Electric blanket automatically protects thermostat circuit

At present, the temperature control of many electric blankets uses manual switches, which are controlled by a fast heating gear and a slow heating gear. In this way, it is difficult to control the internal temperature of the electric blanket to work at a certain constant temperature, which is very convenient for the user. Causes a lot of trouble. This article introduces to you an automatic control circuit for electric blankets, which can work at a predetermined temperature and also function as self-locking and over-voltage protection in case of power outage. 1. Introduction to circuit principle 　　The working principle of the circuit is shown in Figure l. It is mainly composed of a power outage self-locking circuit, an overvoltage protection circuit, and an automatic constant temperature control circuit. When button switch S2 is pressed, the 220V alternating current is stepped down by capacitor C2, stabilized and rectified by V6 and V7, and filtered by C3 for use by relay J. At this time, J is energized and closed, and the normally open contact J1 is connected, making the circuit self-locking. At this time, even if the button switch S2 is released, the power supply to the electric blanket and relay J can be maintained. When the electric blanket encounters a power outage during use, the relay J will be powered off and released, and the normally open electric shock J1 will be disconnected, so the electric blanket will not work. When the power grid is restored, the relay and the electric blanket cannot be automatically turned on. You can only press S2 again to make the relay J close, and the electric blanket will be powered on again. Rt is a positive temperature coefficient thermistor, and its internal resistance changes with the temperature it feels. That is, the internal resistance increases when the temperature is high, and vice versa. The electric blanket constant temperature control circuit is composed of rectifier diodes V2~V5, one-way silicon controlled thyristor SCR, two-way trigger diode V1, positive temperature coefficient thermistor Rt, capacitor C1, and potentiometer RP. The control process is: the alternating current is changed into direct current after bridge rectification by V2 ~ V5, and is charged to C1 through RP and Rt. When the voltage on both ends of C1 reaches a certain value, the bidirectional trigger diode breaks down and conducts and is in a discharge state. At this time, the one-way thyristor is triggered to conduct, making the electric blanket energized to work. When the internal temperature of the electric blanket rises to a certain value, the internal resistance of the thermistor Rt is very large, and the voltage falling on C1 is small and cannot cause V1 to break down, then the SCR is turned off, and the corresponding electric blanket does not work when it is powered off. , at this time the electric blanket is in cooling state. When the temperature of the electric blanket drops to a certain value, the above control process is repeated again and again to keep the temperature of the electric blanket at a certain constant value. If an overvoltage occurs in the power grid, the varistor MY is internally short-circuited (that is, it is in a conductive state) and the fuse RD is quickly blown, thus providing automatic overvoltage protection. Figure 2 is a printed circuit board assembly diagram of the device. 　　2. Selection of main components SCR uses small plastic-encapsulated one-way thyristor MCR100-8. Rt is the positive temperature coefficient thermistor PTC, and its resistance can be determined in experiments. V6 uses a Zener diode with a voltage regulator value of 12V and a rated power of 1W. The capacity of C2 can be selected in the range of 0.47~1uF (withstand voltage 630V). MY uses varistor with varistor voltage of 470V~480V. J selects the JZC-22F ultra-small electromagnetic relay with a rated operating voltage of 12V, a winding resistance of 400Ω, and a rated current of 30mA. Other component parameters are selected according to Figure 1. 3. Installation and instructions According to the different heating conditions of various parts of the electric blanket and the impact of the human body on the compression factors of the electric blanket, it is best to install the thermistor Rt in the middle of the foot area of ​​the electric blanket so that you can fully feel the electric blanket temperature changes to achieve good temperature control effect. Since this circuit uses a thyristor non-contact electronic switch, the conduction can only be triggered by a control signal. This can prevent arcing and ignition problems after the heating wire breaks to a certain extent, thereby playing a safety protection role. The signal input wire of the thermistor Rt should use a shielded wire to ensure firmness and reliability, and to directly transmit the temperature change signal felt by Rt to the control circuit. RP is installed on the panel of the thermostat, and the corresponding temperature scale is marked on the plastic knob so that the required temperature control gear can be selected. In order to achieve the protection effect and service life, the voltage value of the varistor MY should be selected according to 2.2 times the effective value of the rated AC voltage of the electrical appliance.