Maintaining the temperature of the coolant. Encyclopedia of radio electronics and electrical engineering

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

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The heat carrier is water with a high temperature (not lower than 56 ° C), which is used in heating networks for space heating, and is also consumed in apartments and cottages for domestic needs. The lack of hot water makes it necessary to heat it on household electric and gas stoves, which creates certain inconveniences, causes excessive consumption of gas and electricity, and a violation of safety regulations. In some cases, it is possible to heat water in simple containers (boilers) by installing an electric heater in them. When the set temperature of the water in the tank is reached, the heater must be promptly turned off so that the water does not boil and break the boiler.

The classic water heater is made according to a simple scheme: a power switch and a heating element. At best, a pressure sensor and a temperature sensor (regulator) are added to them. The pressure sensor protects the boiler from increased water pressure, and the temperature sensor is triggered by an increase in temperature above a predetermined limit.

A bimetallic one is often used as a heater temperature controller, which is not much different from an iron controller. When the set water temperature is reached, the sensor opens the heater power supply circuit, the water temperature decreases naturally or as a result of consumption and addition of cold water, and the regulator contacts close again, turning on the heating. The simplicity of such a circuit often leads to malfunctions of the heater due to burning of the regulator contacts, which switch high currents.

To improve the reliability of the system, I suggest using an electronic temperature controller (Fig. 1). It allows you to set the desired temperature of the coolant and maintain it automatically.

(click to enlarge)

All sensors are located in a low-voltage circuit and are galvanically isolated from the network by optocouplers and a power transformer. The device consists of:

• temperature sensor (thermistor) RK1 to control and maintain water temperature;
• optocoupler linear signal amplifier VU1, which allows you to increase the sensitivity of the input circuit;
• analog programmable timer on the DA1 chip;
• power amplifier transistor VT1;
• optocouplers VU2 for galvanic isolation of control circuits and heater supply circuits;
• control key on triac VS1;
• power circuits on the transformer T1 and the diode bridge VD3.

Optoelectronic devices provide galvanic isolation of input and output circuits. The circuit uses two types of optocouplers: VU1 - diode-transistor optocoupler and VU2 - diode-thyristor. Optocouplers have a high current gain, which eliminates the need for additional amplification circuits at the input of the timer and in the triac control circuits.

The sensitivity of the thermistor (resistance change with temperature) when using an optocoupler increases from 2...5%/°С to 12...15%/°С. Diode-transistor optocoupler VU1 operates in linear mode. Changing the radiation of its LED changes the resistance of the collector-emitter of the internal transistor VU1. which is included in the timing circuit of the timer DA1. Accordingly, the charge time of the capacitor C2 of the external timer circuit changes.

Temperature regulation and setting are performed by variable resistors R1 and R7. which allows you to maintain any temperature values ​​of the heat carrier. Resistor R1 sets the heating temperature, R7 - heater power. The initial water temperature affects the resistance of the thermistor and, accordingly, the duration of the positive pulse at the output of the timer. At a low coolant temperature, the pulse duration at the output is maximum. The use of an integral timer makes it quite easy to execute a pulse generator. To operate the microcircuit in the oscillator mode, pins 2 and 6 are interconnected and connected to capacitor C2. In steady state, the interval Tj, during which a high level is active at the timer output, is determined by the relation T1=0l69(RVUi+R3)C2. When the internal transistor of the microcircuit opens, the capacitor C2 is discharged through the resistors R4 and R5, forming the second time interval T2 with a low level at the output DA1. Its duration is determined by the formula: T2=0,69(R4+R5) C2. The value of T2 does not change with temperature. The total pulse time T is T=T,+T2.

The duty cycle Q of pulses (Q=T/T1) increases with increasing temperature, thereby reducing the voltage on the heater and the temperature of the coolant. The frequency of the generator on the timer can be adjusted by changing the voltage at pin 5 of DA1. When the voltage drops, the timer generation frequency increases, and the heater power decreases.

The rectangular signal from the output 3 DA1 through the limiting resistor R6 is fed to the input of the power amplifier on the transistor VT1. Resistor R8 in its collector circuit limits the pulsed current through the VU2 optocoupler LED. The use of a transistor VT1 with a high gain makes it possible to generate the output signal of a transistor switch with minimal distortion. This signal is fed to the LED of the optocoupler VU2, amplified by a photodynistor and controls the operation of the power regulator on the triac VS1. Opening VS1 pulses of both polarities are formed by the diode bridge VD4. Optocoupler VU2 provides galvanic isolation of low-voltage and high-voltage circuits of the device.

If the dinistor of the optocoupler is open, the triac turns on at the beginning of the mains voltage subperiod, when the current through the control electrode reaches the threshold value, which reduces the level of interference of the triac converter.

To improve the accuracy of setting the temperature, the bridge and the timer are powered by a stabilized voltage from the DA2 stabilizer. Diode VD2 protects the stabilizer chip from possible reverse voltage breakdown. Capacitors C3 and C5 eliminate the ripple of the rectified voltage, capacitor C1 eliminates the interference that occurs when adjusting the resistor R1. Capacitor Sat. installed in parallel with the load, reduces the level of interference of the triac converter. The contacts of the pressure sensor P close the base VT1 to the body, stopping the heating of the coolant in case of emergency pressure in the heater.

The device uses widely used radio components. Fixed resistors - type MLT-0,125. variables - SP-Ill, thermistor - MMT-4. Oxide capacitors - K50-38, high-voltage (C6) - K73-17. the rest - KM. Timer series 555. The power transformer is used with a secondary winding voltage of 10 ... 12 V. The SA1 switch is automatic, for a current of 25 A. The pressure sensor is used from a Zhiguli car.

The device is assembled on a printed circuit board, the drawing of which is shown in Fig.2.

Temperature regulator R1 and power regulator R7 are installed on the front panel of the device for ease of use. The pressure sensor P and thermistor RK1 are mounted in the water heater housing using a threaded or welded connection.

The electric heater (TEH) is fixed with a flange through a rubber gasket at a small distance from the bottom of the heater tank. The drain cock should be above the heater, and the cold water inlet should be on top. The pressure sensor is installed in any convenient place, and the thermistor - just below the drain cock.

You can adjust the circuit by using an electric kettle instead of a water tank. This will speed up the setup work. The kettle plug is connected to the terminals of the heating element "and the circuit case. The temperature sensor RK1 is placed in boiling water, and after a few minutes the temperature controller R1 is used to make the HL1 heating indication LED go out. The voltage on the heater will drop to almost zero. The position of the R1 slider (100 ° C) The voltage and power at the load can be adjusted additionally by changing the resistance R7. The R7 slider is set to the maximum power position before temperature calibration. extreme temperatures are applied intermediate.

Wires suitable for the heater and triac should have a cross section of 4...5 mm2 (corresponding to a load current of 25...30 A). The wires to the sensors must be laid separately from the mains wires to prevent interference. The heater tank must be grounded.

By the brightness of the HL1 LED, you can visually determine the power in the load. When the LED goes out, the heater is off or the pressure in the tank is critical.

Literature

1. M.A.Shustov. 450 useful schemes for radio amateurs, 2007.
2. G. Schreiber. 400 new electronic circuits. 2006.

Author: V.Konovalov, Irkutsk

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