Diodes are quin most electronics. They are mainly used for ensuring that current flows in one direction. Do you know that diodes can also be used as thermometers? Yes, you can use them for measuring temperature.
In this article, we are going to discuss tips for using a diode as a thermometer.
What are the characteristics of a diode?
Key characteristics of diodes that you should know include:
VF refers to the voltage that a diode needs to be forward biased.
You can measure this in a circuit by applying a known voltage and measuring the current.
VF of silicon diodes ranges from 0.6V to 0.3V, while Zener diodes have VF values ranging from 3V to 30V.
This is the reverse breakdown voltage of a diode.
When a diode is reversed biassed, the voltage required to cause the diode to conduct is called VR.
VR of silicon diodes ranges from 30V to 300V, while Zener diodes have VR values ranging from 30V to 1000V.
The volt drop across a diode is known as its drop or I”.” This can be determined by measuring the voltage across an ideal diode while it conducts current.
The equation for calculating this value is:
I” = V/I = VF/IF, where IF is forward current and VF is forward voltage of the diode.
Voltage and temperature of a diode
For the diode to function as a thermometer, you have to establish the relationship between voltage and temperature.
The voltage drop across the diode is directly proportional to the current flowing through it.
The relationship is given by:
I” = kT/q, where “k” is the Boltzmann constant (1.38 x 10 J/K),
“T” is the temperature in Kelvin, and “q” is the electron charge (1.6 x 10 C).
The power dissipated in a diode can be found from the following equation:
P = I2R, where R is the resistance of the diode and P is the power dissipated in watts.
Since for a silicon diode, I=V/R, we can write:
P = V2/R = (VF+VR)/R, where VF is a forward voltage drop and VR is reverse voltage.
During the forward bias, the current remains constant while the voltage declines. On the other hand, the temperature rises on the diode. The relationship between voltage, current, and temperature is given by:
I” = kT/q.
We can find the temperature from the following equation:
T = VF/(VR-VF).
If we substitute for VF and VR in the previous equation, we get:
T = kI/q = k(I/q)(VR-VF)/q. This equation can be rearranged to give: q = k(VR-VF)/T.
From the above equation, an increase in forwarding voltage will make the diode work as a thermometer.
Reverse voltage (V) across a diode increases the temperature of the diode. If the reverse voltage is increased, the current becomes zero, and the power dissipated in it will be given by:
P = V2/R = (VF+VR)/R. Since for silicon, I=V/R, we have: P = VF2/R.
If we substitute for VF in this equation, we get P = kT/q.
The above equation shows that if the reverse voltage is increased, the temperature rises. This property can be used to measure the temperature of a diode with a thermistor or thermocouple.
A diode will be used as a thermistor and will read the temperatures accurately.
How thermal diodes work
The diode temperature will go up and the voltage will go down. Meanwhile, the heat will flow in one direction. Naturally, heat will flow from one terminal of the diode to the next. This will cool down the diode. This is how a thermal diode works.
A thermal diode is any semiconductor device that will conduct current in one direction only when there is a temperature difference between its two ends. Thermal diodes are used for temperature sensing and for switching.
Thermal diodes are also known as thermocouples or thermistors. A thermistor is any semiconductor device that has a positive temperature coefficient of resistance and thus acts as a variable resistor whose resistance increases with increasing temperature.
A thermocouple is a pair of dissimilar conductors joined together to produce an electromotive force in response to a temperature difference between them, or vice versa; they are used primarily to measure the temperature of objects.
Operation of thermistors
A thermistor is a resistor whose resistance varies with temperature. Thermistors are available in two types: positive and negative. Thermistors have a high resistance in the cold state and low resistance in the hot state, i.e., they behave like an ordinary resistor at room temperature but their resistance changes with change in temperature
Thermistors are used in electronic circuits as voltage-to-temperature converters. They are also used to protect sensitive components such as integrated circuits from high temperatures due to failure of cooling systems or thermal runaway in semiconductor devices.
Application areas of diodes as a thermometer
There are different areas in which diodes serve as thermometers. Some of them are mentioned below:
1.Temperature sensing in liquid-filled thermometers is done by using thermistor diodes. Thermistor diodes are low-cost and have good sensitivity and linearity. They have a very small temperature coefficient, which helps to maintain the accuracy of the liquid-filled thermometer over the full range of temperatures.
2.Diodes find application as temperature sensors in precision temperature measurement systems such as:
a.Infrared temperature sensors
In the above-mentioned applications, diodes are used in conjunction with an amplifier circuit to obtain a high-level output voltage. The output voltage is proportional to the temperature of the object.
3.Diodes are used in the temperature sensing circuitry of microprocessors and digital integrated circuits, which operate at high temperatures.
4.Diodes are used as temperature sensors in car engines, for protecting against overheating of the engine.
5.Diodes are used as temperature sensors in electric stoves, for protecting against overheating of the stove.
6.Diodes are used in a mercury-in-glass thermometer, which protects against the breakage of the glass due to the expansion of mercury at high temperatures.
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