A bimetallic strip is a simple mechanical temperature switch made from two bonded metals with different thermal expansion rates. When heated, the strip bends, making or breaking an electrical contact. Found in thermostats, circuit breakers, and motor protectors, bimetallic switches provide reliable, power-free temperature sensing and switching for electronics projects.
What Is a Bimetallic Strip?
A bimetallic strip consists of two layers of different metals (typically brass and steel, or Invar and brass) bonded together. When temperature changes, the metal with higher thermal expansion (brass) expands more than the other (steel/Invar), causing the strip to bend. This bending motion can open or close electrical contacts.
Key advantage: no external power required. The bimetallic strip is both the sensor and the actuator. It works purely on thermal energy from its environment.
How Bimetallic Strips Work
The bending force depends on:
- Metal pair: Greater difference in thermal expansion coefficients = more bending. Common pairs: Invar (very low expansion) + brass/copper (high expansion).
- Strip length: Longer strips bend more at the tip.
- Strip thickness: Thinner strips are more sensitive but carry less force.
- Temperature change: More temperature change = more bending. The response is approximately linear over a useful range.
A typical bimetallic thermostat trips at a set temperature (adjustable by changing the contact gap) and resets when cooled by 5-15°C (hysteresis).
Types and Configurations
- Snap-action disc: A concave bimetallic disc “snaps” between two positions at specific temperatures. Used in room thermostats, kettles, and motor thermal protectors. Provides crisp on/off action with built-in hysteresis.
- Creep-action strip: Gradually bends with temperature change. Used in temperature gauges and slow-response applications.
- Normally Closed (NC): Contact opens at high temperature. Used for overtemperature cutoff.
- Normally Open (NO): Contact closes at high temperature. Used for activating cooling fans or alarms.
Applications in Electronics
- Motor thermal protectors: Snap-action bimetallic discs embedded in motor windings open the circuit when winding temperature exceeds safe limits. Auto-reset when cooled.
- Iron and hair dryer thermostats: Maintain temperature by cycling the heating element on and off.
- Circuit breakers: Bimetallic strips in thermal circuit breakers bend under sustained overcurrent (I²R heating), tripping the breaker. This provides overcurrent protection without fuses.
- Room thermostats: Traditional wall thermostats used a coiled bimetallic strip with a mercury switch (now replaced by electronic sensors).
- Temperature compensation: Bimetallic strips in clock mechanisms and instrument movements compensate for thermal expansion errors.
Building a Temperature Switch Circuit
A simple temperature-activated fan circuit using a bimetallic thermostat disc:
- Wire the NC bimetallic thermostat (rated for your desired temperature) in series with the fan power
- Mount the thermostat in thermal contact with the component being protected
- Below the set temperature: thermostat is closed, fan runs
- Wait — that is backwards! For a cooling fan that turns ON when hot, use a NO thermostat:
Correct circuit:
- NO bimetallic thermostat (rated at fan-start temperature) in series with the fan
- Below set temp: thermostat open, fan off (saving power)
- Above set temp: thermostat closes, fan runs
- Hysteresis: fan stays on until temperature drops 5-10°C below the set point, then turns off
No Arduino, no power supply for the controller, no programming. Just a thermostat and a fan. Elegant simplicity.
Industrial Applications
In Indian industrial settings, bimetallic devices are everywhere:
- Thermal overload relays: In motor starter panels, bimetallic elements monitor motor current and trip if sustained overcurrent is detected.
- Transformer protection: Temperature sensors in oil-filled transformers use bimetallic elements.
- HVAC controls: Many older Indian buildings still use bimetallic room thermostats for AC control.
Recommended Components
Temperature Sensing and Switching Components
Advantages and Limitations
Advantages:
- No external power required
- Extremely reliable (no electronics to fail)
- Self-resetting (for thermostats)
- Low cost (₹20-50)
- Long lifespan (100,000+ cycles for quality units)
Limitations:
- Low precision: ±5-10°C typical accuracy
- Fixed or limited adjustment of set point
- Slow response compared to electronic sensors
- Contact bounce can cause arcing with inductive loads — use with a snubber or relay
- Limited current capacity (typically 5-15A) for direct load switching
Frequently Asked Questions
What metals are used in bimetallic strips?
Typically Invar (low expansion, 36% nickel-iron alloy) bonded to brass or copper (high expansion). Some use steel/brass combinations. The greater the expansion difference, the more the strip bends.
How accurate is a bimetallic thermostat?
±5-10°C for standard devices. Precision bimetallic thermostats can achieve ±2-3°C. For better accuracy, use electronic sensors (DS18B20 is ±0.5°C at ₹53).
Can I adjust the temperature on a bimetallic switch?
Some bimetallic thermostats have an adjustment screw that changes the contact gap, allowing you to set the trip temperature. Fixed-temperature models cannot be adjusted.
Are bimetallic strips used in modern electronics?
Yes, widely. Motor thermal protectors, circuit breakers, and some automotive temperature sensors still use bimetallic technology. For precision sensing, electronic sensors have largely replaced them.
Where can I buy bimetallic thermostats in India?
Available at electrical wholesalers (SP Road, Lamington Road) and online. Snap-action disc thermostats in various temperatures cost ₹20-50 each. For electronic alternatives, DS18B20 sensors are available on Zbotic.in.
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