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If at least one of the internal measured (CPU, SFP, Switch, Board etc.) temperatures exceed fan-target-temp, the fans will start to spin. The higher the temperature, the faster the fans will spin. For devices with PWM fans, as the internal measured temperatures exceed fan-target-temp, the fans will linearly increase their RPM to try to keep the temperature at fan-target-temp if possible and will get to their Max RPM when the temperature is equal or exceeds fan-full-speed-temp. For devices with DC fans, as the internal measured temperatures exceed fan-target-temp, the fans will start spinning but at a higher minimum RPM by default. This may result in cooling the device to the point where the fans turn-off completely if fan-min-speed-percent is set to 0 0%, while with the default value of 12 12% fans will never go to a full stop therefore reducing the noise and On/Off peaks that may occur. The temperature then may slowly increase to fan-target-temp and the fans will turn on again. Currently, there is one exception. The S+RJ10 modules have a temperature threshold of 65C before they trigger the fans. Since it's a higher temperature threshold, the fans will start spinning at a higher initial speed to cool the device. All the above mentioned functionality is directly related to the fan-control-interval parameter value as it will determine how often FAN controller monitors all sensor data and triggers changes in fan-control.
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