The four wire fan connectors are the same original 3 wires (pwr, gnd, tach) with the fourth wire being a digital pwm control signal to the fan. The way that it works is to take advantage of the power MOSFET (switch) that is already in every DC fan. Add the PWM input to the circuit that controls when the switch is turned on so that it *doesn't* turn on when the PWM is active. The result is that the fan slows down. This is much simpler, cheaper to implement, and resolves several issues with using low-side or high-side MOSFETs on the motherboard. CPU fans on the new Intel CPUs (Conroe, Dempsey, Woodcrest) will probably all be four wire. Penguin Computing uses four wire case fans in its Tempest 2150 for better control and monitoring. :v) -----Original Message----- From: Henrique de Moraes Holschuh [mailto:hmh at debian.org] Sent: Sun Jun 04 11:10:28 2006 To: Jean Delvare Cc: Hans de Goede; LM Sensors Subject: Re: sysfs interface for fan present capability On Sun, 04 Jun 2006, Jean Delvare wrote: > Definitely not the LM75, as it is a temperature-only sensor chip. But I My bad, I should have typed lm85 (which would not be strictly correct, either: the ADM1027 does it. It is a LM85 compatible chip, but I don't think the lm85b/c can do it). > indeed. Strangely enough, some vendors are doing the opposite and > propose 4-wire fans, the idea being that tick counts tend to be lost > when fan speed control is in use on 3-wire fans. As soon as you drive fans using PWM you screw up any sort of simple tick counters completely, as you have a spike on every PWM cycle start, plus no spikes generated during the second phase of the PWM cycle. Probably they want four wires so that you can have Vcc, GND, Vcc-sense and sense, so that PWM does not disable the sensor. -- "One disk to rule them all, One disk to find them. One disk to bring them all and in the darkness grind them. In the Land of Redmond where the shadows lie." -- The Silicon Valley Tarot Henrique Holschuh _______________________________________________ lm-sensors mailing list lm-sensors at lm-sensors.org http://lists.lm-sensors.org/mailman/listinfo/lm-sensors -------------- next part -------------- An HTML attachment was scrubbed... URL: http://lists.lm-sensors.org/pipermail/lm-sensors/attachments/20060604/84cdd296/attachment.html
