inrush current

Inrush current is the brief, large gulp of current a supply pulls in the first instant you switch it on, before everything settles to its normal running level. The big culprit is the reservoir capacitor: at switch-on it is empty, and an empty capacitor looks like a momentary short circuit. Picture turning a tap onto a bone-dry bucket — there is a sudden rush as it fills, then the flow drops to a trickle once it is full. The supply behaves the same way for a few milliseconds.

How big is it? The peak is roughly the available voltage divided by the tiny total series resistance in the charging path — transformer winding, wiring, capacitor ESR. Example: charging a 4700 uF cap through just 0.5 ohm of resistance from a 17 V peak gives an initial surge near 17 / 0.5 = 34 A, even though the steady load is only 1 A. It lasts only while the capacitor charges (a few time constants of R times C), but in that flash it can blow a fuse, weld relay contacts, pit a switch, or trip a breaker.

Why it matters: inrush is a classic field failure that bench testing at steady state never reveals — the supply runs fine, then nuisance-trips a breaker every few power-ups or slowly destroys a switch. The cures limit that first surge: an NTC thermistor in series (high resistance when cold, falling as it warms), an active soft-start that ramps the input up gradually, or a series resistor briefly bypassed by a relay once the caps are charged. Fuses and switches must also be rated for the inrush, not just the running current.