Silicon diode temperature sensors are widely used for accurate and reliable temperature measurement due to their low cost, absolute temperature measurement capability, and high sensitivity. The voltage drop across a silicon diode varies linearly with temperature, making it an ideal choice for temperature sensing applications.
Understanding the Basics of Silicon Diode Temperature Sensors
The threshold voltage for silicon diodes is typically around 0.7V, but the specific value is still a topic of debate among experts. This voltage drop is influenced by various factors, including the semiconductor material properties, doping concentrations, and the operating conditions of the diode.
One of the key advantages of silicon diode temperature sensors is their ability to provide absolute temperature measurements. Unlike resistance-based temperature sensors, which require calibration, silicon diode sensors can directly measure the temperature without the need for complex calibration procedures.
Factors Affecting Measurement Accuracy
The temperature measurement system’s total uncertainty is a more quantifiable term than accuracy. This total uncertainty includes various factors, such as:
- Thermal noise
- Thermal voltages and zero drift
- Electromagnetic noise
- Calibration uncertainty
- Interpolation uncertainty
- Combined uncertainties
The self-heating of temperature sensors can also affect the measurement uncertainty. Minimizing the temperature measurement uncertainty requires balancing the uncertainties due to self-heating and output signal measurement.
Comparison with Resistance Temperature Sensors
The measurement accuracy of diode sensors is not as strongly dependent on the current source accuracy as resistance temperature sensors. The forward voltage variation with changing current for diodes is smaller than for resistance temperature sensors, which have linear current-voltage characteristics.
Interestingly, the sensitivity (dV/dT) of Lake Shore diode temperature sensors increases by an order of magnitude below 30K, while the slope (dV/dI) of the I-V curves stays relatively constant. This makes diode sensors particularly useful for cryogenic temperature measurements.
Addressing the Disadvantages of Silicon Diode Sensors
While silicon diode temperature sensors have many advantages, they also have some disadvantages. One major disadvantage is the need for individual calibration, as each sensor has slightly different characteristics. This can be time-consuming and expensive, especially when using a large number of sensors.
To address this issue, researchers have developed silicon diode temperature sensors without a kink in the response curve in the cryogenic temperature region. These sensors maintain the advantages of being low cost, having an absolute temperature measurement capability, and high sensitivity, while also providing a more linear response in the cryogenic temperature region.
DIY: Building a Silicon Diode Temperature Sensor
To use a silicon diode as a temperature sensor, you will need a stable constant current source and a method to measure the voltage drop across the diode. Here’s a step-by-step guide:
- Obtain a silicon diode with known characteristics, such as the forward voltage drop and temperature coefficient.
- Design a constant current source using a resistor and a stable power supply. The current should be in the range of 10-100 μA to minimize self-heating effects.
- Connect the silicon diode in series with the constant current source and measure the voltage drop across the diode using a high-impedance voltmeter or an oscilloscope.
- Calibrate the diode by measuring the voltage drop at known temperatures, such as the freezing and boiling points of water.
- Use the calibration data to create a lookup table or a linear equation to convert the measured voltage drop to temperature.
It is crucial to ensure that the current source is stable and accurate to minimize errors in the temperature measurement. Additionally, consider the effects of self-heating and electromagnetic interference on the sensor’s performance.
Conclusion
Silicon diode temperature sensors offer a unique combination of low cost, absolute temperature measurement capability, and high sensitivity, making them a popular choice for a wide range of applications. By understanding the factors affecting measurement accuracy, comparing them to resistance temperature sensors, and addressing their disadvantages, you can effectively utilize silicon diode temperature sensors in your projects.
References:
- Silicon diode temperature sensors – A review of applications
- Silicon diode threshold voltage 0.7
- Lake Shore Cryotronics Temperature Sensor Catalog
- Silicon diode temperature sensor without a kink of the response curve in cryogenic temperature region
- Diode Calibration for Temperature Measurement
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