Choose limiter values for capacitive loads, transformer energization, and supply stress with fast engineering estimates. Protect components, reduce nuisance trips, and improve startup reliability.
| Scenario | Voltage (V) | Steady Current (A) | Capacitance (µF) | Max Inrush (A) | Suggested Cold R (Ω) |
|---|---|---|---|---|---|
| Industrial PSU | 230 | 4.0 | 470 | 18 | 15.0 |
| Servo Drive Front End | 400 | 6.5 | 820 | 25 | 18.8 |
| Lab Rectifier Bank | 120 | 2.2 | 330 | 12 | 11.7 |
| Soft Start Module | 48 | 8.0 | 2200 | 20 | 2.5 |
Required total resistance: R_total = V / I_max
Recommended cold resistance: R_cold = (R_total - R_source) × safety factor
Peak inrush estimate: I_peak = V / (R_cold + R_source)
Warm resistance: R_warm = R_cold × warm resistance ratio
Steady voltage drop: V_drop = I_steady × R_warm
Steady power: P = I_steady² × R_warm
Capacitor stored energy: E_cap = 0.5 × C × V²
Limiter pulse energy approximation: E_limiter = (I_peak² × R_cold × t_pulse) / 2
These equations give a practical first-pass sizing method for NTC limiters, resistor soft-start stages, and bypass relay timing checks.
Inrush current appears when a supply energizes a capacitive input, transformer, or mixed front end. A 230 V source feeding 470 µF can draw current far above normal load current during the first milliseconds. This calculator compares steady current, source impedance, pulse duration, and allowable peak current so designers can choose a limiter that reduces fuse operation and relay wear.
The main sizing step uses required total resistance, calculated from supply voltage divided by maximum allowable inrush current. If 230 V must be limited to 18 A, the circuit needs 12.78 Ω total. After subtracting 0.35 Ω of source impedance, the remaining resistance is assigned to the limiter. A safety factor then raises cold resistance to provide margin for tolerance and variation.
Current limiting alone is not enough. The limiter must also absorb startup energy without cracking or aging too quickly. Capacitor energy equals one half of capacitance times voltage squared. At 470 µF and 230 V, stored energy is 12.43 J. The calculator also estimates limiter pulse energy from peak current, resistance, and pulse width. This helps compare surge ratings and pulse limits.
Every limiter introduces warm resistance after startup. That resistance creates voltage drop and continuous power loss. For a 4 A load and 3.33 Ω warm resistance, the drop becomes 13.32 V and the dissipation reaches 53.28 W. These values show when an NTC alone may be inefficient.
The RC surge time constant gives a fast way to judge how quickly current decays after energization. Higher capacitance or higher total resistance lengthens startup. Engineers can compare the calculated time constant with line period, relay pickup delay, and fuse I²t behavior. If surge current remains elevated for several cycles, stress rises and a longer bypass delay may be necessary.
Use calculated values as a first-pass screen, then verify them against manufacturer curves, thermal limits, enclosure temperature, repetition rate, and abnormal conditions. Check cold tolerance, warm resistance spread, ambient rise, and restart behavior after short outages. For production designs, confirm performance with measurements and worst-case mains testing so limiter sizing remains reliable across voltage bands, aging, and wiring variation.
It first estimates the total resistance required to hold startup current below your allowable inrush limit at the selected supply voltage.
No. It also helps with fixed resistor pre-charge paths, relay bypass schemes, and mixed soft-start stages needing a quick first-pass value set.
Existing wiring, transformer resistance, and upstream impedance already limit surge current, so the limiter should only add the remaining needed resistance.
Warm limiter resistance causes voltage drop and I²R heating during normal operation. High current equipment often benefits from a bypass relay after startup.
No. The graph is an engineering estimate. Final selection should still be checked against manufacturer curves and measured startup waveforms.
Verify ambient temperature range, restart behavior, fuse coordination, repetitive surge duty, enclosure heating, and worst-case mains voltage tolerance.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.