Electric water heater temperature controller

Photocoupler

The electric water heater temperature controller introduced in this example has the functions of water level indication, constant temperature and anti-dry burning, automatic electric leakage protection, etc. It is safe and reliable to use. Circuit Operation Principle The electric water heater temperature controller circuit consists of a power supply circuit, a leakage protection circuit, a temperature control circuit, a water level indication and an anti-dry circuit, as shown in Figure 3-81.

The power circuit is composed of a power transformer T, a rectifier bridge stack UR, a filter capacitor Cl, C2, and a three-terminal voltage regulator integrated circuit IC1.

The leakage protection circuit is composed of a current transformer TA, a time base circuit inside the dual time base integrated circuit IC2 (lC2a, IC2b) and related peripheral components.

The temperature control circuit is composed of another time base circuit inside the dual time base integrated circuit 1C2, the thermistor RT, the transistor V, the relay K, the potentiometer RP and related peripheral components.

EH is an electric heater; VL1 is an operating state indicating light emitting diode, and VL6 is an electric heater working indicating light emitting diode.

After the power switch S1 is turned on, the AC 220V voltage is subjected to T step-down, UR rectification, Cl filtering, and ICl voltage regulation, and a +l2V voltage (Vcc) is generated across C2 as leakage protection, temperature control, water level indication, and anti-drying. The operating power of the burning circuit. When the temperature in the water tank is lower than 40 °C (the lower limit temperature of the water temperature is set), the voltage of pin 8 of lC2 is lower than Vcc/3, and the output of the 3 pin is high level, so that the transistor V is turned on, and the relay K is pulled in, which is normally open. When the contact is turned on, the electric heater EH is energized, and the VL6 is illuminated. When the water temperature is heated to 45 ° C (the upper limit temperature of the set water temperature), the 9 pin of IC2 becomes a low level, so that V is turned off, K is released, EH stops heating, and V is extinguished. When the water temperature drops below 40 °C, EH starts working again, VL6 emits light, and the temperature in the tank is constant between 40-45 °C.

When the electric heater EH is working normally, the working current of the W1 and W2 windings flowing through the current transformer TA is relatively stable, the induced voltage on the W3 winding is 0, the voltage of the 6 pin of the IC2 is greater than Vcc/3, and the voltage of the 2 pin is lower than 2Vcc/3, 5 feet output high level, LED VLl lights up, indicating that EH works normally, no leakage phenomenon.

When leakage occurs in the EH, the operating current in the W1 and W2 windings flowing through the TA suddenly increases, and an induced voltage is generated on the W3 winding, causing IC2's 2 pin to go high and 5 to become low. Flat, VLl goes out, indicating that EH has leakage. At the same time, the diode VD2 is turned on, so that the 10 pin of lC2 becomes a low level, and the 9 pin outputs a low level, so that V is cut off, K is released, and the working power of the EH is cut off, thereby realizing leakage protection.

S2 is the leakage protection test button. When the button is pressed, the resistor R1 is connected to the circuit to make a short-circuit current in the W1 and W2 windings of the TA, and the W3 winding generates an induced voltage, and the 2 pin of the IC2 becomes a high level, 5 The pin changes from high level to low level, and VL changes from lit state to off, indicating that the leakage protection circuit can protect against leakage.

The water level indicating circuit and the anti-drying circuit are composed of a water level detecting electrode AE, a resistor R9, an Rl2-Rl9, a light emitting diode VL2-VL5, and an analog electronic switch integrated circuit IC3. The four control terminals (5-pin, 6-pin, 12-pin, and 13-pin) of IC3 are connected to the water level detecting electrode AD, respectively. When the water tank is filled with water, when the water level rises to the electrode D, the main electrode E (which is grounded and connected to +Vcc) is connected to the D electrode through the resistance of the water, so that the 13 pin of IC3 is at a high level, and the foot is connected internally. The analog electronic switch is turned on, so that the 1 pin of IC3 becomes high level, the low water level indicates that the LED VL2 is lit, and the 10 pin of IC2 becomes high level, and the temperature control circuit controls the EH operation (the water level is lower than the electrode D When EH does not work, it prevents EH from being damaged due to dry burning.) When the water level continues to rise to the electrodes C, B, and A, the 12-pin, 6-pin, and 5-pin of IC3 also become high level one after another, and the water level indicating LEDs VL3, VL4, and VL5 are also sequentially illuminated.

Component selection

Rl selects lW wirewound resistor; R3-Rl9 selects 1/4W or l/8W carbon film resistor; R2 selects variable resistor (or replaces it with fixed resistor after debugging).

RT uses a negative temperature coefficient thermistor (adhesive tape attached to the outer surface of the tank), and its resistance should be about 15kΩ at room temperature.

RP selects a variable resistor or potentiometer (to adjust the water temperature at any time, you should use a potentiometer; to fix the water temperature, you can use a variable resistor).

Cl uses aluminum electrolytic capacitors with a withstand voltage of 25V; C2, C5 and C6 select aluminum electrolytic capacitors with a withstand voltage of 16V; C3 and C4 use polyester capacitors or monolithic capacitors.

Both VDl and VD2 use the lN4148 silicon switching diode.

VLl-V has a high-brightness light-emitting diode of φ5mm.

UR selects the rectifier bridge stack of lA and 5OV.

V selects C8050 or 2N5551 type silicon NPN transistor.

ICl selects LM7812 type three-terminal integrated voltage regulator; IC2 selects NE556 type dual time base integrated circuit; IC3 selects CD4066 type analog electronic switch integrated circuit.

T selects 3-5W, the secondary voltage is I5V power transformer.

TA adopts "E" type iron core or core self-made: Wl and W2 windings are wound with 2 or 3 turns of φlmm enameled wire, and W3 windings are wound around 2000匝 with dO?Ogmm enameled wire. The W1 and W2 windings can be wound around the outer layer to facilitate heat dissipation.

The electrode AE ​​was replaced with a copper (or stainless steel) screw of φ3-φ4 mm.

Circuit debugging

After the circuit components, water level detection electrodes, and electric heaters are installed, the controller should be debugged. First adjust the resistor R2 to the maximum value (lOkΩ), then turn on the power switch Sl, press and hold the test button S2, the LED VLl should be lit (if VLl does not emit light, the resistance of the resistor R4 should be increased). Slowly reduce the resistance of R2 until VL1 goes out. If VLl is illuminated after S2 is released, VLl is turned off when S2 is pressed, indicating that the leakage protection control circuit is working normally.

Then adjust the resistance of RP to the maximum value, then add water to the water tank. As the water level rises, the light-emitting diodes VL2-VL5 should emit light one by one. When VL2 emits light, the relay K should be closed, EH is energized, and VL6 is illuminated.

If the VM still does not emit light when the water level reaches the low water level electrode D, a 1-5kΩ resistor should be connected in series between RT and RP to make VL6 emit light.

When the water level rises to the required temperature, adjust the resistance of RP, so that relay K is released, RL is powered off, and VL6 is extinguished.