Can a Diode Function as a Temperature Sensor? Exploring the Possibilities

A diode can indeed function as a temperature sensor, offering a cost-effective and flexible solution for moderate-precision temperature measurement applications. The base-emitter voltage (VBE) of a bipolar junction transistor (BJT) diode exhibits a linear temperature coefficient, typically around -2mV/°C, making it a valuable tool in temperature sensing circuits.

Understanding Diode-Based Temperature Measurement

The simplest diode-based temperature measurement system consists of a current source for diode excitation and another current source for offsetting. However, this basic circuit has some limitations, such as the need for interactive span (GAIN) and zero (OFFSET) adjustments, as well as the inversely proportional relationship between temperature and voltage output.

To design a more advanced temperature measurement system with a 0-V to -1-V output for a 0 to 100°C temperature range, the following parameters can be used:

• VBE25 = 0.6 V (base-emitter voltage at 25°C)
• TC = -0.0021 V/°C (temperature coefficient)
• RZERO = 1 kΩ (zero-offset resistor)
• IOFF = IE = 0.1 mA (offset and excitation current)
• TMIN = 0°C (minimum temperature)
• δVO/δT = -0.01 V/°C (output voltage temperature coefficient)

Using these parameters, the values of the resistors R1 and R2 can be calculated as 9.525 kΩ and 35.833 kΩ, respectively, to achieve the desired output range.

Diode-Based Temperature Measurement Circuits

There are various diode-based temperature measurement circuit topologies, each with its own advantages and applications:

1. Simple Diode-Based Temperature Measurement Circuits: These circuits utilize a single diode and a current source for excitation, providing a straightforward and cost-effective solution.

2. Temperature Measurement Circuits with Independent Gain and Offset Adjustment: These circuits allow for independent adjustment of the gain (span) and offset, enabling more precise temperature measurement.

3. Positive Temperature Measurement Circuits with Negative Supply: These circuits operate with a negative supply voltage, providing a positive output voltage that increases with temperature.

4. Positive Transfer Function Temperature Measurement Circuits with Positive Supply: These circuits offer a positive transfer function, where the output voltage increases with increasing temperature, using a positive supply voltage.

Cryogenic Silicon Diode Temperature Sensors

The CY670 Series silicon diodes from Omega Engineering offer a more accurate reading of temperature ranges compared to previously marketed silicone diodes. These sensors conform to the CY670 standard voltage vs. temperature response curve, making them interchangeable and not requiring individual calibration for many applications.

The CY670 Series diodes are designed for cryogenic temperature measurement, with a temperature range of 1.4 K to 475 K (-272°C to 202°C). They provide outstanding DC precision, low offset, low drift, and a 10-MHz bandwidth, making them suitable for a wide range of temperature sensing applications.

Advantages of Diode-Based Temperature Sensors

Diode-based temperature sensors offer several advantages, including:

1. Cost-Effectiveness: Diodes are relatively inexpensive components, making diode-based temperature sensors a cost-effective solution.
2. Flexibility: Diode-based temperature sensors can be easily integrated into various circuit designs and applications.
3. Linearity: The base-emitter voltage of a BJT diode exhibits a linear temperature coefficient, simplifying the design and calibration of temperature measurement systems.
4. Wide Temperature Range: Diode-based temperature sensors can operate across a wide temperature range, from cryogenic temperatures to high-temperature environments.
5. Reliability: Diode-based temperature sensors are generally robust and reliable, with a long lifespan and minimal maintenance requirements.

Conclusion

In summary, a diode can indeed function as a temperature sensor, offering a versatile and cost-effective solution for moderate-precision temperature measurement applications. By understanding the principles of diode-based temperature measurement, electronics engineers and enthusiasts can design and implement a wide range of temperature sensing circuits to meet their specific needs.

Reference:

1. Diode-Based Temperature Measurement – Texas Instruments: https://www.ti.com/lit/an/sboa277a/sboa277a.pdf?ts=1705394459583
2. Cryogenic Silicon Diode Temperature Sensor – Omega Engineering: https://www.omega.com/en-us/temperature-measurement/temperature-sensing-elements/p/CY670
3. SIM922 & SIM923 – Temperature Monitors – thinkSRS.com: https://www.thinksrs.com/products/sim922923.html