Inrush Current Calculator

Capacitor inrush became a serious engineering problem in the 1970s as switched-mode power supplies replaced linear transformers. A linear supply draws a moderate magnetizing inrush spread over one line cycle; an SMPS's rectifier-capacitor front-end slams charge into 100+ µF of bulk capacitance through only line impedance and cable resistance, producing peak currents 30-60× steady-state within microseconds. Early PC power supplies popped AC line fuses on cold starts and welded contact sets shut.

Negative-temperature-coefficient (NTC) thermistors — invented in the 1930s as surge-limit devices for lamps — became the dominant inrush solution in the 1980s. A cold 5-10 Ω NTC limits peak current during bulk-cap charging, then self-heats to a few hundred milliohms once steady-state current flows through it. Downside: the NTC dissipates 1-3 W continuously and doesn't re-cool fast enough for hot-restart scenarios (a brief brownout can allow full inrush on recovery).

Modern high-efficiency designs use NTC + relay bypass: the NTC protects during the 100 ms charging window, then a mechanical or solid-state relay shorts across the NTC once the bulk cap reaches steady-state voltage. This removes the continuous I²R loss and prepares the NTC to cool for a potential restart. Active inrush controllers (LT8331, LTC4313) use GaN/SiC switches to replace the mechanical relay entirely.

Enter peak input voltage (325 V for 230 V AC rectified, ~170 V for 120 V), bulk capacitance, and total series resistance (ESR + NTC + wiring + rectifier bridge dynamic resistance). The tool computes instantaneous peak current Ipk = V/R at t = 0, RC time constant τ, stored energy ½CV², and time to 99% charge (5τ).

Real inrush scenarios add complexity: line impedance, residual voltage on the bulk cap if hot-restarting, and non-ideal NTC dynamics. Use this as a starting point — the peak current value governs fuse selection (fuse I²t must exceed the charge pulse energy) and NTC sizing (NTC steady-state current must not overheat the device). Everything runs client-side; no values leave your browser.