Temperature coefficient#Positive temperature coefficient of resistance

Thermistor, PTC resistors or PTC thermistor ( Positive Temperature Coefficient English ) are electrically conductive materials that can conduct electricity better at lower temperatures than at high temperatures. Their electrical resistance increases with increasing temperature. This type of resistor thus has a positive temperature coefficient. In principle, all metals have a positive temperature coefficient, so are PTC; In contrast to the devices discussed here, however, its temperature coefficient is substantially smaller and substantially linearly, similar to the used as the temperature sensors due to their linearity " PT100 ".

The opposite of PTC thermistors are thermistor (also NTC resistors or NTC thermistors ), the conduct better at higher temperatures, ie, have a negative temperature coefficient.

Types and applications

In electronics, there are three fundamentally different classes of PTC Application:

Material

When electronic parts are usually made ​​of PTC semiconducting polycrystalline ceramics ( for example, BaTiO3 ), which establish a barrier layer at the grain boundaries in a given temperature range. Recent developments have resulted in components having a very steep increase of the resistance at a characteristic temperature (approx. 80 .. 130 ° C). Furthermore, the materials have been improved, that self-resetting fuses could also be created for mains voltage application.

Temperature-resistance characteristic curve

Silicon-based

PTC thermistors based on doped silicon can be used in a temperature range of -50 ° C to 150 ° C and are characterized by small size, response time, close tolerances and good long-term stability. For KTY11 - 6 is a parabolic relationship between the resistance and the temperature T is considered:

With the constants

In applications normally RT is measured and the temperature T sought. The resolution ( " reversal " ) these formulas together with the associated linearization is described here.

Barium titanate

In the preparation of mixtures of barium carbonate and titanium ( IV) oxide together with other materials that provide the desired electrical and thermal properties, milled, mixed, and then pressed into discs, rod or tube form, depending on the intended use. Thereafter, the bodies are sintered at high temperatures ( between 1000 ° C and 1400 ° C).

Through located at the grain boundaries electron acceptors are bound from the grains. This leads to the formation of depletion boundary layers at the grain surfaces, which cause potential barriers. Below the Curie temperature, this potential barrier will be offset by the spontaneous polarization largely. Below the Curie temperature of the conduction mechanism, therefore, resides essentially in the carrier density which increases with increasing temperature. This is initially a typical thermistor behavior. With increasing temperature, the polarization decreases until it above the Curie temperature eventually disappears completely. Although now takes the charge carrier density with increasing temperature continues to increase, but by the now missing polarization, the insulating effect of the depletion boundary layers comes into its own, so that the resistance rises sharply exponentially.

The individual symbols stand for the following sizes:

• R - resistance at the temperature T
• R0 - Nominal resistance at nominal temperature T0
• T - Operating
• T0 - temperature rating
• B - material constant in K-1

, The thermistor further heated to increase the carrier density effect again the insulation by the depletion layers opposite edge, so that there is a slight decrease of the resistance. This behavior again resembles that of a thermistor.

Metals

Pure metals have a fairly linear with temperature increasing electrical resistance. A well-known representative is the Pt100, a platinum resistance ( wire or sheet ) for temperature measurement.

Light bulbs are also suitable for overload protection; they possess when cold a cold resistance of only a few percent of the resistance at operating at nominal voltage.

Small bulbs were used in RC generators for amplitude stabilization.

Previously iron -hydrogen resistors for current stabilization in the heating of tube equipment were used. They had a voltage range from about 1:3, with a substantially constant current consumption.

Alloys have a much smaller temperature coefficients of the in certain temperature intervals may even be zero ( see constantan ).

Circuit

It is rarely useful to provide PTC with constant current, since no stable temperature is then possible. If this rises a little, also the resistance of the thermistor increases. Therefore increases due to the formula P = I ² · R and the thermal power generated, the temperature rises further. Physically, it is called an unstable equilibrium. It makes sense is the mode with constant voltage. Then decreases with increasing temperature because P = V ² / R, the heat power generated and the PTC cools down again. Therefore, PTC can not be connected in series without further ado, a parallel circuit, however, is not critical.

For temperature measurement in conjunction with microprocessors, the PTC is often connected via a series resistor at the constant operating voltage, whereby the connection temperature - voltage S- shaped manner and with a suitable choice of the series resistor the possibility of linearization around the inflection point results. The condition is that the temperature to be measured must be located in the vicinity of the inflection point of the curve.