How does the fan tach (FG) signal work?
A fan's tach (FG) output pulses a fixed number of times per shaft revolution — two pulses per revolution is the most common convention, but it is model-specific. The host converts frequency to speed with RPM = 60 × f / PPR. Get the pulses-per-revolution wrong and the displayed speed is off by a whole-number factor — the fan runs fine while the monitoring lies.
The conversion and the classic mismatch
RPM = 60 × f_tach / PPR. Example: a tach at 100 Hz with 2 PPR means 3,000 rpm; the same 100 Hz on a 4 PPR fan means 1,500 rpm. When a replacement fan has different PPR than the incumbent, the host displays double or half the true speed — a pure reporting error that looks exactly like a performance change. Count pulses with a meter for 30 s during qualification and derive PPR empirically; never inherit it from the old part's datasheet.
| Tach frequency | PPR = 2 | PPR = 4 | PPR = 1 |
|---|---|---|---|
| 50 Hz | 1,500 rpm | 750 rpm | 3,000 rpm |
| 100 Hz | 3,000 rpm | 1,500 rpm | 6,000 rpm |
| 150 Hz | 4,500 rpm | 2,250 rpm | 9,000 rpm |
Electrical interface
Most tach outputs are open-collector or open-drain: the host supplies a pull-up (commonly 10 kΩ to 3.3 V or 5 V) and the fan pulls the line down at each pulse. Check the output's maximum sink current (a strong pull-up can overload it), the host input's logic threshold at your pull-up voltage, and cable length — long unshielded leads pick up noise that shows up as phantom pulses and jumpy RPM readings.
- Pull-up: 10 kΩ typical; confirm against the fan's sink rating.
- For long leads, prefer shielded cable and a lower-value pull-up at the host end.
- Debounce or rate-limit in firmware: a single glitch spike should not alarm the fleet.
What a healthy tach tells you
Beyond speed display, the tach is a health instrument: a rising speed at constant duty and rail suggests the load is falling or the fan is slipping out of its operating point; a jittery tach at steady command can precede bearing failure; and a dead tach on a spinning fan means a failed output or a wiring fault. Trend tach against duty in your fleet telemetry — deviations from the unit's own baseline are more informative than any absolute threshold.
Send MEGA Tech your airflow and static pressure target, voltage, size limit, control interface, quantity and environment. We reply with a selection proposal and a sample plan — selection by operating condition, samples validated on your unit.
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