Fan Curve

A fan curve is the relationship between a cooling fan's speed and a control input — almost always chip temperature — defined in a miner's firmware. As an ASIC heats up, the curve dictates how aggressively the controller ramps the fans; as it cools, the curve lets them throttle back. It is the logic that turns a temperature reading into a PWM duty-cycle command, and it quietly decides whether a miner runs cool and loud, warm and quiet, or — badly tuned — hot and fragile.

The control loop

The pieces are concrete. Hashboards carry I2C temperature sensors (parts like the LM75A on older boards, TMP1075 and similar on newer ones) mounted on the board where they track nearby chip groups; some boards place sensors at both the air inlet and outlet ends to capture the gradient across the heatsink. The control board polls these sensors continuously, feeds the readings into the curve, and drives the fan headers with a pulse-width-modulation signal — on Antminer-class hardware the fan PWM output and tachometer input run through the control board's logic, with the tach line confirming each fan actually spins at the commanded rate. When measured temperature crosses the curve's thresholds, duty cycle rises; when it falls back, duty cycle drops, cutting noise and the non-trivial power the fans themselves draw. Missing tach feedback triggers protection, because a dead fan on a running board destroys hardware in minutes.

Reading the curve's shape

Two parameters define an operator's experience: where the ramp starts and how steep it climbs. A lazy curve lets chips drift toward limits before reacting, inviting thermal throttling and long stretches at high temperature that age silicon and dry out interface materials. An aggressive curve holds chips cold but wastes fan power and makes a home installation unlivable. Oscillation — fans surging up and down rhythmically — signals thresholds set too close together or airflow that cannot actually satisfy the demand, and is worth fixing rather than tolerating. Stock firmware ships conservative curves biased toward hardware protection; tuning-oriented firmware exposes the curve directly so an operator can trade acoustics, longevity, and hashrate deliberately for their environment via the miner web UI.

Fan curves meet physics

A curve can only command what the fans can deliver. ASIC heatsinks are dense, and pushing air through them takes static pressure, not just free-air CFM — which is why replacing stock fans with quiet consumer fans of similar rated CFM so often ends in throttling: the replacement cannot maintain flow against the resistance, and no curve can fix that. The curve also interacts with ambient temperature and delta-T: a setting tuned in a winter garage will behave differently in July.

Watching it work

The fan curve leaves fingerprints an operator can read. Fan RPM and board temperatures in the dashboard, and fan events in the kernel log, tell you whether the curve is coping: fans pinned at 100% mean the cooling budget is exhausted (dirty heatsinks, hot intake air, or dying fans), while fans that never rise under load deserve suspicion of a sensor or control fault. A five-minute glance at those numbers each season is cheap insurance on hardware that earns its keep by staying alive.

Two deployment styles bend the usual rules. In cold climates, the curve's low end matters as much as its high end: air well below freezing can overcool a miner into condensation risk and startup problems, so winter operators shape curves — or duct recirculated exhaust — to keep intake air in a sane band rather than simply maximizing cooling. In liquid deployments the fan disappears but the curve's job does not: hydro and immersion machines replace fan PWM with pump and flow management, and firmware built for air will object to missing tachometer signals unless it is immersion-aware. Both cases make the same point from opposite ends — the fan curve is one implementation of a general contract between heat produced and heat removed, and an operator who understands the contract can adapt it to any climate, enclosure, or cooling medium without guessing.