Fooling a server motherboard into thinking the fans (6) are spinning fast? - Page 1
Forum section: electronics / beginners

author: petreza
author_rank: Contributor
timestamp: October 17, 2015, 12:37:08 am
content: Hi,I am want to replace the noisy fans of a server with quieter ones for home lab use. There are 6 fans and each one has 4 wires - black, red, blue, and yellow.Google says that the blue wire is a for control of the fan speed by changing the duty cycle and the yellow is for sensing the speed of the fan (tachometer).If I just replace the fans with slower spinning ones I could trip the motherboard's sensor and get a fan failed warning. Also, the fans that I like have only 3 wires.Previously I have used PWM Generator like this for a similar project:http://www.ebay.com/itm/1PC-Nano-Signal-Generator-Module-555-pulse-frequency-adjustable-square-wave-PWM-/291556640248?hash=item43e22145f8:g:sQUAAOSws65ToWO7but in that case I used one per fan for quieting a noisy router.Hopefully, the motherboard fan speed sensor will not give me a warning if the fan is running faster than it expects it to.How much draw would there be on the yellow sensing wire? Can I use only one PWM Generator to emit the signal for all 6 fans?

author: jwm_
author_rank: Frequent Contributor
timestamp: October 17, 2015, 01:03:39 am
content: Quote from: petreza on October 17, 2015, 12:37:08 amHave you tried hooking it up without the wire? I think most motherboards will detect if there is no tach wire and behave just fine without tach data. (perhaps with a BIOS flag switch) They will just pwm based on temperature which is what you want.

author: petreza
author_rank: Contributor
timestamp: October 17, 2015, 01:25:51 am
content: I have not tried that. The parts are on order now but I don't know how many PWM Generators I would need, if any, in this case. Not that they cost that much but because it will take more effort to hook up 6 instead of just 1.

author: Nerull
author_rank: Frequent Contributor
timestamp: October 17, 2015, 02:16:37 am
content: Fan alarms are usually configurable as well, have you checked the BIOS?

author: Messtechniker
author_rank: Frequent Contributor
timestamp: October 17, 2015, 07:26:06 pm
content: I've used this circuit successfully many times.Yours Messtechniker

author: janoc
author_rank: Super Contributor
timestamp: October 17, 2015, 08:55:20 pm
content: If the mobo has RPM detection for failed fans then the RPM tresholds are very likely configurable in the BIOS. Just change the setting there.

author: crispy_tofu
author_rank: Super Contributor
timestamp: October 17, 2015, 09:08:54 pm
content: Not sure if this is relevant, but there are fan header extensions that include an inline resistor...

author: petreza
author_rank: Contributor
timestamp: October 18, 2015, 04:23:39 am
content: Thank you all for the replies! Good info here but the original question still stands - even if it is possible to adjust fan settings in BIOS etc., what if that was not an option?Will the sensing circuitry for 6 fans overload a single PWM Generator based on a single 555 timer?Sorry if this is too much a "beginner" question but I am not an electronics guy.If nobody can tell me, how can I find out? Can a good multi-meter measure accurately the amperage (average?) of a PWM signal?Or maybe the way this circuit works is that the PWM Generator will emit as strong a signal as it can and I can hook up as many "receivers" as I want and nothing will get overloaded and blow up. Maybe the worst that can happen is that the signal will be too weak to detect if I hook it up to, lets say, 100 "receivers".I don't want to just try it and fry something on the motherboard.Thanks!

author: brobbuilder
author_rank: Contributor
timestamp: October 18, 2015, 04:36:14 am
content: Sensing circuitry doesn't usually draw much current (<1 milliamp) so it won't matter how many sensing circuits you connect to the single 555 gadget. It would be good to reply once you fit the new fans and tell us whether the BIOS even cares about higher fan speed, let alone there being sensing data at all.

author: bagnoz
author_rank: Newbie
timestamp: October 19, 2015, 11:32:49 am
content: The fan interface specification, if it helps:http://formfactors.org/developer%5Cspecs%5C4_Wire_PWM_Spec.pdfQuoteSo your circuit must survive more than 5V. (Sadly I can't seem to find a specification for the sourced current for the tachometer signal)