Why are conductors called positive temperature coefficients

PTC - PTC thermistor

PTC thermistors are semiconductor resistors that are temperature-dependent. PTC thermistors have a positive temperature coefficient (TK) and are therefore also called PTC resistors (PTC = Positive Temperature Coefficient).
With this type of semiconductor, the lattice arrangement of the atoms gives one free valence electron per atom. These electrons are easy to move. When connected to a power source, the free valence electrons move to the positive pole and cause electrical conductivity.
Almost all metals are PTC thermistors, as they conduct better at lower temperatures. PTCs consist of polycrystalline titanate ceramic types that are contaminated with foreign atoms (doping).

This can be shown in an experiment:

The resistance value of a wire is determined by measuring the current and voltage. The wire is then heated and the resistance determination is repeated.

Resistance determination before heating:

U = 0.5V
I = 4 A
R = 0.125 Ω

Resistance determination after heating:

U = 1 V
I = 3 A
R = 0.33 Ω

The result of this experiment shows that PTC thermistors have a low resistance when cold, i.e. good electrical conductivity. When heated, the conductivity decreases, the resistance increases (see measurement results).

The diagram describes the resistance curve as a function of the temperature of a PTC resistor.
The resistance value starts at the initial temperature T.A. to rise. This point is the initial resistance RA.. Charge carriers are released as a result of the increase in temperature. Up to the nominal temperature TN the resistance increases non-linearly. From the nominal resistance RN the resistance increases sharply. The barrier layer formation between the material crystals is responsible for the increase in resistance. Up to the final temperature TE. extends the work area of ​​the PTC.

Above a certain voltage, the PTC has a relatively high level of self-heating. These are used for measurements and in control technology. Before that, it reacts like a perfectly normal linear resistance. It only reacts to external warming. This area is shown a little wider in the diagram.

Circuit symbols

Applications

  • Liquid level sensor (liquid cools down the self-heated PTC)
  • Temperature control for a heater
  • Power PTCs are used as an alternative to fuses to protect against overcurrent. Advantage: Power PTCs are reversible.

Other related topics:

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