| 摘要 |
Methods and circuits detect operational information about DC fans powered by pulse width modulation, such as detecting fan presence in a device and/or detecting rotational speed. Output pulses produced by the fan related to the rotational speed are utilized to produce temporary reductions of voltage at the input of a control circuit. The control circuit may count the temporary reductions per unit of time to detect the speed. A pull down resistor may be coupled to the input to pull the input to a continuously low voltage when the DC fan is not present to otherwise provide a pull up at the input, and the control circuit may detect a missing fan from the continuously low voltage. Additionally, or alternatively, a secondary voltage may be provided to the DC fan in addition to the pulse width modulation while a consistently high voltage is provided to the input of the control circuit. Accordingly, the DC fan continues to produce output pulses indicative of speed during the low state of the pulse width modulation, which enables the control circuit to continue to receive the temporary reductions at the input to determine the rotational speed with accuracy.
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