PTC thermistor

A PTC thermistor , PTC resistor or PTC thermistor ( English Positive Temperature Coefficient Thermistor ) is a temperature-dependent resistor that belongs to the group of thermistors . As an essential property, it has a positive temperature coefficient and conducts electrical current better at low temperatures than at high temperatures.

In principle, all metals have a positive temperature coefficient, so they are PTC thermistors; In contrast to the components dealt with here, however, their temperature coefficient is much smaller and largely linear than that of the platinum measuring resistor used as temperature sensors because of its linearity .

The opposite of PTC thermistors are thermistors , which conduct better at higher temperatures and have a negative temperature coefficient .

Various PTC resistors

Types and areas of application

Circuit symbol of a PTC thermistor

Three fundamentally different classes of PTC thermistors are used in electronics:

Resistance increases roughly linearly with temperature: Temperature measurement, temperature compensation , resistance thermometer -Material: thick film technology on different carrier substrates or structure similar to an incandescent lamp; see also iron-hydrogen resistance
Ceramic base with non-linear resistance curve: Overcurrent protection (as a fuse element), over-temperature protection ( thermal fuse ), self-regulating heater, switching element (keywords "engine start PTC", "PTC degaussing" in combination with a heated window for demagnetization of picture tubes ) Material: barium titanate
Polymer base with non-linear resistance curve: Self-resetting fuse (fuse element for overcurrent protection) Material: plastic filled with tiny soot particles

material

As electronic components, PTC thermistors are usually made of semiconducting , polycrystalline ceramics (for example BaTiO ₃ ), which build up a barrier layer at the grain boundaries in a certain temperature range . More recent developments have led to components with a very steep increase in resistance from a characteristic temperature (approx. 80 ... 130 ° C). Furthermore, the materials were improved so that self-resetting fuses could also be created for mains voltage applications.

Temperature-resistance curve

On silicon basis

Resistance of the temperature sensor based on n-doped silicon as a function of temperature

PTC thermistors based on doped silicon are used in the temperature range from −50 ° C to +150 ° C and are characterized by their small size, response time, tight tolerances and good long-term stability. For the KTY11-6 there is a parabolic relationship between resistance and the Celsius temperature : ${\ displaystyle t}$

{\ displaystyle R_ {t} = 2000 \, \ Omega \ cdot \ left [1+ \ alpha \ cdot (t-25 \, {} ^ {\ circ} {\ text {C}}) + \ beta \ cdot (t-25 \, {} ^ {\ circ} {\ text {C}}) ^ {2} \ right]}

with the constants

{\ displaystyle \ alpha = 7 {,} 88 \ cdot 10 ^ {- 3} \ cdot 1 / ^ {\ circ} \ mathrm {C}}

{\ displaystyle \ beta = 1 {,} 937 \ cdot 10 ^ {- 5} \ cdot 1 / ^ {\ circ} \ mathrm {C} ^ {2}}

In applications, measurements are normally taken and the temperature sought. The resolution ("inversion") of these formulas along with the associated linearization is described in Wikibooks. ${\ displaystyle R_ {t}}$ ${\ displaystyle t}$

Barium titanate

Characteristic curve of a PTC

During production, mixtures of barium carbonate and titanium (IV) oxide are ground, mixed and then, depending on the intended use, pressed into disc, rod or tube form together with other materials that produce the desired electrical and thermal properties. The bodies are then sintered at high temperatures (between 1000 ° C and 1400 ° C) .

Due to the grain boundaries located acceptors are electrons bound from the grains. This leads to the development of depletion marginal layers on the grain surfaces, which cause potential barriers . Below the Curie temperature , these potential barriers are largely compensated for by the spontaneous polarization . The conduction mechanism is therefore essentially based on the charge carrier density , which increases with increasing temperature. This is initially a typical thermistor behavior . The polarization decreases with increasing temperature until it finally disappears completely above the Curie temperature. It is true that the charge carrier density continues to increase with increasing temperature, but due to the lack of polarization, the insulating effect of the depletion outer layers comes into its own, so that the resistance increases strongly exponentially.

{\ displaystyle R = R_ {0} \ cdot e ^ {b (T-T_ {0})}}

The individual symbols stand for the following quantities :

${\ displaystyle R}$ - Resistance at absolute temperature ${\ displaystyle T}$
${\ displaystyle R_ {0}}$ - Nominal resistance at nominal temperature ${\ displaystyle T_ {0}}$
${\ displaystyle b}$ - material constant

If the PTC thermistor is heated even further, the increase in charge carrier density now again counteracts the insulation through the depletion edge layers, so that there is a slight decrease in resistance. This behavior is similar to that of an NTC thermistor.

Metals

Characteristic curve of the Pt100

Resistance curve of an incandescent lamp at different operating voltages; the cold resistance is only about 7% of the resistance at nominal voltage.

Pure metals have an electrical resistance that increases linearly with temperature. The platinum resistor Pt100 (wire or layer) is common for temperature measurement and is standardized .

Incandescent lamps are also suitable as overload protection; When cold, they have a cold resistance of only a few percent of the resistance when operated at nominal voltage.

Small incandescent lamps were also used in RC generators for amplitude stabilization.

In the past, iron-hydrogen resistors were used to stabilize the current in the heating circuit of tube devices. They had an almost constant power consumption over an operating voltage range of around 1: 3.

Alloys have a much smaller temperature coefficient, which can even be zero in certain temperature intervals (see constantan ).

circuit

It rarely makes sense to supply PTC thermistors with constant current, since then a stable temperature is not possible. If this increases a little, the resistance of the PTC thermistor also increases. Therefore, because of the relationship P = I² · R, the generated heat output and the temperature continue to increase. Physically one speaks of an unstable equilibrium. Operation with constant voltage makes more sense. Then when the temperature rises because of P = U² / R, the generated heat output drops and the PTC thermistor cools down again. For this reason, components with PTC thermistor characteristics for the simultaneous supply of a common constant voltage cannot easily be connected in series, but a parallel connection is not critical. Conversely, motor protection sensors with several PTC thermistors only make sense with constant voltage supply with a series connection or with individual evaluation.

For temperature measurement in connection with microcontrollers , the PTC thermistor is often connected to the constant operating voltage via a series resistor, whereby the relationship between temperature and voltage is S-shaped and there is the possibility of linearization around the turning point. The prerequisite is that the temperature to be measured must be near the turning point of the curve. The series resistor must limit the current to such an extent that no relevant self-heating of the sensor occurs.

Individual evidence

↑ Like all semiconductors, pure silicon has a negative temperature coefficient and is therefore an NTC thermistor, cf. Earl D. Gates: Introduction to Electronics . Cengage Learning, 2000, ISBN 0-7668-1698-2 , pp. 181 ( limited preview in Google Book search).
↑ data sheet of the KTY11

Web links

Wikibooks: Linearization of Resistive Sensors - Learning and Teaching Materials

Commons : Positive temperature coefficient thermistors - Collection of images, videos and audio files

Wiktionary: PTC thermistors - explanations of meanings, word origins, synonyms, translations

Electronics compendium: PTC thermistors

[1] Like all semiconductors, pure silicon has a negative temperature coefficient and is therefore an NTC thermistor, cf. Earl D. Gates: Introduction to Electronics . Cengage Learning, 2000, ISBN 0-7668-1698-2 , pp. 181 ( limited preview in Google Book search).

[2] ta sheet of the KTY11