Skip to content
RFrftools.io
Power

Inrush Current Limiter (NTC) Calculator

Calculate NTC thermistor requirements for inrush current limiting, including cold resistance, peak inrush current, time constant, and energy absorbed

Loading calculator...

Formula

Rcold=Vs/Iinrush,τ=Rcold×Cf,E=0.5×Cf×Vs2R_cold = V_s / I_inrush, τ = R_cold × C_f, E = 0.5 × C_f × V_s²
V_s— Supply voltage (V)
I_inrush— Target inrush current (A)
C_f— Filter capacitance (F)
R_cold— NTC cold resistance (Ω)
τ— Time constant (s)

How It Works

The inrush current limiter calculator determines thermistor resistance, power rating, and thermal recovery time for soft-start protection — essential for AC-DC power supplies, motor starters, and capacitor bank charging circuits. Power systems engineers, industrial equipment designers, and UPS developers use this tool to prevent nuisance tripping and component stress during power-on. According to Ametherm application note AN-1005, inrush current in a capacitor-input rectifier reaches Ipeak = Vpeak/Rsource = 325 V/0.5 Ω = 650 A for a typical 230 VAC supply with 0.5 Ω mains impedance. Inrush current limits and immunity requirements are specified in IEC 61000-3-3 (Electromagnetic compatibility — Limitation of voltage changes) and IEC 62368-1 (Audio/video, information and communication technology equipment — Safety requirements), which mandate inrush current limiting for mains-connected equipment. This 1-5 ms surge stresses rectifier diodes (typically rated for 30-50 A surge), trips 10-20 A circuit breakers, and causes EMI conducted emissions. NTC thermistors reduce inrush by providing high cold resistance (5-50 Ω at 25°C) that decreases to 0.5-2 Ω when self-heated during steady-state operation. Per Vishay BC Components application note, thermistor steady-state resistance drops to 10-20% of cold resistance at rated current, dissipating 1-5 W continuously. Recovery time between power cycles depends on thermal mass — 30-60 seconds for standard disk thermistors, limiting applications with frequent on/off cycling.

Worked Example

Design inrush current limiting for a 500 W ATX power supply. Requirements: limit inrush to <30 A peak (within rectifier surge rating), steady-state dissipation <3 W, 120 VAC operation. Step 1: Calculate worst-case inrush without limiter — Ipeak = Vpeak/Rwiring = 170 V/0.3 Ω = 567 A (assuming 0.3 Ω mains impedance). Step 2: Calculate required cold resistance — For Ipeak < 30 A: R_cold > 170 V/30 A - 0.3 Ω = 5.4 Ω. Select 10 Ω NTC for margin. Step 3: Verify steady-state current — Iin = 500 W/(120 V × 0.65 PF × 0.85 η) = 7.5 A RMS. Step 4: Calculate hot resistance — At 7.5 A, 10 Ω NTC drops to ~1.5 Ω (per Ametherm characteristic curves). Pdiss = 7.5² × 1.5 = 84 W — unacceptable! Step 5: Redesign — Use bypass relay (contacts rated 10 A AC). NTC only active during 100 ms startup. Select 5 Ω NTC (Epcos B57364S509M): 5 Ω cold limits to 34 A peak, 3 W continuous rating handles worst-case if relay fails.

Practical Tips

  • ✓Per Epcos application note, use bypass relay (activates 100-500 ms after startup) for supplies >200 W — reduces steady-state loss from 2-5 W (thermistor) to <0.1 W (relay contact resistance)
  • ✓Select NTC thermistors with 2× the steady-state power rating for reliability margin — a thermistor dissipating 2 W should be rated for 4+ W to maintain <85°C surface temperature
  • ✓Implement active inrush control (TI TPS2490) for DC applications — MOSFET-based limiters achieve 10× faster recovery time and programmable current limit versus passive thermistors

Common Mistakes

  • ✗Undersizing thermistor energy rating — inrush energy E = ½×C×Vpeak² must be absorbed without exceeding maximum allowable joules; a 1000 µF capacitor at 400 VDC stores 80 J, requiring thermistor rated for >100 J single pulse
  • ✗Ignoring thermal recovery time — NTC thermistors require 30-60 seconds to cool after power-off; rapid cycling causes cumulative heating and permanent resistance shift
  • ✗Using thermistors without bypass relay in high-power applications — continuous power dissipation at rated current can exceed 10-20 W, reducing efficiency and requiring heatsinking

Frequently Asked Questions

Per IEEE Std 1100, inrush limiters protect against: (1) Circuit breaker nuisance tripping — standard 15 A breakers trip at 5-10× rated current for >10 ms, (2) Rectifier diode stress — typical 1N5408 rated for 200 A surge maximum, (3) Contact welding in switches/relays — arcing current >100 A can permanently fuse contacts, (4) EMI conducted emissions — fast di/dt (10-100 A/µs) generates broadband noise violating CISPR limits.
Per Ametherm design guide: NTC thermistors exploit temperature-dependent resistance — cold resistance R(25°C) provides current limiting, self-heating during operation reduces resistance to R_hot ≈ 0.1-0.2 × R(25°C). Time constant τ = thermal mass/cooling rate determines warmup (10-50 ms) and cooldown (30-60 s). For example: Ametherm SL32 10015 provides 10 Ω at 25°C, dropping to 0.6 Ω at 15 A steady-state, with 80 J single-pulse energy rating.
Required applications per UL 60950 and IEC 62368: (1) Capacitor-input rectifiers >50 W — filter capacitors present near-short-circuit to AC mains, (2) Transformer-input supplies — magnetizing inrush reaches 5-15× steady-state current, (3) Motor starters — locked-rotor current is 6-8× running current, (4) Hot-swap modules — charging output capacitance without limiting exceeds connector current rating. Exception: resonant converters with inherent soft-start may not require external limiting.
Per Vishay application note, energy rating scales with thermal mass: E = m × Cp × ΔT, where m = mass (larger disk = more mass), Cp = specific heat (~0.9 J/g·°C for NTC material), ΔT = temperature rise limit (typically 200-300°C). Example: 15 mm disk (5 g) with 250°C rise: E = 5 × 0.9 × 250 = 1125 J. Standard 10 mm disk: ~400 J. For 80 J capacitor bank, minimum 10 mm disk required.
Per Epcos selection guide: (1) Calculate peak inrush without limiter: Ipeak = Vpeak/Zmains (typically 0.2-0.5 Ω), (2) Determine maximum allowable inrush from rectifier/breaker specs, (3) Calculate minimum cold resistance: R_cold > Vpeak/Imax, (4) Verify energy rating exceeds ½×C×V², (5) Verify steady-state dissipation acceptable. Example: 230 VAC, 470 µF capacitor, 20 A max inrush: R_cold > 325/20 = 16 Ω. Energy = 0.5×470µ×325² = 25 J. Select 22 Ω NTC rated for 50+ J.

Shop Components

As an Amazon Associate we earn from qualifying purchases.

DC-DC Buck Converter Modules

Adjustable step-down converter modules for bench and prototype use

Amazon →

LDO Voltage Regulator Kit

Assorted low-dropout linear regulators for prototyping

Amazon →

Electrolytic Capacitor Kit

Aluminum electrolytic capacitor kit for power supply filtering

Amazon →

Power Inductor Kit

Assorted shielded power inductors for switching supply designs

Amazon →

Related Calculators

Power

MOSFET Power Loss

Calculate MOSFET conduction loss, switching loss, total power dissipation, junction temperature, and efficiency for power electronics design

Power

LDO Dropout

Calculate LDO regulator power dissipation, junction temperature rise, minimum input voltage, efficiency, and headroom for linear regulator design

Power

PWM Duty Cycle

Calculate PWM duty cycle, frequency, average voltage, off-time, and RMS voltage from on-time and period parameters

Power

Voltage Divider

Calculate voltage divider output voltage, current, Thévenin impedance, and power dissipation from Vin, R1, and R2. Ideal for bias networks and level shifting.