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A temperature-sensitive bimetal coil is a component made of two dissimilar metals bonded together. These metals expand and contract at different rates when subjected to temperature changes, causing the coil to flex. This flexing motion can be used to operate various temperature-sensitive devices.

Detailed Description:

  • Purpose:
  • Temperature Control: Used to regulate the operation of temperature-sensitive devices by responding to changes in ambient temperature.
  • Automatic Operation: Provides automatic control in systems where manual adjustment is impractical.
  • Components:
  • Bimetal Strip: The core of the bimetal coil, consisting of two different metals with distinct coefficients of thermal expansion.
  • Coil Shape: The bimetal strip is formed into a coil shape to enhance its sensitivity and range of motion in response to temperature changes.
  • Operation:
  • Temperature Increase: As the temperature rises, one metal expands more than the other, causing the coil to unwind or flex in one direction.
  • Temperature Decrease: Conversely, when the temperature drops, the coil flexes in the opposite direction as the metal with a higher contraction rate contracts more.
  • Mechanical Action: The flexing motion of the bimetal coil can be used to activate or deactivate mechanical devices, switches, or valves.
  • Applications:
  • Carburetor Chokes: In automotive carburetors, a bimetal coil adjusts the choke valve to ensure the engine gets the right air-fuel mixture during startup and warm-up.
  • Radiator Fans: Many non-electric radiator fans use a bimetal coil to control the hydraulic circuit, engaging the fan when the engine temperature reaches a certain point.
  • Thermostats: Commonly used in household thermostats to control heating and cooling systems.
  • Temperature Gauges: Employed in various gauges to provide temperature readings by moving a needle based on the coil’s flexing.
  • Advantages:
  • Self-Regulating: Automatically adjusts to temperature changes without the need for external power or manual intervention.
  • Durable: Made from robust materials that can withstand continuous expansion and contraction.
  • Cost-Effective: Provides an inexpensive and reliable method for temperature regulation.
  • Example:
  • Automotive Carburetor Choke: In an older car with a carburetor, a bimetal coil is connected to the choke valve. When the engine is cold, the coil contracts, closing the choke valve to enrich the air-fuel mixture for easier starting. As the engine warms up, the coil expands, gradually opening the choke valve to allow more air in, optimizing the fuel mixture for normal operation.


  1. Initial State (Cold):
  • Ambient Temperature: 5°C
  • Bimetal Coil Position: Coiled tightly
  • Device Action: Choke valve closed
  1. Warming Up:
  • Ambient Temperature: 20°C
  • Bimetal Coil Position: Partially expanded
  • Device Action: Choke valve partially open
  1. Normal Operating Temperature:
  • Ambient Temperature: 80°C
  • Bimetal Coil Position: Fully expanded
  • Device Action: Choke valve fully open

By understanding the function and applications of temperature-sensitive bimetal coils, it becomes clear how these simple yet effective components play a crucial role in many temperature-regulated systems, ensuring efficient and reliable operation without the need for complex electronics or manual adjustments.

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