers
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In the proposed device, positional control with noiseless load switching is used when the mains voltage passes through zero. In addition, its distinguishing feature is the ability to quickly switch between the "Heating" or "Cooling" operating modes.
The proposed heat stabilizer in the winter will help to better preserve fruits and vegetables:
- on a balcony or loggia in a heat-insulated container with a built-in heater;
- in the pantry, basement using a collector motor with a fan that blows cold outside air.
The stabilizer circuit is shown in fig. 1, and the drawing of the printed circuit board - in fig. 2. The required temperature is set by a variable resistor R7. The device contains a switch that allows you to set cooling or heating modes.
(click to enlarge)
Temperature sensor - thermistor RK1. It is powered by a parametric stabilizer HL1R5. The sensor signal is amplified by the transistor VT2 and acts on the inputs of the element DD1.2: high temperature corresponds to a high level at its output, and low - low. Resistor R9 provides some hysteresis for the sensor response when the temperature changes, which is necessary for positional control.
Further, the direct or inverted element DD1.3 signal of the state of the temperature sensor is fed to the lower input according to the circuit (pin 12) of the match element DD1.4, at the upper input of which (pin 13) there are synchronization pulses corresponding to the moments when the mains voltage passes through zero. Only the presence of a high-level voltage at the lower input allows the supply of control pulses to the triac, therefore, the "Heating" position of switch SA1 corresponds to turning on the triac when the temperature decreases, and the "Cooling" position corresponds to increasing it. The НL2 LED indicates the operation of the regulator: the red glow corresponds to the disconnected state of the load, and the green glow corresponds to the switched on state.
For reliable operation of the triac at low temperatures (up to 2 ... 4 ° C), the control electrode current is increased to 80 mA, and the pulse duration is up to 0,7 ms (0,3 ms until the mains voltage passes through zero and 0,4 ms after). For such a pulse and a load power of 75 W, the instantaneous values of the current through the triac at the pulse fronts somewhat exceed the normalized values of the holding current. However, in addition to such coordination, the load of the regulator should provide the required thermal regime in the storage with a margin. With the same quality of thermal insulation, a change in power only affects the ratio of the time of the on and off state of the load and does not affect the regulated average temperature and the electrical modes of the regulator elements. Therefore, it is advisable to increase the load power when heated (rheostat, incandescent lamp, etc.) so that the worst examples of triacs with an increased holding current can be used. For cooling, the fan power may not be high and a better example of a triac with a small current will be required. It is important to ensure that this condition is met. The fact is that if, due to the low load current, the triac does not open in one of the half-cycles, a unipolar current will flow through the load, which is completely unacceptable for an AC motor.
The following parts were used in the temperature stabilizer: variable resistor - RP1-64A, thermistor - MMT-1, resistor R9 - KIM with a power of 0,125 W, capacitor C2 - K50-29. Switch SA1 - a jumper that is set to the desired position before using the stabilizer.
Establishing a thermal stabilizer comes down to setting a controlled temperature in the storage using a thermometer with a variable resistor R7. During operation, the temperature in the storage should be periodically monitored so that the products do not deteriorate due to power outages, malfunctions, severe frosts, etc.
The device has a galvanic connection with the electrical network. This should be remembered during the manufacture and adjustment of stabilizers and precautions should be taken: all changes to the design should be made only in a state disconnected from the mains.
Author: V. Zhgulev, Serpukhov, Moscow region
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