Thermostat for water tank. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, temperature stabilizer

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The author uses this device to control the heating of water in the summer shower tank. Automatic shutdown of the heater is provided in case of insufficient amount of water in the tank. This regulator can be used in everyday life and in other cases when it is required to maintain the set water temperature.

Rice. 1 (click to enlarge)

The device, the scheme of which is shown in Fig. 1, maintains the set temperature of the water in the tank or other reservoir. Water is heated using a heating element, the protection unit of which from its insufficient level is implemented on transistors VT1 and VT2. Electrodes E1, E2 form a level sensor. When it is sufficient, the electrical resistance of the gap between the electrodes is small due to the presence of water in it. Transistor VT1 in this case is closed, and transistor VT2 is open, current flows through the winding of relay K1, included in its collector circuit. The power circuit of the temperature control and heater control unit, built on a parallel integrated voltage regulator DA2, is closed by contacts K1.1.

A voltage divider of resistors R1, R7 and thermistor RK8 is connected to the control electrode (pin 1) of the stabilizer, and the relay winding K3 is connected to the cathode circuit (pin 2). When the water temperature drops, the resistance of the thermistor RK1 increases, the voltage between the control electrode and the anode (pin 2) of the stabilizer increases. As soon as it exceeds 2,5 V, the stabilizer cathode current increases sharply and becomes sufficient to operate relay K2. Its K2.1 contacts turn on the heater. The temperature at which this happens is regulated by a variable resistor R7.

As the water in the tank warms up, the resistance of the thermistor RK1 decreases, the voltage between the control electrode and the anode of the stabilizer drops below 2,5 V and the current of its cathode decreases to 0,3 ... 0,4 mA, which is not enough to keep the K2 relay in the triggered state. Opened contacts K2.1 break the power supply circuit of the heater.

Resistor R6 provides positive feedback, providing an avalanche switching of transistors VT1 and VT2, which reduces the bounce of the contacts of relay K1. Diodes VD2 and VD3 protect the transistor VT2 and the stabilizer DA2 from self-induction voltage surges on the relay windings K1 and K2.

The power supply unit of the thermostat from the 220 V AC network is built according to the usual scheme on a step-down transformer T1 with an overall power of several watts. The HL1 LED indicates that the device is connected to the 220 V network. The HL2 LED on indicates that there is enough water in the tank, and HL3 indicates that the heater is working.

A 1x30 mm plate made of stainless steel sheet 50 mm thick was used as electrode E3, and the metal case of the tank was used as electrode E2. The plate is set to 3 ... 4 cm

above the heater at a level corresponding to the minimum allowable water level to ensure fire-safe operation. The insulation of the plate from the housing must be waterproof. The thermistor RK1 is fixed in the tank next to the plate E1. It is placed in a glass tube to protect it from direct contact with water.

Fig. 2

The thermostat is assembled on two single-sided printed circuit boards. The first of them, the main one, is shown in Fig. 2, and the second, on which three LEDs with current-limiting resistors are located, is in fig. 3.

Ris.3

The boards are designed for the installation of fixed resistors MLT and the like. Trimmer resistor R3 - PV-36W (POT3106W), variable R7 - R-24N1. Thermistor RK1 - MMT-1, it can be replaced by MMT-4, KM7-1, KMT-4 and others with negative TCR and resistance Yu ... 22 kOhm at a temperature of 25 CC. Oxide capacitors - K50-35 or their imported counterparts. The remaining capacitors are ceramic or film.

Instead of KT315A transistors, you can use KT315V, KT315G or other low-power npn silicon structures. Analogs of the integrated voltage regulator 7812 - KR142EN8B, KR142EN8D, and the parallel stabilizer TL431A - KR142EN19. Diode bridge VD1 can be any with a forward current of 0,5 ... 1 A. LEDs can be domestic or foreign production with a permissible forward current of about 20 mA, HL1 - green, HL2 - yellow, HL3 - red glow. Relays K1, K2 - JZC-412 with a rated voltage of 12 V. If the heating element has a power of more than 500 W, it must be connected to the thermostat through an intermediate relay of the appropriate power. As T1, any low-power network transformer with a secondary voltage of 12 V is suitable.

Fig. 4

The thermostat boards are placed in a plastic case (Fig. 4). LEDs HL1 - HL3 are displayed on its front panel. It also has a variable resistor R7 (temperature controller), a terminal block for connecting a water level sensor and a thermistor, a socket for a heater. The power cord with a power plug is removed from the rear panel.

Establishing the regulator comes down to setting the trimmer resistor R3 to the level of the Schmitt trigger on transistors VT1 and VT2. ensuring reliable operation of the relay K1 in the presence of water between the electrodes E1 and E2. The desired water temperature in the tank is set using a variable resistor R7 empirically.

Author: A. Titarenko

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