The electrical temperature control switch

The electrical temperature control switch (temperature controller) is a device that automatically controls the on-off of the circuit according to the change of the ambient temperature. Its core principle is to sense the temperature change through a temperature-sensitive element and trigger the action of a mechanical or electronic switch. The following are its detailed working principles and classifications:

 I. Core Components

1. **Temperature Sensor**

   - **Bimetallic Strip**: It is made by laminating two metal layers with different thermal expansion coefficients. When heated, it bends due to the expansion difference and triggers the mechanical switch.

   - **Thermistor**: Its resistance value changes significantly with temperature (Positive Temperature Coefficient PTC or Negative Temperature Coefficient NTC).

   - **Thermocouple**: Based on the Seebeck effect, a small voltage signal is generated when the temperature changes.

   - **Liquid Expansion Type**: The liquid in the sealed tube expands when heated and pushes the mechanical device.

2. **Control Circuit** (Electronic Temperature Control Switch)

   - Comparator or Microprocessor: Compares the sensor signal with the set value and outputs a control signal.

   - Relay/Solid State Switch: Turns the circuit on or off according to the signal.

3. **Actuator**

   - Mechanical contacts (Normally Open/ Normally Closed) or electronic switches (such as MOSFET, Thyristor).

 II. Working Principles

 1. Mechanical Temperature Control Switch (Bimetallic Strip Type)

- **Temperature Rise**: The bimetallic strip bends towards the side with a smaller expansion coefficient when heated.

- **Trigger Action**: When it bends to the set angle, it pushes the contacts to separate (Normally Closed type) or close (Normally Open type), cutting off or connecting the circuit.

- **Temperature Drop**: After cooling, the bimetallic strip returns to its original state and the contacts reset.

- **Application Scenarios**: Simple on-off control of electric kettles, electric irons, etc.

 2. Electronic Temperature Control Switch

- **Signal Acquisition**: The thermistor or thermocouple converts the temperature into an electrical signal (such as resistance change or voltage change).

- **Signal Processing**: The control circuit amplifies the signal and compares it with the set temperature value.

- **Switch Control**:

   - **Analog Control**: Drives the relay through an operational amplifier.

   - **Digital Control**: The microprocessor outputs a PWM signal or a digital switch signal.

- **Example**: The air conditioner automatically starts and stops the compressor according to the room temperature, and the accuracy can reach ±0.5℃.

 III. Key Parameters

1. **Operating Temperature**: The critical temperature value that triggers the switch (such as disconnecting at 60℃).

2. **Reset Temperature**: The temperature at which the original state is restored after the temperature drops (such as closing at 55℃).

3. **Hysteresis**: The difference between the operating and reset temperatures to prevent frequent on-off.

4. **Rated Current/Voltage**: The maximum electrical parameters that the switch can safely carry (such as 10A/250V).

5. **Response Time**: The time from the temperature change to the switch action (usually from a few seconds to a few minutes).

# IV. Classification and Applications

 1. According to the Action Type

- **Normally Open (NO)**: Closes the circuit when the temperature reaches the standard (such as over-temperature protection of heaters).

- **Normally Closed (NC)**: Opens the circuit when the temperature reaches the standard (such as temperature control of refrigerators).

 2. According to the Reset Method

- **Automatic Reset**: It restores itself after the temperature drops (such as overheat protection of motors).

- **Manual Reset**: Requires manual intervention for reset (such as preventing mis-triggering of safety equipment).

 3. Typical Applications

- **Home Appliances**: Anti-dry burning protection of electric water heaters, temperature control of ovens.

- **Industrial Equipment**: Overheat protection of motor windings, cooling systems of transformers.

- **Automobiles**: Control of engine cooling fans, battery temperature management.

- **New Energy**: Heat dissipation of solar inverters, temperature control of electric vehicle charging piles.

 V. Key Points for Selection and Installation

1. **Match the Temperature Range**: Ensure that the operating temperature of the switch covers the actual requirements (such as -20℃~150℃).

2. **Load Capacity**: Select the appropriate specifications according to the circuit current and voltage to avoid damage due to overloading.

3. **Environmental Adaptability**: The waterproof and dustproof grades (such as IP67) should meet the requirements of the installation environment.

4. **Calibration and Testing**: Regularly check the operating temperature to ensure long-term reliability.

 VI. Fault Troubleshooting

- **No Action**: Sensor damage, contact oxidation, deviation of the set value.

- **False Action**: Environmental interference (such as vibration), too small hysteresis, poor heat dissipation.

Through accurate temperature sensing and response, the temperature control switch realizes the automatic protection and energy-saving control of electrical equipment, and is an essential key component in both industrial and civil fields.