"EFB-09-12-HHE" means EFB series, 90 mm, 12 volt, extra high power. (I do not know if 92 mm fans are used on the Mac Pro, but AFB1212HHE is the 120 mm equivalent: ) It was for sale on Newegg from 2007 to 2014. Evidently the 92 mm fans used as the CPU fans and the rear fans in the G5 are model EFB0912HHE. The fans used in the Mac Pro and the Power Mac G5 are made by Delta Electronics, Inc. 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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