How to Select NTC Thermistors for Inrush Current Limiting

There are 3 major criteria for selecting the best NTC Thermistor inrush current limiter, surge suppressor for an application:
¡¤ Rated resistance (R25)
¡¤ Maximum permissible continuous current under rated operating conditions (Imax, DC or RMS values for AC)
¡¤ Maximum capacitance C_{T} to be switched

The rated resistance is a measure of the damping of inrush current. Under rated
operating conditions after stabilization, the maximum continuous current must under no
circumstances be exceeded. Otherwise the component can be both thermally and
electrically overloaded and thus destroyed.
The test method shown in the following figure 1 is used to indicate the pulse strength. A capacitor C_{T}
discharges across a series resistor R_{S} and an NTC thermistor. The charge voltage V_{C} is
selected so that the voltage VNTC applied to the thermistor at the start of discharge is 345 V,
corresponding to (230V +¦¤V) x ¡Ì2. The capacitance C_{T} indicates the energy absorption capacity and thus the pulse
strength of the NTC thermistor.

Fig.1 Test circuit for evaluating the pulse.

The maximum capacitance to be switched represents a criterion for comparing the level
and duration of the current pulse at the moment of turn-on at which the inrush current
limiter can be loaded without sustaining damage.
Knowing the capacitance C_{T} allows an NTC thermistor to be scaled for optimum cost and
geometry (space requirement). However, different C_{T} values can be compared only if
equivalent test methods are used to determine them (see Fig.1).
A particular rated resistance R25 can be implemented in disks of different sizes. This
means that for the same value of R25 different figures can be obtained for continuous load
and pulse strength. So by referring to test patterns, the user can decide which inrush
current limiter is best suited to the application.
As a rule of thumb, the larger the diameter of the thermistor, the higher is the current in
continuous operation. This also applies to energy absorption capability and pulse
strength at the moment of turn-on.

Fig.2 Typical current characteristics of inrush current limiting NTC Thermistor on load after turn-on

(1) Reduction rate of inrush current determines R25 value.
(2) The operating current determines Imax value.