Hold current / Trip current | Basic Knowledge of PTC Thermistor (POSISTOR)

PTC Thermistors (POSISTOR)

PTC Thermistors (POSISTOR)Hold current / Trip current

Basic Knowledge of PTC Thermistor (POSISTOR)

Hold current
Trip current
Protective Threshold Current Range
Basic Knowledge of PTC Thermistor
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Hold current

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

Figure 1 : 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 1,

when the current flowing through the PTC thermistor is less than the trip current,
Shown in figure 2, 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 2 : 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 1.

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 3.

Figure 3 : 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. 5.

Figure 4 : Change horizontal axis to voltage at PTC

Figure 5 : 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.