This circuit generates speed pulses from the speed-dependent voltage spikes generated by commonly used types of PC power supply fans, which are superimposed on the supply voltage. The pulse signal can be used by the motherboard to monitor the speed of the fan. For this purpose, the pulses are tapped off from a fan connected to K1 via capacitor C1 and amplified by opamp IC1a (one half of a TL082 dual opamp). The second opamp in the Tl082 (IC1b) transforms the resulting signal into a clean rectangular clock signal and passes it to a binary counter circuit in the form of IC2a (4520), which reduces the frequency by a factor of 2. A BC547C transistor (T1) connected to the counter output provides an open-collector output at K2 for connection to the motherboard.
K2 can be connected directly to a fan connector on the motherboard. The 12-V supply voltage for the circuit is also taken from the motherboard via this connector. Components C3, C4, R8 and R9 create an artificial ground potential at half the supply voltage (6 V), which serves as a reference voltage for the opamp. Diode D1 should have the lowest possible voltage drop to minimize the voltage loss to the fan. The circuit is suitable for use with CPU fans and fans for graphics cards in addition to power-supply fans.
Author: Stefan Schwarck
(Elektor Electronics Magazine –7-8 /2006)
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