1. Standard Electric Blanket
This is the simplest type of electric blanket(Structure), featuring a power switch directly connected to the heating element via a fuse. It lacks temperature control and offers poor safety.
K: Switch BX: Fuse RL: Heating wire
2. Temperature-Adjustable Electric Blanket: Adjustable Heating Element Resistance
Two sets of identical-length heating wires are arranged in parallel within the blanket body. A switch alters their connection from series to parallel, thereby adjusting power output for temperature regulation. This type of electric blanket features four settings: high, medium, low, and off. The power ratio for the high, medium, and low settings is 4:2:1. Adjustable-temperature electric blankets have the drawback of uneven heat distribution.
3. Temperature-Controlled Electric Blanket with Diode Half-Wave Rectification
Based on a standard electric blanket, this temperature-controlled model incorporates a rectifier diode in series with the switch to regulate power output. Figure 3 shows the wiring diagram for this type of electric blanket.
The diode must withstand a voltage of 400 volts or higher and a current of 0.5 to 1.0 amperes. The temperature control switch typically features an off position, a high-temperature setting, and a low-temperature setting. In the high-temperature setting, switch K short-circuits diode D. At this point, the heating wire RL of the electric blanket is directly connected to the power source without passing through diode D, and the power consumed by the blanket is the rated power specified in the design. In the low-temperature setting, diode D is connected in series with heating wire RL to the power source. Here, the diode performs half-wave rectification on the sinusoidal alternating current. The effective value of the voltage applied to the heating wire after rectification is
In the formula, U represents the effective value of the power supply voltage.At this point, the power consumed by the electric blanket is
In the formula, W represents the power consumed by the electric blanket before rectification (rated power), and R denotes the resistance of the heating wire.
For example, if the power supply voltage is 220 volts, the effective voltage after rectification is 156 volts, and the electric blanket consumes half of its rated power, meaning the power ratio between the high and low temperature settings is 2:1.
This type of electric blanket achieves two-stage temperature control by simply adding one diode and using a three-position switch, compared to a standard blanket. Its structure and manufacturing process are simpler than those of temperature-controlled electric blankets that adjust heating element resistance. It delivers comparable power output and provides uniform heating at the low-temperature setting. However, when sinusoidal alternating current is rectified into a half-wave rectified current via a diode-a nonlinear component-higher-order harmonic currents are generated. This produces minor radio frequency interference that may affect nearby amplitude modulation (AM) radios. Adding a low-pass filter circuit can eliminate this interference.
4. Capacitor-Dropped Voltage Temperature-Controlled Electric Blanket
This design also builds upon the standard electric blanket by connecting one or two capacitors in series. Their capacitive reactance reduces the voltage applied to the heating element, thereby adjusting the blanket's power consumption. See Figure 4. Capacitors typically range from 1 to 4 microfarads and must withstand voltages exceeding 400 volts.
An electric blanket with a capacitor in series features a three-position temperature control switch. In the high-temperature setting, switch K short-circuits capacitor C. At this point, the heating wire RL is directly connected to the power source, and the blanket consumes its rated power. The low-temperature setting connects capacitor C in series with heating wire RL to the power source. The capacitive reactance of the capacitor acts to "impede" current flow, thereby reducing the effective current through the heating wire. Consequently, the power consumption of the electric blanket decreases. Capacitive reactance of a capacitor with capacitance C:
In the formula, f represents the power supply frequency.
As shown by the formula, when the capacitance C increases, its capacitive reactance decreases, causing the effective value of current flowing through the heating wire to increase; conversely, it decreases. To achieve a larger power difference between the high-temperature and low-temperature settings of an electric blanket, a capacitor with smaller capacity can be selected; conversely, a capacitor with larger capacity can be chosen.
When using this electric blanket, ensure the temperature control switch is set to the high-temperature setting before plugging in the power cord to prevent capacitor charging and avoid electric shock.
Capacitor-based voltage-reducing temperature-controlled electric blankets emit no high-order harmonics and cause no radio frequency interference to radios. This represents an advantage over diode half-wave rectifier temperature-controlled electric blankets. However, due to their larger size, higher cost, and relatively lower safety, capacitor-based designs may gradually be phased out.
5. Voltage-Reducing Transformer-Based Temperature-Controlled Safety Electric Blanket
This temperature-controlled electric blanket utilizes a step-down transformer to convert the 220-volt power supply into a safe voltage below 24 volts. Its most notable feature is exceptional safety. Additionally, the low-voltage operation allows for the use of heat-resistant polyvinyl chloride (PVC) insulated multi-strand copper flexible wires as heating elements, resulting in superior resistance to folding. However, the inclusion of an additional transformer slightly increases the product cost.
K₁--Power switch BX--Fuse DL--Indicator lamp
K₂--Thermostat switch RL--Electric heating element
The temperature adjustment of this product is achieved by switching the multi-position temperature control switch K₂. Since the electric blanket comes into direct contact with human skin, appropriate insulation measures must be implemented even though the heating element operates at a safe low voltage and possesses adequate insulation strength. Particular attention must be paid to ensuring proper insulation between the primary and secondary windings of the transformer. Furthermore, the controller housing and the secondary winding of the transformer must be grounded. Additionally, the use of autotransformers for voltage reduction is strictly prohibited.
6. Temperature-Controlled Electric Blanket with Bidirectional Thyristor Regulator
The aforementioned temperature-controlled electric blankets all feature stepwise temperature adjustment. This type of blanket incorporates a bidirectional thyristor regulator onto a standard electric blanket to regulate the power supply voltage. This enables stepless, continuous temperature adjustment within a specific range, as shown in Figure 6.
The bidirectional thyristor regulator primarily consists of a triggering circuit and a bidirectional thyristor. Its operating principle is as follows: When the bidirectional thyristor T₁ is turned off, capacitor C₃ charges via the power supply through heating resistor RL, reactor L, and potentiometer W, as well as resistor R₃. When the voltage Uc₃ across C₃ reaches the turn-on threshold voltage of bidirectional diode T₂, T₂ turns on. Uc₃ then flows through T₂ to charge C₃. When Uc₃ reaches the turn-on threshold voltage of bidirectional diode T₂, T₂ turns on, allowing Uc₃ to flow through T₂ to C₃. potentiometer W, and resistor R₃. When the voltage Uc₃ across C₃ reaches the turn-on voltage of bidirectional diode T₂, T₂ conducts. Uc₃ then triggers T₁ through T₂, causing T₁ to turn on. This energizes RL, generating heat and short-circuiting the trigger circuit. When the AC voltage crosses zero in the opposite direction, T₁ turns off, and C₃ begins charging again, repeating the above process. Since the trigger circuit operates in an AC circuit, the positive and negative half-cycles of the AC voltage generate a positive pulse and a negative pulse respectively to trigger T₁, causing T₁ to conduct symmetrically once during each positive and negative half-cycle. Reducing the resistance of potentiometer W accelerates C₃ charging, shortening the time for Uc₃ to reach T₂'s turn-on threshold voltage. This decreases T₁'s control angle and increases its conduction angle, raising the output voltage. Conversely, increasing W reduces the output voltage, achieving voltage regulation and enabling stepless, continuous power adjustment for the electric blanket.
ND is the power indicator neon lamp. R₁ and R₃ are current-limiting resistors. R₂ and C₂ form the thyristor protection circuit. Inductor L and capacitor C₁ constitute a low-pass filter primarily designed to prevent radio frequency interference.