Understanding Capacitor Inrush Current
Capacitor inrush current appears when an uncharged capacitor is connected to a supply. At the first instant, the capacitor voltage may be far below the source voltage. The capacitor then behaves almost like a short circuit. Current is limited mainly by equivalent series resistance, wiring resistance, switch resistance, and any added soft start part.
Why It Matters
High inrush current can stress rectifiers, relays, switches, fuses, connectors, traces, and power supplies. It can also cause voltage dips that reset nearby circuits. Large input capacitors in amplifiers, motor drives, LED drivers, and DC bus systems need careful checks before hardware is built.
Calculation Method
This calculator models the charging event with a simple RC network. It combines all resistance terms into one effective resistance. It also converts capacitor units, handles capacitor banks, and considers initial capacitor voltage. The peak current is found from the voltage difference divided by total resistance. The current after a chosen time is estimated with the exponential decay equation. The time constant shows how quickly current falls. After about five time constants, charging is usually close to complete.
Advanced Use
Use measured ESR when possible. Add fuse resistance, cable resistance, MOSFET on resistance, contact resistance, and source resistance. For a bank, select parallel or series operation. Parallel banks increase capacitance and reduce total ESR. Series banks reduce capacitance and increase total ESR. Real circuits may include NTC thermistors, active precharge, current limited converters, or foldback protection. Those effects can lower the actual peak current.
Design Notes
Compare peak current with the surge rating of every device in the path. Check energy because contacts and resistors may heat during charging. Repeat calculations at low temperature if ESR changes. Also test with maximum supply voltage and minimum resistance. The result is an engineering estimate, not a replacement for oscilloscope measurement. Use it to choose precharge resistance, verify margins, and decide whether a soft start circuit is required.
For AC rectified supplies, remember that line phase, rectifier drop, transformer impedance, and capacitor tolerance change the first pulse. Simulate or measure critical equipment. Keep safety spacing and discharge paths. Never assume a discharged capacitor is safe after power is removed. Record assumptions for later troubleshooting and review.