Basic Characteristics | Basic Knowledge of PTC Thermistor (POSISTOR)

Resistance - Temperature Characteristics (R-T characteristics)

Curie Point (C.P.) is defined as a temperature that is twice the resistance value at 25°C.
Between room temperature and the Curie point, the resistance decreases slightly or is almost constant, but above the Curie point the resistance increases logarithmically.

Figure 1 : PTC Res.-Temp. Characteristic Curve and Curie Point

[Reference]

There are types of Curie point (c.p.) as shown in Fig. 2.

Figure 2 : PTC with different Curie points Res.- Temp. characteristics

If the temperature continues to rise beyond the Curie point, it loses its positive temperature characteristics and the resistance decreases. This point is called the TN point.
The rated value of the PTC thermistor is designed with a margin so that it does not exceed the TN point.

Figure 3 : TN point of PTC thermistor

Current-voltage characteristics

I-V characteristics, static characteristics

The I-V characteristics of the PTC thermistor are shown in figure 4.
When the internal heat generation and the heat dissipation to the outside are in equilibrium with applied voltage, the relationship between the applied voltage and the stable self-current is determined. In the constant resistance range, V = IR, and the PTC thermistor does not self-heat. It has a maximum point of current and a constant power range.

Figure 4 : Current-Voltage characteristics

Relation with R-T characteristics

Relationship between resistance and temperature characteristics
Exceeding the maximum point of the current is called “tripping”.

Figure 5 : Relationship between PTC I-V and R-T characteristics

Change in I-V characteristics

The characteristics of overcurrent protection PTC are adjusted by controlling the “Initial resistance”, “Curie point”, and “Heat dissipation” in Figure 6.

Change of initial resistance

Change of Curie point

Changes of heat dissipation

Changes of ambient temperature

Figure 6 : Changes in I-V characteristics due to each factor

For a PTC with fixed characteristics, the I-V characteristics will fluctuate due to changes in the ambient temperature, as shown in Fig. 6, “Changes in ambient temperature.” The maximum current point is the trip current. From this graph, it can be confirmed that the trip current value differs depending on the ambient temperature. Therefore, the trip current is defined at a temperature other than 25°°C (e.g. −10°C) at the rating.

Hold current

Indicates a current that does not reach a current maximum due to factors other than ambient temperature.

Figure 7 : PTC connection diagram

Trip current

In current-voltage characteristics, the maximum point of the current is called the trip current.

In the circuit of figure 7,

when the current flowing through the PTC thermistor is less than the trip current,
Shown in figure 8, the load curve (a) and the current voltage characteristic of the PTC thermistor are stabilized at point (A), and act as a simple fixed resistor.
When a current larger than the trip current of the PTC thermistor flows,
it stabilizes at the intersection point (B) with the load curve (b).
This means that if a current larger than the trip current flows through the circuit, the resistance of the PTC thermistor increases, attenuating the circuit current to a value less than the trip current and protecting the power supply and load sides.

Figure 8 : I-V characteristic and Load Curve

[Reference] Load Curve

The load curve in the PTC I-V characteristic graph represents the current flowing through the circuit as the voltage to the circuit resistance gradually decreases due to the increasing voltage drop across the PTC in the connection diagram of Figure 7.

Under normal conditions (there is no fault in the circuit and it is operating normally)
If the normal current is normal I, the power supply voltage E / normal I = normal circuit resistance.
When there is an abnormal conditions (some kind of circuit abnormality)
If the current at the time of abnormality is set to Abnormal I, the power supply voltage E/Abnormal I = Abnormal circuit resistance.

When a voltage is applied to these circuit resistors, it is shown in the figure 9.

Figure 9 : I-V characteristics of circuit resistance

As definition, the horizontal axis is defined as “voltage drop at PTC =Power supply voltage - voltage drop at circuit resistance”, As the definition, if the horizontal axis is changed to “voltage drop at PTC = power supply voltage - voltage drop at circuit resistance” and both vertical and horizontal axes are displayed logarithmically, the characteristics will be as shown in Fig. 11.

Figure 10 : Change horizontal axis to voltage at PTC

Figure 11 : Change both axes to logarithmic

Protective Threshold Current Range

The trip current of PTC thermistor varies depending on the ambient temperature, resistance value, temperature characteristics and shape. The current range above the upper limit of the trip current is called the trip current range, it below the lower limit is called the hold current range, and it between the upper and lower limits is called the Protective threshold current range.
That is, when a current is smaller than the hold current, PTC works only as a fixed resistor. When larger than the trip current flows, however, PTC protects the circuit from overload.

Figure 12 : Relationship between Protective Threshold current range and I-V Characteristic Variation

[Reference] Calculate Protective Threshold Current Range

Thermal equilibrium formula: Wattage (I2R) ＝ Thermal Energy（DT）

D: Typical Dissipation Constant
R: Resistance
Ia: Trip current at ambient temperature Ta
Tcp: Curie temperature

Since D and R are the same value for the same element, the ratio with the ambient temperature of 25°C,,,

As an example, the trip current is calculated at Curie point 120°C, ambient temperature 60°C & 10°C.

→ 0.795 times the trip current at 25°C

→ 1.17 times the trip current at 25°C

Figure 13 : Protective Threshold current range

Operating curve

Temperature of tripping PTC thermistor can be estimated by the operating curve.

How to create the operating curve

Typical Dissipation Constant
The typical dissipation constant is confirmed by measuring the current and the element temperature under the applied abnormal voltage in the environment where the set is used.
(This is necessary because chip PTC thermistors have different the dissipation depending on their mounting condition.)
Resistance value calculation
The temperature of PTC thermistor calculate the resistance value using the following formula.
Rx=(Abnormal Voltage)2/(typical dissipation constant ×(PTC heating temperature − Ambient Temperature))
Check the intersection with resistance-temperature characteristics
This is temperature of PTC thermistor that is tripped by an abnormal voltage.

Figure 14 : R-T characteristics and Operating curves

Current-time characteristics

It represents the relationship between the current and the time until the internal heat generation and external heat dissipation reach an equilibrium state for the application of a voltage above the maximum current.

Figure 15 : Operating time when applying exceeding the trip current

The larger applied voltage (current), the greater the instantaneous power consumption and the shorter the time to thermal equilibrium.

Operating Time

It is defined as the time until the inrush current reaches 1/2 value.

Figure 16 : Operating Time characteristics

Figure 17 : Inrush Current and Operating Time

Maximum voltage / Maximum current

It indicates the maximum voltage / current that can be applied to the PTC thermistor at all times within the operating temperature range.

Withstand voltage

Withstand voltage indicates the voltage that can withstand application for 3 minutes in still air at 25 degrees Celsius. The voltage is applied by increasing the voltage from 0V to the withstand voltage by slow boosting.
The higher the thickness of the element and the longer the distance between the electrodes, the higher the breakdown voltage. In other words, the more grains there are, the higher.