Contents
How does a diode measure temperature?
The principle of using a diode as a temperature sensor is straightforward. With a constant current applied, the voltage across a diode or PN junction will decrease by approximately 1 to 2 mV/°C over temperature. An expression for the diode voltage over temperature is derived by using the point-slope form of equation 2.
What is the temperature coefficient of a diode?
Most diodes, therefore, have a negative temperature coefficient, typically −2 mV/°C for silicon diodes. The temperature coefficient is approximately constant for temperatures above about 20 kelvin.
What is a diode thermometer?
Diodes are typically used in electronics to move current in one direction, but this common component can also be used as a thermometer. If the circuit can amplify the diode’s forward voltage, then the diode begins to serve as a thermometer. The diode will generate voltage fluctuations as ambient temperature changes.
What are two conditions under which a diode is operated?
Diode operation: (a) Current flow is permitted; the diode is forward biased. (b) Current flow is prohibited; the diode is reversed biased. When the polarity of the battery is such that current is allowed to flow through the diode, the diode is said to be forward-biased.
What causes a diode to get hot?
Zener diode is heating due to power dissipation. More the power dissipation more the heat, similar to resistor, for example: 2 ohms resistor carrying current of 1A is dissipation 2W of power. Smoking is due to “excess power dissipation”, i.e you are crossing the limit of maximum power withstanding capacity of Zener.
So first let’s look at a plot of diode voltage vs. temperature. As you can see, we have a nice linear relationship between voltage and temperature. However, the amplitude of the voltage response is quite small. Over a range of 60°C, the voltage changes only by ~70 mV.
How is the reverse current of a diode measured?
The reverse current of a diode lays in the order of magnitude between a few nanoamperes to milliamperes. The reverse current is temperature as well as reverse voltage dependent. While measuring these small currents one must be aware of external leakage currents by contaminants.
What happens to the voltage of a circuit as the temperature decreases?
The voltage value that corresponds to a given current value increases as temperature decreases. In other words, if a circuit maintains a diode current of, say, 15 mA, the diode’s voltage drop at 10°C will be higher than its voltage drop at 20°C.
Where did the diode for thermometer come from?
I started with a circuit that was taken directly from an old textbook of mine. The LTspice schematic was identical to the circuit in the book, even down to the part numbers used for the diode and BJT. And as far as I can tell, the textbook version is barely functional.