Thermistor

The thermist is a special type of variable resistant element that changes its physical resistance when exposed to changes in temperature.

The thermist is a solid state temperature sensing device that acts as an electrical resistance but is sensitive to temperature. The thermists can be used to produce an analog output voltage with changes in ambient temperature and therefore can be called a converter. This is due to an external and physical change in temperature, which creates a change in its electrical properties.

The thermist is basically a two-terminal solid state thermally precise converter made using precision semiconductor-based metal oxides with metallized or sintered connectors created in the form of ceramic discs or beads.

This allows the thermistor to change the resistance value in proportion to small changes in ambient temperature.In other words, its resistance changes as its temperature changes, and therefore its name is the combination of the words "Theristor" THERM-ally res-ISTOR.

Although the change in resistance due to heat in standard resistors is usually undesirable, this effect can be used well in many temperature sensing circuits.Therefore, thermists, which are nonlinear variable resistant devices, are widely used as temperature sensors with many applications for measuring the temperature of both liquids and ambient air.

In addition, as a solid state device made of highly sensitive metal oxides, they work at the molecular level with the activation of the outermost (precious) electrons, produce a negative temperature coefficient or become less active, producing a positive temperature coefficient as the temperature of the thermistor.

This means that they have a very good resistance to temperature characteristics that allow it to operate at temperatures up to 200 o C .

thermistor

Although the main use of the thermists is resistant temperature sensors, they can be serially connected with another component or device to control an electric current passing through them.In other words, they can be used as thermally sensitive current limiting devices.

The thermisors are available in a wide range of types, materials and sizes, characterized by response times and operating temperatures.In addition, hermetically sealed thermists offer high operating temperatures and a compact size, while eliminating errors in resistance readings due to moisture penetration.The three most common types are: Bead thermisters, Disc thermisors and Glass encapsulated thermisors.

Resistances due to this heat can work in one of two ways by increasing or decreasing resistance values with changes in temperature.Then there are two types of thermists: the negative temperature coefficient of resistance (NTC) and the positive temperature coefficient of resistance (PTC).

Negative Temperature Coefficient Thermist (NTC)

Negative temperature coefficient of resistance thermists, or NTC thermistsfor short, reduces resistance values as the operating temperature around them increases.In general, NTC thermis are the most widely used temperature sensors, as they can be used in almost any type of equipment in which temperature plays a role.

NTC temperature thermists have a negative electrical resistance to temperature (R/T) relationship.The relatively large negative response of an NTC thermist means that even small changes in temperature can cause significant changes in electrical resistance.This makes them ideal for accurate temperature measurement and control.

We have previously said that a thermist is an electronic component whose resistance depends largely on temperature, so if we send a constant current from the thermistor and then measure the voltage drop on it, so that we can determine its resistance at a certain temperature.

An NTC thermist reduces its resistance with an increase in temperature and is available in various base resistors and temperature curves.NTC thermists are characterized by base resistances at room temperature, usually 25 o C (77 o F), as they provide a suitable reference point, for example: 2k2Ω at 25 ° C.

Another important feature of a thermistor is the value "B". A substance determined by the ceramic material for which the B value is made is fixed. defines the gradient of the resistance (R/T) curve at a specific temperature range between the two temperature points.Each thermisor material will have a different material constant and therefore a different resistance to the temperature curve.

Thus, the B value will define the resistant value of the thermisors at the first temperature or base point called T1 (usually 25oc) and the resistant value of the thermisors at a second temperature point called T2(e.g.100 o C).

Therefore, the value B will define the material constant of the thermisors between the T1 and T2 range. This has the following B values for the typical NTC thermisor, which is given anywhere from about 3000 to about 5000: BT1/T2 or b25/100.

Note, however, that both T1 and T2 temperature points are calculated in Kelvin temperature units of0 0C = 273.15 Kelvin. Thus, the value of 25oc equals 25o + 273.15 = 298.15 K, and equals 100oC 100o + 273.15 = 373.15 k.

Therefore, knowing the B value of a particular thermistor (obtained from the manufacturers' datasheet), it is possible to produce a temperature and resistance table to create an appropriate graph using the following normalized equation:

TheristOr Equation

thermistor
  • Here:
  • T 1 is the first temperature point in Kelvin
  • T 2 is the second temperature point in Kelvin
  • R 1 is thermisor resistance in Ohm at T1 temperature
  • R 2 is the thermisor resistance in Ohm at T2 temperature

Therist Question Sample 1

The 10kΩ NTC thermist has a value of 3455 "B" between 25 o C and 100oC temperature. Calculate the resistant value at 25oc and again at 100oc.

Given data: 25o'Cat B = 3455, R1 = 10kΩ. To convert the temperature scale from celsius to kelvin, let's add the mathematical constant 273.15.

The R1 value is already given as 10kΩ base resistance, so the R2 value at 100oc is calculated as follows:

thermistor

By giving the following colon characteristic graph:

thermistor

Keep in mind that in this simple example there are only two points, but usually the thermists exponentively change their resistance with changes in temperature, and therefore their characteristic curves are not linear, so it would be more accurate to calculate more temperature points.

Temperature (o C)10202530405060708090100110120
Resistance (Ω)1847612185100008260574040802960218816451257973765608

and these points can be drawn as shown to give a more accurate characteristic curve for the 10kΩ NTC Theristor with a value of B of 3455.

NTC TheristOr Properties Curve

thermistor

Note that it has a negative temperature coefficient (NTC), that is, its resistance decreases with increasing temperatures.

Using a Thermistor to Measure Temperature

According to ohm law, if we pass a current over it, a voltage drop will be generated on it.Since a theristor is a passive sensor type, that is, it requires a warning signal to operate, any change in its resistance as a result of changes in temperature can be converted into voltage change.

thermistor

The simplest way to do this is to use the thermistor as part of a potential divisive circuit, as shown.During the resistance and theristor series circuit, a constant supply voltage is applied with the output voltage measured throughout the theristor.

For example, if we use a 10kΩ thermistor with a serial resistance of 10kΩ, the output voltage at base temperature of 25 o C will be half the feed voltage of 10Ω/(10Ω+10Ω) = 0.5.

When the resistance of the theristor changes due to changes in temperature, the ratio of the feed voltage throughout the theristor will also change, producing an output voltage proportional to the fraction of the total serial resistance between the output terminals.

Therefore, the potential dividing circuit is an example of a simple voltage converter resistance, in which the resistance of the thermistor is controlled by temperature, and the output voltage produced is proportional to the temperature.Thus, the hotter the thermist, the lower the output voltage.

If we reverse the positions of serial resistance R S and theristor R TH, the output voltage will change in the opposite direction, that is, the hotter the thermistor, the higher the output voltage.

thermistor

We can use NTC thermisors as part of the basic temperature detection configuration using a bridge circuit, as shown.The relationship between resistances R 1 and R 2 sets reference voltage, V REF to the desired value.R1 both, for example, and R 2 have the same resistance value, the reference voltage will be equal to half the supply voltage as before.This is Vs/2.

As the temperature and therefore the resistance value of the thermistor changes, the voltage in the V TH will also change, producing a positive or negative output signal to the connected amplifier, which will be higher or lower than in V REF.

The amplifier circuit used for this basic temperature sensing bridge circuit can act as a differential amplifier for high sensitivity and amplification, or as a simple Schmitt trigger circuit for ON-OFF switching.

The problem with passing such a current from a thermist is that the thermists experience what is called the self-warming effect, that is, the I 2 *R power loss may be high enough to generate more heat than the therist can dissipate, affecting the resistance value that produces incorrect results.

Therefore, if the current passing through the theristor is too high, it is possible that it results in increased power loss and decreases its resistance as the temperature increases, causing more currents to flow, which further increases the temperature and causes the condition known as Thermal Leakage .In other words, due to the measured external temperature, we want the thermist to warm up and not to warm up on its own.

The serial resistance value, the above R S value , should be selected to provide a reasonably wide response over the expected temperature range that the thermistor is likely to use, and at the same time limit the current to a safe value at the highest temperature.

One way to improve this and more accurately convert temperature resistance (R/T) is to drive the thermist with a constant current source.The change in resistance can be measured using a small, measured direct current or DC that passes through the theristor to measure the output voltage drop produced.

Thermist used to Suppress Takeoff Current

Here we have seen that thermists are used as resistant temperature-sensitive converters, but the resistance of a thermist can be replaced by external temperature changes or changes in temperature caused by an electric current passing through them, after all, these are resistant devices.

Ohm's Law tells us that when an electric current passes through R resistance, as a result of the voltage applied, the power is consumed in the form of heat due to the I 2 *R heating effect.Due to the self-heating effect of the current in a thermist, a thermist can change its resistance with changes in the current.

Inductive electrical equipment such as motors, transformers, ballast lighting, etc. are exposed to excessive sudden currents when they are first placed in the "ON" position.However, serially connected thermists can also be used to effectively limit any high initial current to a safe value.NtC thermists with low cold resistance values (at 25 o C) are usually used for such current regulation.

Departure Current Limiting Thermist

thermistor

Sudden current suppressors and overvoltage limiters are types of serially connected thermisors whose resistance is very low because they are heated by the load current passing through them.At first start, the cold resistance value (basic resistance) of the thermists is quite high, controlling the first sudden current to the load.

As a result of the load current, the thermistor heats up and reduces its resistance relatively slowly until it is sufficient to maintain the low resistance value with most of the applied voltage developed throughout the load.

Due to the thermal inertia of its mass, this heating effect lasts for a few seconds, during which the load current increases gradually rather than at once, so any high sudden current is restricted and the power it draws accordingly decreases.Due to this thermal effect, sudden current suppression thermists can therefore work very hot in low-resistance conditions.Therefore, it requires a cooling or recovery period when the power is cut off, so that the resistance of the NTC thermistor is allowed to be sufficiently ready the next time it is needed.

The reaction speed of the current limiting thermist is given by the time constant.That is, 63% of the total change (i.e. from 1 to 1/ε) is the time it takes to resist change.For example, suppose the ambient temperature changes from 0 to 100 o C, then the 63% time constant will be the time it takes for the therist to have a resistance value of 63 o C.

NTC thermists provide protection from unwanted high currents while their resistance remains negligiblely low during continuous operation that powers the load.The advantage here is that they can effectively process sudden currents much higher than standard constant current limiting resistors with the same power consumption.

TheristIst Summary

Here, a thermist is a two-terminal resistant converter that can change the resistance value with changes in the surrounding ambient temperature, hence thermal resistance or simply called the "thermistator".

Thermissors are inexpensive, easy-to-obtain temperature sensors made using semiconductor metal oxides.They are available with negative temperature coefficient, (NTC) resistance or positive temperature coefficient (PTC) resistance.The difference is that NTC thermists reduce their resistance as the temperature increases, while PTC thermists increase their resistance as the temperature increases.

The NTC thermisor is most widely used (especially 10KΩ NTC) and combined with an additional series of resistance, R, S can be used as part of a simple potential divider circuit.Thus, changes in resistance due to changes in temperature produce an output voltage due to temperature.

However, the operating current of the thermist should be kept as low as possible to reduce the effects of self-warming.If the operating currents are too high, they can heat up faster than they can distribute, resulting in incorrect results.

In addition to using thermaries to measure an external temperature, we found that they can also be used to control an electric current as a result of the I 2 R heating effect caused by the current passing through it.By serially connecting an NTC thermist to a load, it is possible to effectively limit any high currents.