Thermal sensors work by detecting changes in temperature and converting them into measurable electrical signals. The specific mechanism of conversion varies depending on the type of thermal sensor.
Different types of thermal sensors rely on different physical properties that change with temperature. Here's a breakdown of how some common types work:
- Thermocouples: These sensors exploit the Seebeck effect, where a temperature difference between two different metals creates a voltage. The voltage is proportional to the temperature difference.
- Resistance Temperature Detectors (RTDs): RTDs utilize the principle that the electrical resistance of a metal changes with temperature. As the temperature increases, the resistance also increases. RTDs are often made of platinum, nickel, or copper.
- Thermistors: Similar to RTDs, thermistors also measure temperature by detecting changes in resistance. However, thermistors are typically made of semiconductor materials and exhibit a much larger change in resistance per degree Celsius than RTDs. They can be Negative Temperature Coefficient (NTC), where resistance decreases with increasing temperature, or Positive Temperature Coefficient (PTC), where resistance increases with increasing temperature.
- Infrared (IR) Sensors: These sensors detect thermal radiation emitted by objects. All objects above absolute zero emit infrared radiation, and the amount of radiation increases with temperature. IR sensors use materials that convert the infrared radiation into an electrical signal.
- Semiconductor-based Temperature Sensors: These sensors, as mentioned in the reference, often use diodes or transistors. The voltage drop across a diode is temperature-dependent. As temperature increases, the voltage drop across the diode changes in a predictable manner. This change in voltage is then measured to determine the temperature.
Here's a table summarizing the key differences:
| Sensor Type | Working Principle | Material | Advantages | Disadvantages |
|---|---|---|---|---|
| Thermocouple | Seebeck effect (voltage generated by temp diff) | Dissimilar metals | Wide temperature range, robust | Low accuracy, requires cold junction compensation |
| RTD | Resistance changes with temperature | Platinum, Nickel, Copper | High accuracy, stable | Slow response time, more expensive than thermistors |
| Thermistor | Resistance changes with temperature | Semiconductor materials | High sensitivity, fast response time | Limited temperature range, non-linear response |
| IR Sensor | Detects infrared radiation | Various materials sensitive to IR radiation | Non-contact measurement, fast response | Affected by ambient radiation, accuracy depends on emissivity of the object |
| Semiconductor-based | Voltage changes in diode or transistor | Silicon | Small size, low cost, can be integrated with other circuitry | Limited temperature range, less accurate than RTDs or thermocouples |
In summary, thermal sensors work by leveraging different physical phenomena related to temperature changes. These phenomena are converted into electrical signals that can be measured and interpreted to determine the temperature. The choice of sensor depends on the specific application, considering factors like temperature range, accuracy, response time, and cost.
