PWM vs Tacho (FG/RD): Speed Control and Monitoring Options for DC Fans

A DC fan is not necessarily a fixed-speed device. Depending on how many wires it has, it can be throttled, monitored, or both. The wire count is the first thing to confirm in any fan specification — here is what each configuration does.

Two-wire: power only

The simplest fan: positive and negative. Speed follows voltage, so the only way to slow it down is to lower the supply voltage or insert a series resistor / thermistor. Two-wire fans are right for cost-driven products with a predictable thermal load, such as adapters and small appliances.

Three-wire: add a tacho (FG)

The third wire is a speed-feedback signal, usually called FG (frequency generator) or tacho. It typically outputs 2 pulses per revolution as an open-collector square wave — so a fan at 3,000 RPM produces a 100 Hz signal your MCU can count. What you get:

• Verification that the fan is actually spinning (a stalled fan reports zero)
• Trend monitoring — bearing wear shows up as RPM drift long before failure
• Closed-loop verification of voltage-based speed control

A tacho tells you the speed but cannot change it. For that you need PWM.

Four-wire: PWM speed control

In the four-wire configuration — the same one used by standard PC and server fans — the fan is fed full voltage on two wires and receives a PWM signal on a dedicated control wire. Key parameters to align with your design:

• Duty cycle range: typically 20–100%; below the minimum duty the fan may stop, so define your floor
• PWM frequency: 21–28 kHz is the industry standard — above audible range, so the fan never whines
• Logic level: 3.3 V or 5 V logic, open-collector or push-pull — confirm with the fan maker
• Zero-RPM option: some of our models support a "stop at 0% duty" mode for silent standby

PWM control in the fan (rather than voltage control in your circuit) keeps the fan's internal driver at optimal efficiency and gives a clean, linear speed curve — which is why it is the default choice for EV chargers, energy-storage systems and telecom power.

RD: the alarm variant

An RD (rotation detect) signal is a status line rather than a pulse train: it holds one level while the fan rotates and flips when the fan stalls or drops below a threshold speed. Where FG is for logging and trend analysis, RD is for protection circuits — trigger a shutdown or an alarm relay when RD changes. Many of our industrial models can supply RD instead of FG on request.

Configuration	Wires	Control	Monitoring	Typical use
Basic	2	Voltage only	None	Adapters, appliances
Tacho	3	Voltage only	RPM (FG)	Servers, medical, monitoring systems
PWM	4	PWM duty	Optional FG/RD	EV chargers, energy storage, telecom

Wiring and connector notes

We build fans with the common harness standards — JST XH2.54 / PH2.0 / VH3.96 and Molex 2510 — and match the wire-color conventions your system already uses. When requesting a quote, send your connector part number and wire-definition table; if you don't have one, a photo of your current fan's plug is enough for us to match it.

Rule of thumb: if the product has a user-facing noise target or a fan-failure alarm requirement, specify PWM + FG from the start — retrofitting signal wires into a two-wire design later costs far more.