Practical Tips for Selecting and Using TDK NTC Thermistors

Many electrical and electronic devices, such as switching power supplies, motors, or transformers, have significant inrush currents when these devices are turned on, which can damage other components or trip (trip) circuit breakers. Specially designed to limit inrush currents, TDK's NTC thermistors are connected in series with the load and can significantly reduce these currents. A feature of NTC thermistors is their high cold resistance. When the load current flows through the thermistor, it heats up, and its resistance decreases from 10 to 50 times. Thus, NTC thermistors allow you to control the inrush current more efficiently than a resistor with a fixed (constant) resistance at the same power.

Effect of an NTC thermistor on inrush current in a DC circuit

The NTC type thermistor is always connected in series with the load. An example of the simplest protection against inrush current is shown in the figure:

Basic inrush protection circuit with NTC thermistor

If one NTC thermistor is not enough to limit the inrush current, then two or more thermistors are installed in series. Parallelization of several NTC thermistors is unacceptable, since the load current in this case will be unevenly distributed. The thermistor that carries the most current will heat up until all possible current flows through it (which can destroy it), while the other thermistors in the parallel circuit will remain cold. A typical scheme for protecting the input circuit from inrush current, including an alternative installation location, is shown in the figure:

Typical input circuit protection with NTC thermistors

Selecting the most suitable NTC thermistor series and model is a prerequisite for effective circuit protection. The first, and most important, criterion is the maximum continuous current, which is determined by the load.

The self-heating of the inrush current limiter during operation depends on the applied load. Although some heat is dissipated, the temperature of the NTC thermistor can average up to 250°C in extreme cases. The heat generated during operation will also be dissipated through the lead wires. Therefore, when mounting NTC thermistors, it should be taken into account that the contact surfaces can also become very hot at maximum load.

The power control capabilities of the NTC thermistor cannot be fully utilized over the entire temperature range. The current vs. temperature graph below provides information on the extent to which the load current must be reduced at a given ambient temperature (Ta). On the example of NTC thermistors of the S237 series and others:

Current versus temperature curve for S237 series

The calculated value of the current as a percentage of the maximum current according to the graph for these series of thermistors can be determined by the formula:

Calculation formula of the current as a percentage of the maximum current, where Ta is the ambient temperature, more than 25°C.

For NTC thermistors of the S153 , S235 , S236 , S238 , S364 series, the graph will have a dependence with wider temperature limits:

Current versus temperature curve for S153, S235, S236, S238, S364 series

The calculated value of the current as a percentage of the maximum current according to the graph can be determined by the formula:

Calculation formula of the current as a percentage of the maximum current, where Ta is the ambient temperature, more than 65°C.

The value of the maximum current Imax is defined in the NTC Thermistor Series Specification and indicates the maximum allowable continuous current (DC DC or rms RMS for sinusoidal AC AC) over a temperature range of 0°C to 65°C.

When the load is turned off, the NTC thermistor cools down slowly. Its resistance increases steadily, but the full resistance value is reached only after 1-2 minutes (depending on the type). Therefore, in some applications it may be useful to provide a bypass circuit for the thermistor to restart. In this case, the operation of the end device can be resumed faster and the thermistor condition will not affect the performance of the system.

TDK type NTC thermistors are available in a variety of sizes and resistance ratings to best suit the end application. The product line ranges from the small-sized S153 series thermistor with a maximum power of 1.4W to the currently largest S464 series thermistor with a maximum power of 6.7W. In this case, the maximum continuous currents in AC circuits can be up to 20A.