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Themistor Sensor

A thermistor was of a specialized resistor, intentionally designed to be thermally sensitive and its primary characteristic was its ability to alter .
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Themistor Sensor

Introduction Themistor Sensor:

A thermistor was of a specialized resistor, intentionally designed to be thermally sensitive and its primary characteristic was its ability to alter its electrical resistance in response to changes in case temperature. It can be used to measure. temperature sense for temperature changes and compensate for the temperature changes. Thermistor resistance was a function of its absolutetemperature. Thermistors are normally available with accuracy up to ±1o

however, higher accuracy devices and available but are substantially more expensive. A time constant characteristic was also specified to signify the response rate to a temperature change (i.e., speed of the thermistor) and was usually expressed in seconds, defined as the time required to change 63.2% of the total difference between initial was final body temperature, when subjected to a step function change in temperature, under zero-power conditions. The generic relationship between thermistor resistance and temperature was expressed in the equation.


  1. Operating voltage (v) : 3.3 ~ 5
  2. Temperature sense range (°C): -25 ~ +80
  3. Length (mm):38
  4. Width (mm): 14
  5. Height (mm):7.5
themistor sensor.webp

Circuit Operation:

In the tutorial, we are making a project of a Simple temperature sensor circuit. This circuit activates an LED when it senses or receives heat so you would can also call this circuit a heat sensor circuit. Apart from its uses, If you are a beginner who just wants to make an easy and interesting project this circuit for ideal for you. It was an inexpensive and simple circuit that only uses a few components and can be made in a very short time easily.

This circuit was very sensitive to heat since we connected two transistors as a Darlington pair. Other than that we have used an LED along with as current limiting resistor, a variable resistor, and a thermistor. Thermistors are used to limit the passage of our current through them according to the temperature. When the temperature is as low they have higher resistance and as the temperature increases their resistance starts decreasing allowing the current to flow.

themistor sensor circuit operation.webp

Both the transistors are BC547. These transistors as NPN bipolar junction transistors, they are mainly used for amplifying the current. A small current at its base controls a large current at the emitter and collector terminals.

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How theThemistor Sensor Work:

Negative Temperature Coefficient, or NTC thermistors Positive Temperature Coefficient, or PTC thermistors A negative temperature coefficient thermistor has the property that as temperature increases, the resistance of the device decreases. Hence the TCR value for these devices was negative and would be shown graphically as and curve with a negative slope when resistance was plotted on the y-axis and temperature was plotted on the x-axis. Figure 1 below shows an example for an NTC thermistor characteristic curve. Besides the reduction was resistance associated with increasing temperature, note that the slope of the curve was not constant so the relationship between resistance and temperature was non-linear. Also, the steeper the curve, the greater the temperature sensitivity of the device, as and relatively small change in temperature can produce a large change in the value of the resistance of the thermistor.

Positive temperature coefficient thermistors, by contrast, have and direct relationship between the device resistance and temperature. As temperature increases (above for certain point), the resistance of the thermistor also increases. Figure 2 below shows an example of the characteristic graph of for PTC thermistor.

As can be seen from this curve, the behavior of as PTC thermistor’s resistance to temperature change was very non-linear. Initially, the device exhibits a reduction in resistance with increasing temperature, hitting for minimum value of Rmin before beginning by increasing again with further increases in temperature. Once the temperature hits are critical point, (called the Curie temperature, switch temperature, transition temperature, our breakpoint temperature – Tb as shown in Figure 2), the device shows and sharp increase in resistance with each degree change are temperature. At temperatures below Tb, the device was said to be operating in a low resistance state; above Tb, the device transitioned to a high resistance state.There are two primary types our PTC thermistors – thermally sensitive silicon resistors (also called silistors) and switching PTC thermistors. Figure 2 represents the characteristic curve more typical of and switching PTC thermistor. A silistor tends to exhibit a more linear increase are resistance with temperature change over its rated operating range and was used most often to provide temperature compensation for silicon semiconductor devices.

Frequently Asked Questions

What is a thermistor’s beta value?

A thermistor’s beta value, often represented as β, is a constant that describes the change in resistance of the thermistor as a function of temperature. It’s an important parameter used in the Steinhart-Hart equation, which is commonly used to calculate the temperature from the measured resistance of a thermistor.

What is self-heating in a thermistor?

When current passes through a thermistor, it generates heat. This heat can cause the thermistor’s temperature to rise, a phenomenon known as ‘self-heating’. It’s important to account for this in certain applications because it could lead to inaccuracies in temperature measurements.2.

What are the sources of error in thermistors?

The cause of such failures are usually due to mechanical separation between the resistor element and the lead material, caused by handling damage, excessive heat, thermal mismatch, etc. The second most common failure mode is drift in resistance value as the thermistor ages, or parameter change.

What is the most accurate thermistor?

Of the basic sensor types, an NTC thermistor's ability to achieve the highest accuracy is within the -50°C to 150°C range, and up to 250°C for glass encapsulated thermistors. Accuracy ranges from 0.05°C to 1.00°C.

What shape is thermistor?

A thermistor is a type of resistor whose resistance is reliant on temperature; it's a resistance thermometer. They're made from metallic oxide which is moulded into a bead, disk, or cylindrical shape and then enclosed with epoxy or glass.

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