"Hot cake" heater principle NE555 heater circuit diagram

Brief description of working principle

The electrical schematic diagram of the Haci brand "hot cake" heater is shown in the attached figure. Its working principle is: after plugging in the power plug of the "hot cake", the mains power will get about 7V voltage at both ends of C2 after R1, C1 current limiting, D1 rectification, DW voltage stabilization and C2 filtering. This voltage serves as the operating power supply for the control circuit. The control circuit is mainly composed of time base circuit IC1 (NE555) and peripheral components. It is used to control the heating time. After the power is turned on, the 7V voltage obtained at both ends of C2 begins to charge C3 through R3. Since the voltage at both ends of c3 cannot change suddenly, the ② and ⑥ pins of the time base circuit Ic1 are low level, and at this time, the output terminal ③ pin outputs a high level. Then the working winding of relay J is energized → the normally open contact JK is closed → the heater RL is energized for heating, and the heating indicator LED2 (red) is lit at the same time, indicating that the circuit is in a scheduled heating state. The voltage at both ends of c3 continues to rise with charging. When the voltage of pins ② and ⑥ of Ic1 reaches 2/3 of the working power supply voltage → the time base circuit flips → its output pin ③ outputs low level → the working winding of relay J loses power → Normally open contact JK is disconnected → heater RL stops heating (at this time, the temperature at the bottom of the heating plate is about 75°C). If the normally open contact of the relay cannot be disconnected due to adhesion failure and continues to heat. When the temperature inside the "hot cake" close to the heating part reaches 85°C, the thermostat ST will automatically disconnect and cut off the heating circuit to ensure safe use. . At the same time, the heating indicator LED2 goes out and the heating stop indicator LED1 (green) lights up, indicating that the circuit is in a heat preservation state. Since the heating stop indicator LED1 is lit, a voltage drop of about 3V will be formed at both ends of R5, causing the buzzer Bz connected in parallel to both ends of R5 to obtain working power and emit the musical sound of "Good people have a safe life". At this time, the user can unplug the mains power plug of the Rebao heater (after unplugging the mains power plug, LED1 will go out and the music will stop), and put the "Hotcake" heater into the place where heating is needed, such as In your arms, hands or under the covers.