1．Temperature-controlled bimetallic strip vs. NTC thermistor

Advantages and Disadvantages：

Temperature-controlled bimetallic strip:

l  Advantages: Simple structure, extremely low cost, direct drive of high power, high reliability, and no need for power supply.

l  Disadvantages: Low precision, slow response, only switch function, limited mechanical life, large size.

NTC Thermistor:

l  Advantages: High precision, fast response, continuous temperature reading can be provided, small size, no mechanical wear, and complex control (such as PID) can be achieved.

l  Disadvantages: Requires a matching circuit, cannot directly control the load by itself, is prone to damage under overload, and has nonlinear output.

2．Temperature-controlled bimetallic strip and NTC Thermistor，detailed introductions of the two temperature control methods：

2.1 Temperature-controlled bimetallic strip

l  Definition: A mechanical temperature control switch

It is not a sensor but an actuator that directly controls the on and off of a circuit by using physical principles. You can understand it as an "automatic tripping knife switch".

l  Working principle: The ingenious application of thermal expansion and contraction

It is made by firmly pressing together two metal sheets with different coefficients of thermal expansion. When the temperature changes, the amount of elongation or contraction of the metal sheet with a large coefficient of expansion (active layer) will be greater than that of the metal sheet with a small coefficient of expansion (passive layer). This difference forces the entire metal sheet to bend towards the passive layer side. This bending mechanical force is used to push an electrical contact, achieving the "connection" or "disconnection" of the circuit.

Bimetallic strip relays perform disconnection and contact operations based on the different expansion coefficients of the two materials of the metal strip.

l  Work mode:

Temperature rise → bimetallic strip bending → Push the contact to open → Power off the circuit, heating stops.

The temperature drops → the bimetallic strip is restored → the contacts are reconnected → the circuit is energized and heating begins.

This cycle is repeated to achieve constant temperature control near a set temperature point.

2.2 NTC Thermistor

l  Definition: An electronic temperature sensor

It is an electronic component whose resistance value changes with temperature. It does not have a switching function itself and is only responsible for "sensing" temperature

l  Working principle: Characteristics of semiconductor materials

NTC stands for "Negative Temperature Coefficient". It is made of metal oxide semiconductor materials. When the temperature rises, the number of conductive electrons (carriers) within the material will increase, thereby causing its resistance value to decrease. Conversely, as the temperature drops, the resistance value increases. This resistance change is continuous and measurable.

l  Work mode:

An NTC needs to be connected in series with a fixed resistor to form a "voltage divider circuit". When the temperature changes and causes the NTC resistance value to change, the voltage in the circuit will also change accordingly. This voltage signal can be read by the microcontroller and converted into a specific temperature value through a formula or table lookup method. Then, the system makes decisions based on this temperature value (such as engaging or disengaging the relay, or adjusting the motor speed).

NTC thermistor temperature measurement circuit diagram

3.  Recommended applications of temperature-controlled bimetallic strips and NTC thermistors

3.1 Products that recommends using Temperature-controlled bimetallic trips

The typical characteristics of this type of product are: cost sensitivity, low requirements for control accuracy, simple functions, and the need to directly control large currents.

l  Thermostats/overheat protectors in household appliances

Electric kettles (automatically trip when water boils), electric irons (keep the base plate at the set temperature), coffee machines/water dispensers (keep the base plate at the set temperature), rice cookers (serve as temperature-limiting or heat preservation switches), microwave ovens (mechanical and electrical door switches or overheat protection), etc.

l  Motor/transformer protection:

Embedded in the motor winding, it directly cuts off the power supply when the motor overheats to prevent burnout.

l  Automotive electrical appliances:

Overheating protection for seat heating and rear window defogging.

Core application: Wherever simple switch control such as "power off when the temperature reaches X degrees and power on when it drops below Y degrees" is required, and where volume and precision are not sensitive, bimetallic strips are the most cost-effective choice.

3.2 Products that recommends using NTC thermistors

The typical features of such products are: precise temperature measurement is required, digital display is needed, complex control logic is required, and the degree of product intelligence is high.

l  Battery management system

Smartphones/Laptops/electric vehicles: Precisely monitoring battery temperature to prevent danger caused by overcharging and overdischarging is a key sensor for achieving fast charging.

l  Automotive electronics

Engine water temperature sensor, intake air temperature sensor, and air conditioning internal and external circulation temperature sensor: Provide precise temperature parameters for the engine electronic control unit.

l  High-precision temperature control equipment

Constant temperature boxes/incubators, medical equipment, environmental monitoring stations: They need to be precisely maintained within a narrow temperature range.

l  Smart home appliances

Inverter air conditioners: Precisely sense indoor, outdoor and pipe temperatures, thereby achieving energy-saving and comfortable temperature control.

Smart rice cooker/oven: In combination with MCU, it achieves precise control of multiple cooking curves.

l  Consumer electronics

It is used to monitor the internal temperature of the equipment and prevent chips such as the CPU from overheating.

Core application: Wherever it is necessary to "know the exact current temperature" and conduct complex calculations and precise control based on this temperature value, NTC is the inevitable choice.

In summary, the temperature-controlled bimetallic strip is a "doer". It can complete the switching action by itself, which is simple and straightforward but reliable and durable. NTC thermistors act as "intelligence agents", precisely collecting temperature information, but they need to be handed over to the "brain" (MCU) for decision-making and directing the actions of the "hands and feet" (relays, etc.).

Leiditech Electronics supplies temperature-controlled bimetallic strips and thermistors (NTC). Welcome to contact and choose.

Bimetallic strips include: protective bimetallic strips, action bimetallic strips, jump bimetallic strips, overload bimetallic strips, etc. Customization is also available.

NTC thermistors include: through-hole NTC (20D-9, 8D-9, 1.3D15, etc.), surface mount NTC (CMFA103F3380HANT, CMFB104J4250HANT, CMFA104F4250FBNT, etc.)