SCK-2R55A 2.5Ohm 5A NTC Power Thermistor for PSU Repair

SCK-2R55A 2.5Ω 5A NTC Power Thermistor – ASIC Miner PSU Inrush Limiter

The SCK-2R55A is a 2.5Ω, 5A NTC (negative temperature coefficient) power thermistor sitting in the AC input path of mining PSUs to limit inrush current at switch-on. When the PSU first energises, the bulk capacitors look like a near-short and would otherwise pull a destructive surge through the bridge rectifier and input fuse. The cold SCK-2R55A presents 2.5Ω, capping that surge to a safe value; once current flows the thermistor self-heats and its resistance drops to a fraction of an ohm, removing itself from the loss equation during normal operation. A failed inrush thermistor is one of the most common causes of "PSU blows the input fuse on every power-up".

How the Thermistor Fails and What to Look For

SCK-class thermistors fail open after sustained over-current events — for example, a short downstream that the protection circuit didn't catch fast enough, or a violent power cycle at full load. Visual signs are a cracked or scorched ceramic body and an open circuit on the multimeter. A subtler failure mode is "stuck high": the thermistor reads close to its cold value at any temperature, meaning the PSU runs at full inrush-limited resistance during normal operation, which causes overheating of the part and unexplained efficiency loss. Either failure mode warrants replacement.

Replacement Procedure

Disconnect the PSU from mains and bleed the bulk capacitors (5 minutes minimum, or use a bleed resistor for safety). Identify the failed thermistor — usually a black disc near the input fuse and bridge rectifier. Desolder the failed unit, clean the through-hole pads, and solder the new SCK-2R55A in the same orientation (NTC thermistors are non-polarised but the body should match the original placement to keep the heatsink airflow path clear). Verify the thermistor's cold resistance (~2.5Ω at 25°C) before powering up.

SCK-2R55A Specifications

Related PSU Inrush / Input Stage Parts

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Stocking SCK-2R55A for a PSU repair line? Contact us at contact@lys-sz.com for tray-quantity pricing. Worldwide shipping from our Shenzhen warehouse.