Fan PWM controller - Page 1
Forum section: electronics / beginners

author: adraen
author_rank: Newbie
timestamp: June 10, 2016, 12:48:58 pm
content: Hi everyone,First post on the EEVblog forum, I've been playing around with a fan controller and I'm struggling a bit with something, told myself that Dave would know the answer! (but he obviously has better things to do than reply to stranger's questions)Anyway, I got a server motherboard on Ebay and I've been playing with it, at the moment I'm trying to connect normal 4 pins PWM fans to the header connector on the motherboard, I got most of the things working but I'm struggling with the final pin which is PWM.The pinout of the fan connectors is listed on the datasheet (http://www.intel.com/content/dam/support/us/en/documents/motherboards/server/sb/s2600gzgl_tps_r2_4.pdf) and is attached to this post.Each connector (6 of them in total) has 10 pins. The tachometer is a standard open-drain output and connecting it to the sense pin (pin3) of the fan is fine, connecting the 12V and GND to pin 1 and 2 give the power and the fan spins and the motherboard read the rotation speed perfectly well. The next pin to deal with was the FAN_SYS0_PRSNT_N, after a bit of digging around I came across the patent (https://www.google.com/patents/US20140211353) that describes that this pin should just be tied to ground and the voltage drop across the diode notify of the fan's presence.The last part is the PWM pin, in the 4-wire PWM fan specifications (http://www.formfactors.org/developer%5Cspecs%5C4_Wire_PWM_Spec.pdf) it says that the PWM line should be open-drain and active-high with a maximum voltage of 5.25V. However when I probe the pin on the motherboard I get an active-low, 12V line and when I analyse the line with the scope I get a flat 12V line (no 25kHz PWM signal). I've tried to pull-it down with a 10k resistor but without much success.My current guess is that the PWM output is a open-collector inverter, something along the line ofhttp://www.ti.com/lit/ds/symlink/sn74lvc1g06.pdf,which would explain (I believe so?) why the PWM output is 12V. If I follow the application diagram of the datasheet, I should just pull the output high through a resistance and I should be able to see the PWM signal?Therefore I have few questions: does any of the above make sense or I am making an obvious mistake? any recommendation on how to go about this? And finally I've been trying to use the pin naming as a hint, for present pin the_Nshows that it's active-low but I'm not sure what to make of_R_BUFfor the PWM (well the buf is likely to be a buffer, but what about the R?)Cheers!

author: bitslice
author_rank: Frequent Contributor
timestamp: June 10, 2016, 08:24:17 pm
content: As I understand it, the PWM signal is an optional output from the motherboard. You don't actually need to drive it to get the fan to run, but if you did want to control the speed of the fan then that's the pin to drive.

author: adraen
author_rank: Newbie
timestamp: June 13, 2016, 08:24:33 am
content: Hi bitslice,Thanks for the answer, you're right the PWM signal is optional and there is no need for it to be connected, however it's more interesting to do it that way! Just 12V would have been enough, getting the tachometer working was nice to get some airflow performances and hopefully getting the PWM out working will be good to have some temperature/rpm curves.

author: RogerRowland
author_rank: Regular Contributor
timestamp: June 13, 2016, 08:36:22 am
content: When I messed around with a 4-pin fan while playing with PICs, I found that the fan I used had the PWM input internally pulled up to 12V rather than 5V as described in the spec.In any case, it was straightforward to drive it - just by pulling the PWM pin low at the required duty cycle - and I hooked it up to a proximity sensor so I could vary the speed as it detected my hands moving closer. The fan had a minimum speed, to avoid accidental overheating, so it only responded down to 20% duty cycle, and I had to add an extra MOSFET to switch it off completely.No idea what you need/intend to do with your setup but if it's of any interest or help, I blogged about it here:http://rogerrowland.blogspot.co.uk/2015/09/pic-fan-boy.html