Inrush Current

Inrush current is the instantaneous peak current that floods into a device the moment it is energized. In a switch-mode AC-DC power supply — the type used by every ASIC miner — the large input smoothing capacitors are fully discharged at switch-on and momentarily look like a short circuit to the low-impedance mains. The result is a surge that can be several times the steady-state running current, lasting only milliseconds until the capacitors charge. A miner that draws 15 A running can demand a far larger instantaneous gulp at plug-in, and your breakers, relays, and wiring all feel it.

Why inrush matters when racking miners

A single miner's surge is brief, but energizing many power supplies at once multiplies the combined inrush. That stacked surge can trip a breaker that the running load would never bother, weld PDU relay contacts closed, or sag the supply voltage enough to reboot neighboring equipment mid-hash. This is why breaker selection for high-inrush loads is more involved than simply matching the steady current, and why staggering power-on across a fleet — rather than flipping one master switch after an outage — protects your panel. Home miners meet the same physics at smaller scale: an ASIC on a smart plug or relay-switched circuit can pit or weld the relay contacts over repeated cycles, because most consumer relays are rated for resistive loads, not capacitive inrush.

Taming the surge

Designers limit inrush with a soft-start circuit or an NTC (negative-temperature-coefficient) thermistor in series with the input. The cold thermistor presents tens of ohms at switch-on, capping the initial current; as it self-heats, its resistance collapses to a fraction of an ohm and steady-state losses become negligible. The catch is thermal memory: a hot thermistor that is power-cycled quickly has not recovered its resistance, so rapid off-on cycling defeats the protection — one reason to wait a few seconds before re-energizing a PSU. Larger supplies use active circuits (a resistor bypassed by a relay or triac once the bus charges) for the same job with lower running losses. On the distribution side, time-delay (Type D or motor-rated) breakers ride through legitimate surges without nuisance tripping, and sequenced PDUs bring outlets up in staggered banks.

On the repair bench

Inrush is also diagnostic. A supply such as the APW12 that pops breakers instantly at plug-in — before the fans even twitch — often has a shorted bridge rectifier or failed inrush limiter rather than a downstream fault, while one that starts and then trips under hashing load points elsewhere. Bench technicians use a current-limited AC source or an incandescent-bulb limiter to power suspect supplies safely, precisely because an uncontrolled inrush into a faulted unit turns diagnosis into fireworks.

Sizing upstream equipment

Inrush also constrains anything upstream that is smaller than the utility grid. A generator or UPS sized comfortably for a miner's running watts can stall, fault, or drop its output the instant the miner's capacitors demand their surge, because small sources lack the grid's near-infinite stiffness. Off-grid and backup-power miners size sources with generous surge headroom or bring loads online one at a time, letting each PSU finish charging before the next connects. The same logic applies to extension cords and undersized branch wiring: a marginal conductor that tolerates the running current may see enough voltage sag during inrush to make the PSU brown-out and retry in a loop — a symptom that looks like a bad power supply but is really a bad power path.

Account for inrush whenever you size breakers, PDUs, or a UPS. Read it together with soft start and total harmonic distortion for the full power-quality picture.