Fan replacement of a Dell Tower UPS (J715N) - Page 1
Forum section: electronics / projects

author: junkpete
author_rank: Contributor
timestamp: January 01, 2025, 05:28:14 pm
content: I have a Dell UPS which I want to use to power my A100S server together with my WiFi router in the living room. Both together require less than 25W and although the load on the UPS is very low, the fan is spinning all the time with about 3000RPM and is very loud (55dB). Even when I pull the power plug and let the AC converter do the job to power the 25W load, the temperature inside the UPS barely goes above 30degC.Long story short, I would like to replace the fan with a silent one and I have a Noctua fans which would perfectly do the job. They run at lower speeds and have a very clever design. I get a comparable airflow with 30dB noise.The problem is however: the Dell firmware monitors the fan speed (which is a good thing) but it does not accept the signal or RPM range of the Noctua fan. While the original fan runs at 5400RPM at full speed, the Noctua is rotating with only 2200RPM (it's bigger and has a better aerodynamic style). The UPS shuts down after 10s with "fan error", which can only be due to the wrong RPM measured (or not measured since it expects raw hall sensor output, see below).I though I would outsmart the Dell firmware by doubling the square wave pulse of the hall sensor, which comes through the yellow wire of the fan, and measured it with the oscilloscope but then realised that the signal of the original fan is not a square wave.I always thought that fan manufacturers stick to Intels specs, but the original fan which is an NMB-MAT 2806KL creates four "hall sensor signals" per revolution instead of the standard two square waves per revolution.The attached screenshot shows the fan running with 5V and the frequency of the peaks is 138Hz which corresponds to about 2000RPM (138/4*60). This also matches the data sheet specs, which otherwise does not tell anything useful regarding the output.The TACH output of the Noctua fan and every other fan I found at home do have a clean square wave output.I will have to reverse engineer the PSU board now since I assume that the main board probably has a circuit to create an actual square wave. I thought that maybe I could inject there the signal of the Noctua fan by creating a multiple of the square waves but not sure if that's a good idea.Any thoughts?

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: January 01, 2025, 11:52:31 pm
content: Are there any utilities for adjusting the UPS firmware's settings, including fan speed fault threshold? Either Dell or 3rd party?Other Dell UPS users might have encountered this problem - maybe they have solved it.A UPS sensor misreading the actual temperature might cause the fan to run at full speed. Could that be happening here?

author: junkpete
author_rank: Contributor
timestamp: January 02, 2025, 08:02:26 am
content: Unfortunately there are no settings whatsoever. I search everywhere but the only posts I could find is that people are replacing the fans and are faced the "fan error" issue with the 10s shutdown.It cannot be the temperature since it also happens without any load. It's really just plugging in a different fan and the UPS is striking. The UPS also explicitly shows: fan error. There are temperature sensors and warnings as well, but I have not seen them popping up yet.To me it's quite clear that the RPM reading cannot work when the yellow wire delivers the square wave, which should be the standard but for some reason Dell decided to use a fan with a raw Hall sensor output. It's just really hard to simulate this signal, that's why I thought that at some point they probably do the square wave conversion on the motherboard via a Smitt-trigger or so. I had not time yet to check the board but will report back.All in all, I think I am facing two issues:Find the point where the square wave is generated and inject the proper signal from an "Intel-spec'd" fan at that very point, bypassing the Hall sensor signal conditionerHope that the RPM range is big enough, so that Dell accepts it, otherwise trick it to generate twice (or ore) the frequency of the square wave the etc.A completely different approach would be to simulate the Hall sensor output based on the square wave output (RPM signal) of the new fan, using e.g. an Arduino and an analog output, but that sounds like an overkill