PTC Thermistors For Inrush Current Limiting

Improving Inrush Current Protection

By Mehdi Samii, Ametherm Inc.


Many applications today, including industrial machinery, power tools and other high current equipment, use limiting inrush current as a major design consideration to combat the problematic effects of inrush current. Inrush current occurs when a system powers on and experiences a spike in current. This current can be substantially higher than standard operating current. If not properly managed, it can reduce the effective operating life and impose damage to equipment. For example, inrush current could disable a cooling fan, eventually leading to total system failure.




ametherm inrush ptc thermistor

Applications that are switched on and off quickly, such as welding equipment, present a particular concern for limiting inrush current. The limiting inrush current circuit must reset instantaneously during each power on to protect the system. This further complicates the management of inrush current.



Inrush Current Overview

During power on, a high inrush current can occur because the power supply's link capacitor functions to dampen ripples in the output current. This capacitor acts like a short, causing an inrush of current. The inrush lasts until the capacitor is charged. Length of the inrush current depends upon the power supply and link capacitor.

The low internal resistance of the power supply aggravates this issue. Any resistance in the power supply introduces inefficiencies through heat. To minimize resistance, engineers typically use an inductive load. While this improves the overall operating efficiency of the power supply, the lack of resistance enables the inrush current to pass through to the main system when the power supply switches on.

Temporarily introducing a high resistance between the power supply and system at power on limits inrush current. The resistance switches out when the initial current surge at power on reaches completion.


NTC-based Limiting

For many systems, a negative temperature coefficient (NTC) thermistor can effectively limit inrush current. An NTC thermistor provides variable resistance based on its temperature. Placing an NTC thermistor between the power supply and system limits inrush current (see Figure 1). At first, the initial temperature of the NTC thermistor is low, providing high resistance. When the system is powered on, it energizes the NTC thermistor, causing the temperature to rise, and thus lowering resistance. As resistance drops to a low value, the current passes through without adversely affecting normal operation or power efficiency.


NTC-based limiting circuit

Inrush current limiters are typically installed in either locations A&B or C&D and depending on applications sometimes only on location A or C.

Figure 1:

 NTC-based limiting circuit

To limit inrush current, an NTC thermistor is placed between the power supply and system (see Figure 1).  Upon power on, the NTC thermistor provides high resistance to limit inrush current. As the inrush current drops, the NTC thermistor self-heats and its resistance drops to a low enough value to pass current through.

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For example, consider a system with 10 A continuous current and an inrush current of 100 A. Upon power up, an NTC MS32 100 15 thermistor has an initial resistance of 10 ohms. Instead of passing 100 A, the NTC MS32 100 15 only allows 35 A to pass through. Then, as the NTC MS32 100 15 self-heats, its resistance drops and lowers the current until the inrush current is over. The NTC MS32 100 15 still continues to heat, dropping resistance to as low as 0.05 ohm where it reaches a steady state and passes current through minimum loss in efficiency.


NTC-based limiting has several advantages compared to a surge limiting circuit that uses a fixed resistor and bypass circuit. An NTC-based circuit typically occupies half the board space of a fixed resistor. It also has a very simple selection criteria to design in the circuit. Because resistance drops as it self-heats, no bypass circuit is needed to disable the limiting circuit. Finally, an NTC-based circuit has a lower total cost compared to limiting based on a fixed resistor.


PTC-based Limiting

NTC thermistors are the most commonly used limiter. They have a wide range of uses and applications. However, a few scenarios exist that require a positive temperature coefficient (PTC). If a system meets one of the exceptions listed below, a PTC thermistor is the best choice.


  • Ambient temperature is greater than room temperature: If the ambient temperature is already high, the resistance of the NTC thermistor will be lower when the system is powered on. This lower resistance will reduce the limiting capabilities of the NTC thermistor and could put the system at risk.
  • Ambient temperature is less than room temperature: If the ambient temperature is already low, the resistance of the NTC thermistor will be very high. The high temperature could limit all of the current and prevent the system from actually turning on, even after the initial inrush ends.
  • Reset time needs to be near-zero: Certain types of equipment, such as welding gear or a plasma cutter, switch on and off frequently as part of their normal operation. This creates multiple instances of inrush current. NTC-based limiting operates on the nature of the NTC thermistor to self-heat and lower its resistance. However, when a system is quickly turned off and then on again, the NTC thermistor may not have completely cooled.  It takes time for the NTC thermistor to release its heat and reset, dependent upon the size and mass of the NTC thermistor. If the NTC thermistor has not had sufficient time to cool, it will have a lower resistance when the system is turned on again, reducing its ability to handle the inrush current and protect the system.
  • Short circuit: A short circuit drops the internal resistance of a system to near zero, quickly raising the current the system draws from the power supply. As the NTC thermistor limits this current, it quickly increases in temperature, thus lowering its resistance. This allows more of the current to flow through until it can damage the system. High current from a short can also destroy the NTC thermistor.


PTC-based Limiting Analysis

When the previous scenarios occur, a positive temperature coefficient (PTC) thermistor can provide effective inrush current protection. A PTC thermistor functions opposite to an NTC thermistor: as temperature rises, its resistance increases. Resistance begins to increase rapidly at Curie temperature (Tc). For example, Figure 2 shows the behavior of a PTC MCL20 500 100 thermistor compared to an NTC thermistor. At Tc resistance increases rapidly. At low temperatures resistance stays constant.

Resistance to temperature curve graph

Figure 2:

Resistance for an NTCthermistor drops as it self-heats while resistance increase for a PTC MCL20 500 100 thermistor.  At a specific threshold, 120° C for the PTC MCL20 500 100, resistance increases sharply, enabling the PTC MCL20 500 100 to respond quickly to inrush current.  Also note how the PTC MCL20 500 100 has a flat response at low temperatures, making it effective across the entire temperature spectrum.


PTC Thermistor Tradeoffs 

There are a few tradeoffs when designing in a PTC-based limiting circuit. A PTC thermistor costs about 1.5 times more than an NTC thermistor. Additionally, PTC-based limiting requires an active circuit to bypass the PTC thermistor to prevent shutting the entire system down. As resistance increases, it limits the incoming current. This occurs even after the inrush current has dropped to normal levels.


A bypass circuit is active during power on for a set interval, typically 3 or 4 times the amount it takes for the inrush current to settle (see Figure 3). Then, the bypass circuit shuts itself off and sends current back through the PTC thermistor to protect the system against shorts. If the bypass circuit were always triggered by a high current, the limiting circuit would not provide protection during a short. Overall, the increased responsiveness and advanced protection outweigh the added complexity and cost of a bypass circuit.


PTC-based limiting circuit

Figure 3:

Complete PTC-based limiting circuit, with bypass circuit 

A PTC-based limiting circuit requires a bypass circuit to send current back through the PTC thermistor to protect the system against shorts. By setting the bypass to 3 or 4 times the amount it takes for the inrush current to settle, response time for the PTC-based limiter is extremely fast.

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NTC thermistors limit inrush current by providing low resistance in high temperatures. They are also the most commonly used thermistor because they fit a wide range of equipment. Certain scenarios, however, may require PTC thermistors. These thermistors stop inrush current by providing high resistance in high temperatures. Examples include industrial equipment, power tools, and other fast switching systems (see Table 1). For these cases, PTC thermistors provide cost-effective protection and superior responsiveness. Other benefits include: near-zero reset time, ability to operate in extreme temperature conditions, and effectiveness when limiting high current from shorts.


NTC vs. PTC Comparison Table

PTC vs NTC table

Table 1:

PTC-based inrush current limiting provides many advantages over fixed- or NTC-based limiting for applications such as fast switching and high current industrial equipment and power tools.


Inrush PTC Thermistor

MCL20 500100-A Thermistor 

The News: Ametherm introduces a new PTC circuit protection thermistor with an industry-high 680 V voltage rating.

Mouser Electronics Available at Mouser Electronic!

Key Benefits:

PTC Thermistor for Inrush Current

 High 680 V voltage rating

 Maximum inrush current of 20 A at maximum peak voltage

 Resistance at 25 °C of 50 Ω, with a tolerance of 20 %

 Dissipation constant of 55.0 mW/°C

 Heat capacity of 5.45 J/°C

 Thermal time constant of 62 s

 Radial-leaded for easy PCB mounting

 19 mm diameter, 9 mm maximum profile, and 7.8 mm lead spacing


 Available with straight leads and outside or inside kinked leads


Target Applications:

·         Inrush current limiting and overcurrent protection in applications such as welding equipment and plasma cutters with extremely high voltages from 480 V to 930 V


Key Specifications:

Max. Voltage Rating

680 V

Max. Inrush Current

20 A

Thermal Constant

62 s

Dissipation Constant

55 mW/°C

Heat Capacity

5.45 J/°C

Resistance at 25°C

50 Ω


20 %


19 mm

Operating Temperature Range

-50 °C to +150 °C


The Context: With its industry-high voltage rating, Ametherm’s MCL20 500100-A can withstand hundreds of hits of maximum inrush current without degrading. The device offers a short reset time, and as a PTC thermistor a quick reset will not result in a large inrush current, as its resistance is already at a high state. The result is extremely high reliability and stability in high-voltage applications. With a 19 mm diameter, 9 mm maximum profile, and 7.8 mm lead spacing, the MCL20 500100-A provides designers with a more compact and cost-effective alternative to combining a power resistor, relay, and timer on one circuit to achieve the same functionality.


Availability: Offered direct or through Ametherm's network of distributors (, samples and production quantities of the MCL20 500100-A are available now, with factory lead times of seven weeks.


More info:


Download this New Product Announcement as a PDF:


Request a Sample: Call 800-808-2434 (toll free in the United States) or 775-884-2434 from outside the U.S. and Canada.


AS Series Inrush Current Limiters

Key Benefits:

  • Recognized By Underwriters Laboratories For Ensured Safety
  • Designed To Withstand High Steady Sate Current
  • Absorbs And Minimizes High Input Energy
  • Cost Effective One Component Solution To Inrush Current
  • Wide Temperature Range Of Operation

Available from Ametherm Stocking Distributors

Digi Key


AS32 Inrush Current Limiter


Electrical Specifications

D Max
T Nom
AS32 0R536 Y 0.5 30 300 29 8 570-1103-ND 995-AS32-0R536-100
AS32 0R530 Y 0.5 30 300 29 5 570-1118-ND 995-AS32-0R530-100
AS32 1R030 Y 1.0 30 300 30 8 570-1104-ND 995-AS32-1R030
AS32 1R036 Y 1.0 36 300 30 8 570-1119-ND 995-AS32-1R036-100
AS32 2R025 Y 2.0 25 300 30 8 570-1105-ND 995-AS32-2R025
AS32 5R020 Y 5.0 20 300 32 8 570-1106-ND 995-AS32-5R020
AS32 10015 Y 10.0 15 250 30 9 570-1107-ND 995-AS32-10015
AS32 20010   20.0 10 250 29 9 570-1108-ND  
AS32 50006   50.0 6 250 30 9 570-1109-ND  

Mechanical Specifications

AS32 0R536 29.00 7.80   17.50 22.00 4.80 0.90 AS Inrush Current Limter Drawing
AS32 0R530 29.00 5.30   17.10 22.00 2.60 0.90
AS32 1R030 30.00 8.00   17.10 22.00 2.40 0.80
AS32 1R036 30.00 8.00   17.10 22.00 2.40 0.80
AS32 2R025 30.00 7.80   17.10 21.00 4.80 0.80
AS32 5R020 32.00 8.40   17.10 21.00 5.40 0.80
AS32 10015 30.00 8.50   17.10 21.00 3.70 0.80
AS32 20010 29.00 9.00   17.10 22.00 6.40 0.90
AS32 50006 30.00 9.00   17.10 21.00 4.80 0.80

Inverter Inrush Current Protection

Protect An Inverter From Inrush Current

By: Mehdi Samii
Kaushik Das


Inverters are electrical systems that provide variable voltage (AC output) when connected to a DC input source. Inverters are available in two varieties: three phase and single phase. These inverters are also known as static frequency battery chargers or variable frequency drives.

Inrush Current in Inverters

A common failure of inverters is overloading the inverter due to inrush current . This is due to the fact that most inverters are designed with a minimum amount of resistance to increase their efficiency and minimize losses due to heat.

Inverter Component

Cause of Failure

Failure of electrolytic capacitors

Current and voltage stress

Welding of contactors

Inrush current

Failure of Bridge rectifiers

Inrush currents greater than the rating specified


For example:
An overload condition will occur even if you switch on three appliances–one by one–connected to an inverter.

Consider the following:

  • A 1000W inverter (more specifically, a 1500W inverter with 50% total overload capacity)
  • Three standard appliances, such as a refrigerator of 300W, an LCD Television of 300W, and a computer of 300W. Total load for these appliances: 900W.
  • A 1000W inverter is fully capable of running the above three appliances

The overload condition happens because of energy required for start-up. But, the start-up or inrush current for each appliance could be as high as 900W or 3 times the rated power.


The inverter overloads in the following scenario:

  • Step 1: If we switch on the first appliance, the load is 900W which is less than the rated capacity of the inverter. Thus, no overload situation is encountered
  • Step 2: If you switch the second appliance, the total wattage needed is as follows: First appliance 300W + second appliance 900W = 1200W. No overload situation is encountered.
  • Step 3: If you switch the third appliance, the total wattage needed as follows:
    1st appliance 300W + 2nd appliance 300W + 3rd appliance 900W = 1500W

Notice that an overload condition is encountered as soon as the third appliance is switched on to the inverter. See (a) of Figure 1 below.

Inverter Inrush Overload Diagram
Figure 1


Use a thermistor (See Figure 1(b).) to address the overload scenario of the sample problem:

  • As per Step 3 above, the inverter wattage needed including the overload condition > 1500W
  • Since max output power allowed is 1000W
  • Allowed current: 8.0A, 50 # 2 at 120V
  • Normal continuous current per appliance = 300W/120V = 2.50A
  • Due to inrush current = 2.50A x 3 = 7.50A
  • Duration of inrush = one cycle = 1 x 1/50 sec = 0.02 sec
  • Energy of the thermistor = 120V x 7.50A 0.02 sec = 18.0 J
  • Note: The energy requirement, mentioned above, is needed to handle without self destruction.
  • So, for three appliances that start up at the same time, we need 3.0 x 18.0 J = 54 J
  • Minimum resistance: 120 V x 1.414/8.0A = 21.21Ω
    (This ensures that the current does not exceed 8.0 A.)
  • So if we assume Ambient of 50°C, Min Resistance = 40Ω, so we can reconnect.

Ametherm suggests two methods for solving this situation:

Method (a)


Inrush Current Inverter Schematic
Figure 2

In the above circuit (Figure 2),

  • NTC = SL 22S0004 (50 Ω, 4.0 Amp, 75 Joules), UL (E204153), CSA (CA40663) is used to bypass the surge after one second.
  • Note that the NTC does not interfere with the efficiency of inverter since the relay is also protected from the inrush current by the thermistor. The thermistor will conduct through the relay with 99.2% efficiency loss of current.

Method (b)

As shown in Figure 3, choose Ametherm P/N: MS3220008 x 2 to provide 40Ω, 10 AMP, 500 Joules.

Inrush Current Inverter Schematic
Figure 3
  • Efficiency C 8.0 A = I2R = 14.1 W losses due to thermistor
  • RC 8.0 Amp = 0.22
  • Efficiency = 985.90/1000W = 98.6%

Conclusion: method (b) is more cost effective.

Inverter Circuits with Thermistors

Simple NTC thermistors are shown below in the following three circuits: Figure 4, 5 and Figure 6.
These thermistors minimize the effect of inrush current on components, such as bridge or link capacitors.

Classic Inverter Circuits

Inrush Current Inverter Circuit
Figure 4

Frequency Charger

Inrush Current Frequency Changer Schamatic
Figure 5

Variable Frequency Drive

Inrush Current Variable Frequency Drive Schematic

Figure 6


  • Elliot Sound Products
  • Sinetech Advanced Power Products
  • Rockwell Automation Publication PFLEX_A700lk_EN_P Sept 2011
  • US Patent 2003/0150369A1

AS Inrush Current Limiters

Introducing the AS Series of Inrush Current LimiterAS32 Insrush Current Limiters

The new AS Series inrush current limiters are now available from Ametherm. As our most advanced inrush current limiting power thermistors, the AS series is ideal for applications in MRI equipment and x-ray machines, for both healthcare and security. Key benefits include:

  • Lower current density (as compared to traditional types of inrush current limiters)
  • Faster reset time
  • No hot spots from fatigue, because of lower current density and uniform temperature gradient throughout the disc
  • Wider temperature range of operation with out de-rating