Definition
A soft start is a circuit that deliberately slows the rate at which a power supply energizes, holding the input current to a safe level throughout the startup ramp instead of letting it spike. By controlling how quickly the input capacitors charge, a soft start tames the inrush current surge that would otherwise stress diodes, switches, fuses, and upstream breakers every time the unit is powered on.
How soft start is implemented
The simplest and most common method, found in virtually every computer and ASIC PSU, is an NTC (negative-temperature-coefficient) thermistor in series with the AC input. Cold, it offers enough resistance to throttle the initial current; as it self-heats during operation its resistance drops to a fraction of an ohm, so it barely affects efficiency once running. More sophisticated supplies add active soft-start control that ramps the internal voltage rails up smoothly, sometimes bypassing the limiting resistor with a relay once the capacitors are charged.
Relevance to mining hardware
ASIC power supplies switch thousands of watts, so an unmanaged surge could be enormous. A working soft-start stage is part of why a healthy miner can be plugged into a properly sized circuit without instantly tripping the breaker. When repairing a PSU, a failed soft-start thermistor is a common culprit behind blown input fuses or breakers that trip at power-on the resistor cracks or goes open and the full inrush hits unrestrained.
Soft start works hand in hand with inrush limiting; see inrush current for the surge it is designed to suppress.
In Simple Terms
A soft start is a circuit that deliberately slows the rate at which a power supply energizes, holding the input current to a safe level…