![]() Assuming an open drain / open collector an a pull-up resistor, low voltage would mean full power. ![]() It does not say what is "100% duty cycle", low voltage or high voltage. The Intel specification is vague in other ways too. (See the question "Can I connect a 4-wire fan to a 3-wire connector?" here: ) This is only possible, if they have an inbuilt pull-up resistor. 4-pin fans work when connected to a 3-pin connector. This Intel specification from 2004 speaks of PWM as a "control" signal: \specs\rev1_2_public.pdf Talk of a pull-up resistor on this thread. The argument for an open drain control line: A P-channel MOSFET would be more suited at providing power at 12 volts. The open-drain MOSFET would always be N-channel. This would be simple, as it can be implemented inside an Integrated circuit without the need for a power transistor stage. There is an underlying assumption that the Intel 4-pin fan standard drives the PWM line by a open-drain MOSFET. Both MOSFETS are of similar size and thermal rating fully able to drive a fan with several amps of current. I also checked the 4-pin system fan connector on my GA-EG41MFT-US2H motherboard and saw a PA102FDG P-channel MOSFET next to a 100uF capacitor. Next to a 16V, 47uF capacitor sits a FR5505 P-channel MOSFET. I had I look at the fan control circuits on a uniprocessor Power Mac G5 that I had just pulled out. The typical application circuit 1 is almost identical to your drawing. Building a CustoMac Hackintosh: Buyer's Guide
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