How to Protect Industrial Devices from Power-on Current Surges

When an industrial device is powered on, it often draws a surge of current (known as inrush current) that is way higher than its normal operating level. Depending on the device type, this power-on surge can be on the order of 10 to 30 times the steady-state current. These extreme inrush currents are momentary but can cause significant electrical and mechanical stress.

Without proper control, inrush currents can trip circuit breakers, blow fuses, damage sensitive components, and even degrade power connectors and power supplies. Therefore, an effective inrush current management strategy is crucial for the reliable and safe operation of industrial systems.

One way to manage power-on surges is to add inrush current limiters (ICLs) in series with the device’s power input. Among the various types of ICLs, negative temperature coefficient (NTC) thermistors are widely used for their simple design and ease of integration. An NTC thermistor is a temperature-sensitive resistor whose resistance decreases with increasing temperature.

Figure 1: Panasonic Electronic Components’ ERT-J0EG103FA NTC thermistor with 10 kΩ nominal resistance at 25°C and ±1% resistance tolerance. (Image source: Panasonic Electronic Components)

When the industrial electrical device is off, the NTC element has relatively high resistance. They are placed in series with the load. The high cold resistance slows the initial surge of current at power-on, acting like a shock absorber.

The limited inrush current flowing through the thermistor causes it to self-heat through resistive power dissipation. As the thermistor heats up, its resistance drops dramatically to a small fraction of its cold value. Within moments, the thermistor transitions to a low-resistance state. By this point, the input capacitors have charged, and normal operating current can flow.

Once the inrush event subsides, the NTC effectively disengages, behaving almost like a short circuit during normal operation. For example, an NTC with 10 Ω cold resistance may drop to less than 0.5 Ω when fully heated. This ensures that the industrial machine operates at nearly full voltage in a steady state while minimizing energy loss across the thermistor.

Design considerations while implementing NTC limiters

To ensure reliable and efficient operation, several design parameters must be considered when implementing NTC-based inrush current limiters.

1. Cold resistance

Cold resistance (R₂₅) is the rated resistance at 25°C and determines the initial impedance that limits the inrush current. The minimum required resistance can be estimated from the desired maximum inrush current and the supply voltage. Engineers calculate this resistance using Ohm’s law: R = V_peak/I_{max (inrush)}. For example, in a single-phase 230 VAC system (approx. 325 V_peak), if the inrush current is to be limited to 20 A_peak, a cold resistance on the order of 325/20 ≈ 16 Ω is required.

Manufacturers like TDK Electronics, Vishay Ametherm, and Amphenol Advanced Sensors offer standard NTC values such as 2 Ω, 5 Ω, 10 Ω, 22 Ω, 47 Ω, etc, at 25°C. Selecting the right cold resistance is crucial, as a higher R₂₅ value yields better surge suppression. However, a value that is too high may overly restrict charging currents, increase startup time, and cause excessive initial voltage drop.

Figure 2: EPCOS – TDK Electronics’ B57164K0220K000 leaded NTC thermistor with 22 Ω resistance at 25°C and ±10% resistance tolerance. (Image source: EPCOS – TDK Electronics)

2. Operating resistance

Operating (hot) resistance represents the residual series impedance and continuous dissipation. In practice, the hot resistance will be a small fraction of R₂₅, typically 2 to 5 percent of the cold resistance at nominal current. For instance, an NTC with a 10 Ω cold resistance might drop to approximately 0.3 Ω at its rated current.

Lower hot resistance is desirable for efficiency but achieving it means a larger thermistor. Designers must ensure that, at the application’s steady-state current, the NTC heats sufficiently to reduce its resistance to an acceptably low level. If the device is oversized, it may not self-heat adequately, resulting in higher-than-expected resistance.

For high performance, the normal operating current should be at least 30 percent of the NTC’s maximum rating so that it runs hot enough to reach the flat part of its R-I curve. If the load current is very small relative to the NTC’s capability, the engineer should consider a lower-current thermistor, so it drops to a lower resistance when heated by that current.

3. Maximum continuous current

The NTC must be capable of carrying its rated RMS or DC current continuously in a steady state without overheating. The NTC must be chosen so that the Imax is equal to or greater than the system's normal operating current. If the steady-state current exceeds the NTC’s permissible continuous current rating, the thermistor will overheat beyond its design limits, with the risk of thermal runaway or device damage.

It is important to review the device's derating curve to determine whether the application will run hot in an enclosure or near heat sources. If the design current is close to I_max, it is crucial to use some safety margin or cooling mechanism around the thermistor.

4. Energy surge capability

The thermistor's energy rating is a critical parameter. It must withstand the joule (J) energy of the inrush without damage. For a capacitive input, a first-order estimate of the surge energy is the energy needed to charge the capacitor. For instance, charging a 100 μF capacitor to 325 V requires about 5.3 J. The selected thermistor should have an inrush energy rating above this level, accounting for the worst-case scenario.

Similarly, for motor or transformer loads, the designer can measure the surge current waveform and calculate the integration (∫I²R dt) to ensure the I²t through the thermistor stays within its specification. Manufacturers do provide an I²t or joule rating for one-time surges and sometimes a repeating surge rating if the device sees frequent on/off cycling.

Figure 3: Amphenol Advanced Sensors’ AL03006-535K-145-G1 NTC thermistor with 1 MΩ resistance and operating at up to 250°C with stability. (Image source: Amphenol Advanced Sensors)

When properly chosen and implemented, an NTC thermistor-based inrush current limiter offers a reliable safeguard against power-on surges. The device provides a transient series resistance that auto-regulates itself out of the circuit once its operation is complete.

Conclusion

As industrial systems grow and incorporate power-hungry devices, controlling energy surges at start-ups becomes critical. NTC-based inrush current limiters offer a proven balance between design, cost, and reliability. They aim to enable designers to achieve soft-start protection without adding control complexity, ensuring safe and efficient industrial operation.