PTC Thermistors Structure and Characteristics
Linear silicon PTC thermistor and Switching Ceramic PTC Thermistors
Commercial PTC thermistors (abbreviated for Positive Temperature Coefficient of Resistance) fall into two major categories. The first category consists of thermally sensitive silicon resistors, sometimes referred to as
¡°silistors¡±. These devices exhibit a fairly uniform positive temperature coefficient (about +0.77%/C) through most of their operational range, but can also
exhibit a negative temperature coefficient region at temperatures in excess of 150C. These devices are most often used for temperature compensation of
silicon semiconducting devices in the range of -40C to +150C.
Fig. RT characteristics of linear silicon PTC thermistor and switching ceramic PTC thermistors
The other major category, and the one that we shall concentrate on in this section, are referred to as switching PTC thermistors. These devices are
polycrystalline ceramic materials that are normally highly resistive but are made semiconductive by the addition of dopants. They are most often manufactured
using compositions of barium, lead and strontium titanates with additives such as yttrium, manganese, tantalum and silica.
These devices have a resistance-temperature characteristic that exhibits a very small negative temperature coefficient until the device reaches
a critical temperature, that is referred to as its ¡°Curie¡±, switch or transition temperature. As this critical temperature is approached, the devices begin to exhibit a rising, positive temperature coefficient of resistance as well as a large increase in
resistance. The resistance change can be as much as several orders of magnitude within a temperature span of a few degrees.
Switching Ceramic PTC Thermistors Characteristics
Ceramic PTC Thermistors Resistance vs. Temperature Characteristic
PTC Thermistor Resistance vs. Temperature means the relation of zero-power resistance of PTC thermistor to PTC thermistor body temperature under a specified voltage. Zero-power resistance should be measured in super slot by using pulse power supply with low output impedance an stable output amplitude. Temperature rise of PTC thermistor induced by measuring current should be so limited that it could be ignored.
Rn - Room temperature zero-power resistance
Rmin - Minimum zero-power resistance
Tb - Curie temperature
Rb - Switch resistance value Rb=2Rn
Rmax - Maximum resistance
Tp - Poise point temperature
¦Â - Lift-drag ratio is LgRmax/Rmin
Fig.2 PTC Thermistor Resistance vs. Temperature Curve
The resistance of the PTC thermistor is composed of the grain resistance and the grain boundary transition resistance. Particularly in the hot state, the strong potential barriers are determining resistance.
Higher voltages applied to the PTC thermistor therefore drop primarily at the grain boundaries with the result that the high field strengths dominating here produce a break-up of the
potential barriers and thus a lower resistance. The stronger the potential barriers are, the greater is the influence of this "varistor effect" on resistance. Below the reference temperature, where the
junctions are not so marked, most of the applied voltage is absorbed by the grain resistance. Thus the field strength at the grain boundaries decreases and the varistor effect is quite weak.
Fig 3 shows the typical dependence of resistance on field strength. It can be seen that the difference in resistance is largest between R(E1), R(E2) and R(E3) at temperature Tmax and thus in
the region of maximum resistance.
Fig 3. Influence of field strength E on the PTC thermistor R/T characteristic
(Varistor effect) aR1 > aR2 > aR3
Due to this dependence on the positive temperature coefficient of the field strength, operation on high supply voltages is only possible with PTC thermistors that have been designed for this purpose
by means of appropriate technological (grain size) and constructional (device thickness) measures.
The impedance measured at AC voltage decreases with increasing frequency. The dependence of the PTC resistance on temperature at different frequencies is shown in figure 3. So use of the
PTC thermistor in the AF and RF ranges is not possible, meaning that applications are restricted to DC and line frequency operation.
Fig 4. Influence of frequency on PTC Thermistor RT characteristics.
Ceramic PTC Thermistors Voltage vs. Current characteristic
The properties of electrically loaded PTC thermistors (in self-heated mode) are better described by the I/V characteristic than by the R/T curve (see figure 4). It illustrates the relationship between voltage and current in a thermally steady state in still air at 25C, unless another temperature is
Fig 5. PTC Thermistor Current Voltage Characteristics
Figure 6 shows two I/V characteristics of one and the same PTC thermistor for two different ambient temperatures T1 and T2, with T1 < T2. At the higher temperature the PTC thermistor has a
higher resistance value although the conditions are otherwise the same. Therefore, it carries less current. The curve for T2 is thus below that for T1. The breakdown voltage, too, depends on the
ambient temperature. If the latter is higher, the PTC thermistor reaches the critical temperature where breakdown occurs on lower power or operating voltage. VBD2 is therefore lower than VBD1.
Fig 6. Ambient temperature influence on PTC thermistors VI characteristics
Ceramic PTC Thermistors Current vs Time Characteristic
PTC Thermistor Current vs Time Characteristic means current change characteristic vs. time. It is normally measured with memory oscilloscope by using measuring circuit as shown in Fig.7.
|Fig.7 Measuring circuit of PTC Thermistor Voltage Current Characteristic
||Fig.8 Measuring circuit of PTC Thermistor Current Time Characteristic
Operating time of PTC thermistor decreases dramatically as initial current increases. Furthermore the operating time is in relation to resistance temperature coefficient, applied voltage and heat capacity of PTC thermistor. The following figure illustrate the PTC Thermistor current time characteristics.
Fig.9 Switching times of some PTC thermistors (parameter: different geometries) versus switching
current (measured at 25C in still air)
Fig.10 PTC Thermistors current time characteristics (AC)
Fig.11 PTC Thermistors current time characteristics (DC, Ambient temperature change)