Homemade electric blanket with adjustable thermostat

The weather is cold, but the electric blanket at home has no constant temperature and cannot be adjusted. The temperature of the electric blanket can only be controlled manually, which is inconvenient and unreliable. For this reason, the author made an adjustable constant temperature controller for electric blankets, which can automatically keep the electric blanket within a certain temperature range, and the user can set the temperature range that suits him best. When the temperature is lower than the set lower limit temperature, the electric blanket automatically heats up, and when it is higher than the set upper limit temperature, the power supply is automatically disconnected, thereby achieving the purpose of automatic constant temperature.

1. Working Principle

The circuit is shown in the attached figure. The AC220v power supply is stepped down by R1 and C1, rectified by VD1~VD4, filtered by C2, and then stabilized to DC12V by VT1, R2, DW1, C4, and supplied to the relaxation oscillator composed of VT2, RP1, RP2, R4, R5, RT, C5, etc., to provide a trigger pulse for the control electrode of the bidirectional thyristor VS. RP1 sets the voltage (adjusts the temperature), and its resistance determines the temperature; the thermistor RT (negative temperature coefficient) is used to detect the temperature. When the power is just turned on, the temperature of the electric blanket is low, the resistance of RT is large, the potential of point A is high, the charging time constant of C5 becomes smaller, VT2 oscillates in advance, the conduction angle of the thyristor VS moves forward, and the current flowing through the electric blanket is large, so that it reaches the predetermined temperature setting value as soon as possible. When the temperature of the electric blanket rises, the resistance of RT becomes smaller, the potential of point A decreases accordingly, the current flowing through the electric blanket decreases, and the temperature gradually decreases, thereby achieving automatic constant temperature.

In the attached figure, the pulse isolation transformer T adopts a soft ferrite magnetic ring, such as MX2000~6000, with an inner diameter of 11mm~18mm. The primary and secondary are both wound with φ0.19mm enameled wire for 60 turns. The other component parameters are as shown in the attached figure, and there are no special requirements.

2. Debugging

Solder the components as shown in the attached figure, and debug after checking. When debugging, first adjust the resistance of R2 so that the current flowing through DW1 is about 10mA. At this time, the voltage across capacitor C4 is 12V. RT is led out with a soft, heat-resistant thin wire and hidden in an appropriate part of the electric blanket (pay attention to the insulation between the two legs of RT and the insulation between RT and the heating wire, and pay attention to the protection of RT). Adjust P2 to the minimum value. Adjust the potentiometer RP1, and use a mercury thermometer to measure the position of RP1 when the electric blanket is at 36℃~42℃, and mark it. Then, adjust RP2 to the appropriate position according to the need for control sensitivity.

During installation, remove the power switch of the electric blanket, fix the adjusted circuit board in the small plastic box, fix RP1 on the small plastic box, and then it can be used.