Power NTC Thermistors for Inrush Current Limiting Surge Suppression

What is Inrush Current Limiting (ICL) Surge Suppression Power NTC Thermistor?

Inrush Current Limit NTC Thermistor

Inrush Current Limit NTC Thermistor

Inrush Current Limiting (ICL) Surge Suppression Power NTC Thermistor is polycrystalline mixed oxide ceramics made semiconductor type NTC Thermistor disc soldered with radial metal wire, coated with epoxy resin, function as a power resistor to limit the inrush current surge current in the turn-on stage.

Power NTC thermistor can be a cost effective device to limit the amount of inrush current in a switching power supply or other devices when the power is first turned on.

Power NTC thermistor limits surge current by functioning as a power resistor which drops from a high cold resistance to a low hot resistance when heated by the current flowing through it.

Inrush-current limiters power NTC thermistor protect circuits from undesirably high currents, suppressing high inrush current surges, while its resistance remains negligible low during continuous operation. Thanks to their low resistance in the operating state, Power NTC thermistor have a considerably lower power dissipation than the fixed resistors frequently used for this application.

How Does  NTC Thermistor Inrush Current Protection Work?

In the cold state, i.e. at room temperature, the high initial resistance of the inrush current limiter effectively absorbs the power of peak inrush currents. As a result of the current load and subsequent heating, the resistance of the inrush current limiter then drops by a factor >30 – 50 to a few percent of its value at room temperature. The power consumption of the inrush current limiters is thus negligible in continuous operation – an outstanding advantage of NTC thermistor over fixed resistors.

Resuming operation after cooling down

After a load has been switched off, the NTC thermistor must be allowed to cool down to room temperature if its capacity for inrush current limiting is to be fully used. This can take from 30 seconds to two minutes depending on the disk size. In the case of switched mode power supplies, these cooling times are often a minor consideration because electrolytic capacitors in the circuit usually take longer to discharge fully. The NTC thermistor will therefore be cool enough to resume operation in the event of another short-term turn-on.

Inrush Current Limit Power NTC Thermistor Application

limiting surge current, suitable for the protection of switch mode power supply, UPS power, transformers, motors, various electric heating utensil, energy saving lights, ballast, various power circuit, amplifiers, colored displayer, monitors, color TV, filament protection, etc.
Power NTC thermistor components can also be used for the soft starting of motors, for example in vacuum cleaners with continuous currents of up to 20 A.

Surge Suppression Power NTC Thermistor Advantages:

Comparison curve With Without Inrush Current Limiting Power NTC thermistor

Comparison curve With Without Inrush Current Limiting Power NTC thermistor

·Low cost solid state device for inrush current suppression.
·Minimize line current distortion and radio noise.
·Protect switches, rectifier diodes and smoothing capacitors against premature failures.
·Prevent fuse from blowing in error.

 Power NTC Thermistor Features:

Power NTC Thermistor Load Temperature Characteristics

Power NTC Thermistor Load Temperature Characteristics

·Resin coated disk NTC thermistor with uninsulated lead-wires.
·Suitable for both AC and DC circuits up to a voltage of 265 V(rms).
·Wide range of resistance, current and dimension.
·Excellent mechanical strength.
·Suitable for PCB mounting.

Typical Application of  Power NTC Thermistors for Circuit Protection Diagram

NTC thermistors in a protective circuit mounting positions

NTC thermistors in a protective circuit mounting positions

NTC Thermistor for diode protection

NTC Thermistor for diode protection

Power thermistor application circuits

Power thermistor application circuits

NTC Inrush Current Limiters In Switching Power Supplies

Typical Power Supply Circuit

Typical Power Supply Circuit

The problem of current surges in switch-mode power supplies is caused by the large filter capacitors used to smooth the ripple in the rectified 60 Hz current prior to being chopped at a high frequency. The diagram above illustrates a circuit commonly used in switching power supplies.

In the circuit above the maximum current at turn-on is the peak line voltage divided by the value of R; for 120 V, it is approximately 120 x √2/RI. Ideally, during turn-on RI should be very large, and after the supply is operating, should be reduced to zero. The NTC thermistor is ideally suited for this application. It limits surge current by functioning as a power resistor which drops from a high cold resistance to a low hot resistance when heated by the current flowing through it. Some of the factors to consider when designing NTC thermistor as an inrush current limiter are:

  • Maximum permissible surge current at turn-on
  • Matching the NTC thermistor to the size of the filter capacitors
  • Maximum value of steady state current
  • Maximum ambient temperature
  • Expected life of the power supply

Maximum Surge Current

The main purpose of limiting inrush current is to prevent components in series with the input to the DC/DC converter from being damaged. Typically, inrush protection prevents nuisance blowing of fuses or breakers as well as welding of switch contacts. Since most NTC thermistor materials are very nearly ohmic at any given temperature, the minimum no-load resistance of the NTC thermistoris calculated by dividing the peak input voltage by the maximum permissible surge current in the power supply (Vpeak/Imax surge).

Energy Surge at Turn-On

At the moment the circuit is energized, the filter caps in a switcher appear like a short circuit which, in a relatively short period of time, will store an amount of energy equal to 1/2CV2. All of the charge that the filter capacitors store must flow through the  thermistor. The net effect of this large current surge is to increase the temperature of the thermistor very rapidly during the period the capacitors are charging. The amount of energy generated in the  thermistor during this capacitor-charging period is dependent on the voltage waveform of the source charging the capacitors. However, a good approximation for the energy generated by the NTC thermistor during this period is 1/2CV2 (energy stored in the filter capacitor). The ability of the NTC thermistor to handle this energy surge is largely a function of the mass of the device. This logic can be seen in the energy balance equation for a thermistor being self-heated:

Input Energy = Energy Stored + Energy Dissipated

or in differential form: Pdt = HdT + δ(T – TA)dt


  • P = Power generated in the NTC thermistor
  • t = Time
  • H = Heat capacity of the NTC thermistor
  • T = Temperature of the thermistor body
  • δ = Dissipation constant
  • TA = Ambient temperature

During the short time that the capacitors are charging (usually less than 0.1 second), very little energy is dissipated. Most of the input energy is stored as heat in the thermistor body. In the table of standard inrush limiters there is listed a recommended value of maximum capacitance at 120 V and 240 V. This rating is not intended to define the absolute capabilities of the thermistor; instead, it is an experimentally determined value beyond which there may be some reduction in the life of the inrush current limiter.

Maximum Steady-State Current

The maximum steady-state current rating of a thermistor is mainly determined by the acceptable life of the final products for which the thermistor becomes a component. In the steady-state condition, the energy balance in the differential equation already given reduces to the following heat balance formula:
Power = I2R = δ(T – TA)

As more current flows through the device, its steady-state operating temperature will increase and its resistance will decrease. The maximum current rating correlates to a maximum allowable temperature.

In the table of standard inrush current limiters is a list of values for resistance under load for each unit, as well as a recommended maximum steady-state current. These ratings are based upon standard PC board heat sinking, with no air flow, at an ambient temperature of 77° (25°C). However, most power supplies have some air flow, which further enhances the safety margin that is already built into the maximum current rating. To derate the maximum steady state current for operation at elevated ambient temperatures, use the following equation:
Iderated = Iderated = √(1.1425–0.0057 x TA) x Imax @ 77°F (25°C)

Notes on scaling an inrush current limit  NTC thermistor

A few items of data are needed to scale an inrush current limiter NTC thermistor:

  • Load capacitance of device to be protected (determination of minimum size of the component)
  • Steady-state current and maximum ambient temperature
  • Required reduction of inrush current

Load capacitance of device to be protected

The high inrush current of devices results from the higher energy required to turn on. In power supplies the energy requirement is primarily caused by load capacitors, in transformers by magnetizing energy. The associated turn-on operations load the inrush current limiter as a current pulse. So this energy must be known to select the right component. It can be converted into capacitance for a given voltage.

Steady-state current and maximum ambient temperature

Select the component so that the steady-state current does not exceed the maximum admissible current (Imax) of the inrush current limiter. The maximum admissible current is produced from the figure for Imax and the derating in 2.4 with the maximum ambient temperature.

When scaling a design, remember the possibility of line voltage fluctuations and different operating states (steady-state currents) of the device itself, and incorporate appropriate precautionary measures.

Required reduction of inrush current

Within this component model the maximum steady-state current then determines the highest possible cold resistance (R25) that can be used for an application.
The higher the cold resistance (R25) of the inrush current limiter, the more the inrush current is dampened. If the current limiting effect of a component is inadequate, choose a larger model.

AMWEI Inrush Current Limiting Power NTC Thermistors Dimensions (mm)

Inrush Current Limit NTC Thermistor Dimension

AMWEI Inrush Current Limit Power NTC Thermistors Data Sheet

AMWEI Part No.Resistance
AMF72-3D93 ohm4A0.1201134115.57.5 /5
AMF72-5D95 ohm3A0.2101134115.57.5 /5
AMF72-8D98 ohm2A0.4001132115.57.5/5
AMF72-10D910 ohm2A0.4581132115.57.5/5
AMF72-16D916 ohm1A0.8021131115.57.5/5
AMF72-22D922 ohm1A0.9501130115.57.5/5
AMF72-33D933 ohm1A1.1241130115.57.5/5
AMF72-50D950 ohm1A1.2521130115.57.5/5
AMF72-80D980 ohm>0.8A2.0101130115.57.5/5
AMF72-3D113 ohm5A0.1001343135.57.5/5
AMF72-5D115 ohm4A0.1561345135.57.5/5
AMF72-8D118 ohm3A0.2551447135.57.5/5
AMF72-10D1110 ohm3A0.2751447135.57.5/5
AMF72-12D1112 ohm2A0.4621448135.57.5/5
AMF72-16D1116 ohm2A0.4701450135.57.5/5
AMF72-20D1120 ohm2A0.5121552135.57.5/5
AMF72-22D1122 ohm2A0.5631552135.57.5/5
AMF72-33D1133 ohm1.5A0.7341552135.57.5/5
AMF72-50D1150 ohm1.5A1.0211552135.57.5/5
AMF72-60D1160 ohm1.5A1.2151552135.57.5/5
AMF72-1.3D131.3 ohm7A0.062136015.567.5
AMF72-3D133 ohm6A0.092146015.567.5
AMF72-5D135 ohm5A0.125156815.567.5
AMF72-10D1310 ohm4A0.206156515.567.5
AMF72-15D1315 ohm3A0.335166015.567.5
AMF72-30D1330 ohm2.5A0.517166515.567.5
AMF72-47D1347 ohm2A0.810176515.567.5
AMF72-1.3D151.3 ohm8A0.048186817.5610/7.5
AMF72-1.5D151.5 ohm8A0.052186917.5610/7.5
AMF72-3D153 ohm7A0.075187617.5610/7.5
AMF72-5D155 ohm6A0.112207617.5610/7.5
AMF72-8D158 ohm5A0.178208017.5610/7.5
AMF72-10D1510 ohm5A0.180218517.5610/7.5
AMF72-15D1515 ohm4A0.268207517.5610/7.5
AMF72-30D1530 ohm3.5A0.438187517.5610/7.5
AMF72-47D1547 ohm3A0.680218617.5610/7.5
AMF72-0.7D200.7 ohm12A0.018258922.5710/7.5
AMF72-1.3D201.3 ohm9A0.037248822.5710/7.5
AMF72-3D203 ohm8A0.055248822.5710/7.5
AMF72-5D205 ohm7A0.087238722.5710/7.5
AMF72-8D208 ohm6A0.1422510522.5710/7.5
AMF72-10D2010 ohm6A0.1622410222.5710/7.5

NTC thermistor part with increased max. operating current


AMWEI PartResistance
Max Stable Current(A)Approx. Resistance Value at Maximum Current (Ω)Max Rated Power Pmax.(W)Dissipation Factor (mW/oC)Thermal Time Constant (s)Dimensions (mm)
D15mm 2.5ohm 9.5A2.5 ohm9.5A0.0443.5W22 min75 max17.56
D15mm 5 ohm 8A5 ohm8A0.0583.5W22 min75 max17.56
D15mm 10 ohm 7A10 ohm7A0.0983.5W22 min75 max17.56
D20mm 1 ohm 16A1 ohm16A0.0275W28 min110 max22.57
D20mm 5 ohm 12A5 ohm12A0.0475W28 min110 max22.57
D20mm 10 ohm 8A10 ohm8A0.0855W28 min110 max22.57
D25mm 1 ohm 20A1 ohm20A0.0217W30 min130 max298
D25mm 5 ohm 14A5 ohm14A0.0477W30 min130 max298
D25mm 10 ohm 10A10 ohm10A0.0847W30 min130 max298
D30mm 1 ohm 30A1 ohm30A0.0148W40 min190 max368.5
D30mm 10 ohm 13A10 ohm13A0.0568W40 min190 max368.5

Inrush Current Limiter NTC Thermistor Surge Suppression Reliability Data

Test ItemStandardTest ConditionsΔR25/R25Remarks
Storage in dry heatIEC 60068-2-2Storage at upper category  temperature
Temperature: 125oC
Time: 1000h
<10%No visible damage
Storage in damp heat, steady stateIEC 60068-2-3Temperature of air: 40oC
Relative humidity of air: 93%
Duration: 21 days
<5%No visible damage
Rapid temperature cyclingIEC 60068-2-14Lower test temperature: -55oC
Upper test temperature: 125oC
Number of cycles: 10
<10%No visible damage
Endurance\I=Imax t: 1000h<10%No visible damage
Cyclic endurance\I=Imax, 1000 cycles
On-time=1 min C
ooling time=6 min
Transient load\Capacitance=CT
Number of cycles: 1000
<5%No visible damage

Consideration in Selecting Power NTC Thermistors for Inrush Current Limiting Surge Suppression 

  • Maximum operating current > Actual operating current in the power loop
  • Rated zero power resistance at 25C
  • The larger Beta value, the smaller residual resistance, the smaller operating temperature rising.
  • Generally, the larger product of time constant and dissipation coefficient, the larger NTC thermal capacity, the more powerful NTC Thermistor surge current restraining capacity.

Inrush Current Limit NTC Thermistor Application Precautions

  • For inrush current limiting, the NTC thermistor must be connected in series with the load circuit. Several inrush current limiters can also be connected in series for higher damping.
    Inrush current limiters must NOT be connected in parallel.
  • In general inrush current limiters require time to get back to cold state, in which they can provide adequate inrush current limiting due to their high resistance. The cooling down time depends on ambient conditions.
  • It should be considered that the surrounding area of NTC thermistor may become quite hot. Ensure the adjacent components are placed at sufficient distance from a thermistor to allow for proper cooling time of the thermistor.
  • Make sure that adjacent materials are designed for operation at temperatures comparable to the surface temperature of the thermistor. Make sure that surrounding parts and materials can withstand this temperature.
  • Make sure that thermistor are adequately ventilated to avoid overheating.
  • Avoid contamination of the thermistor surface.
  • Avoid contact of NTC thermistor  with any liquids and solvents. Ensure that no water enters an NTC thermistor

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