Soft Start

A soft start is a circuit that deliberately slows the rate at which a power supply energizes, holding input current to a safe level through the startup ramp instead of letting it spike. At the instant of plug-in, a PSU's large input capacitors look almost like a short circuit; by controlling how fast they charge, the soft-start stage tames the inrush current surge that would otherwise hammer rectifier diodes, switching devices, fuses, relay contacts, and the upstream breaker every single power-on. It is the difference between a supply that eases onto the line and one that arrives like a hammer blow.

How soft start is implemented

The simplest and most common method, found in computer and ASIC power supplies alike, is an NTC (negative-temperature-coefficient) thermistor in series with the input. Cold, it presents enough resistance to throttle the initial charging current; as it self-heats in operation, its resistance falls to a small fraction of an ohm, so it barely costs efficiency once the supply is running. The design has one well-known weakness: a brief power interruption leaves the thermistor hot — and therefore low-resistance — so an immediate re-plug can pass nearly full inrush — the classic reason a supply survives years of service and then dies during a night of grid flickers. Waiting a minute before restoring power is a genuinely useful habit, not superstition. Better supplies add active soft-start: a controller ramps the converter's duty cycle or a switch's conduction gradually, and some designs bypass the limiting element with a relay once the capacitors are charged, eliminating even its small running loss. Downstream, the same principle repeats at board level — DC-DC converters ramp their outputs so the hash domains on a board come up in a controlled sequence rather than all at once.

Relevance to mining hardware

An ASIC PSU such as the APW12 switches thousands of watts, with input capacitance to match, so an unmanaged surge would be enormous. A working soft-start stage is a large part of why a healthy miner can share a properly sized circuit without instantly tripping the breaker at power-on — and why several miners on one panel should still be staggered rather than energized simultaneously, since even limited inrush stacks across units. When a supply reaches the repair bench with a blown input fuse or a breaker that trips the instant it is plugged in, the soft-start path is a prime suspect: a cracked or open thermistor, a failed bypass relay, or a shorted rectifier downstream letting the full unrestrained surge through. Diagnosis is straightforward — visual inspection of the thermistor (they fail dramatically), a resistance check against its cold rating, and verification that the bypass engages.

On the bench

Soft start also matters to how you test. Powering a suspect supply through a current-limited bench power supply or an isolation setup lets the technician observe startup behavior without risking a second failure, and watching how the rails ramp reveals whether the soft-start control is doing its job. Soft start and inrush limiting are two views of one problem — see inrush current for the surge itself. A PSU that trips breakers at power-on is a repairable fault, not a write-off; it is among the more common findings when a "dead" miner arrives at D-Central's repair bench. And one safety note that bears repeating: the same input capacitors that cause inrush hold a lethal charge after unplugging, so a supply opened for soft-start diagnosis must be verified discharged first — the circuit that protects the breaker does nothing to protect the technician. Bleed resistors are supposed to drain the bank within minutes, but they fail too — measure before you touch, every time, on every supply.