Hot-swappable-PSU fan replacement w Noctua?

[pipe2null]

I picked up some old datacenter servers and network gear with industrial-grade noise pollution (as expected).  The equipment is just to play with and for home server/lab use, not for commercial purposes.  I worked on the servers, got them quieted down to easily tolerable normal-desktop noise levels, same with the network gear system board fans.  BUT, the network gear's hot swappable PSUs are giving me trouble.

I swapped out the stock 404028 fan for similar size 404020 Noctua, but now the PSU consistently shuts itself down to standby mode after 10s of applying power.  I scoped both fans, Noctua's tach output is about 165Hz, and stock is 600Hz.  I am assuming there are 2 pulses per rotation, so Noctua measures at about 5kRPM which is spec, and the stock fan is running about 18kRPM for a Nidek Ultraflow Ultraloud screamer of a enterprise network switch cooling fan.  All fans are 4wire, power plus tach plus PWM.

1) I am guessing that the PSU firmware does not like Noctua's lower RPM, so the fw is failing poweron tests due to fan speed?  If so, does anyone know if this fan speed check is pretty much standard for all hotswap PSUs, thus no point in me picking up other ebay PSUs to try and find one that won't shut itself off after I've modified it with a quiet fan?

2) I am assuming the issue is PSU firmware, but could a system board fw setting be causing this behavior?  Just FMI, I don't think those settings are exposed, so I couldn't change them even if on systemboard.

3) Is there any way to fake out the fw, or solder a few passive components together, squeeze them into a cramped PSU case to triple the tach signal output frequency?  I have no idea on this part.

Thanks!

[thm_w]

I assume you checked every setting in the BIOS to see if there was no fan speed option for the PSU right? Probably does run its own firmware as you say.

You can use a 555 to generate 600Hz, which should fool the PSU:
https://www.aliexpress.us/item/3256804173112769.html
https://www.aliexpress.us/item/3256803165906615.html

quadrupling the existing signal is also possible, but I don't think its going to be easy at this frequency with passive components.

[pipe2null]

Good tip on the mini signal generators for faking the tach signal, will keep that as a backup plan, thanks!

The network switch has a somewhat obscure OS/firmware, part of why I got a fully functional 48p10Gb/4p40Gb managed switch for cheap.  I dug through options exposed to the serial console terminal, can display temps and system board fan speeds and detect hotswap PSU info, but haven't found any fan speed related settings.  OS/firmware treat PSU sensors separately from system board sensors (typical).  On my old enterprise servers it isn't a problem, most fan settings are exposed via IPMI, plus fan idle speed for server PSUs is much more sane (aka, doesn't cause insanity due to noise).  I do not have a sound-proof room for this gear, and at lease parts of it are going next to my bench.

I'm thinking perhaps sticking a small MCU between the PSU and the fan might be the best approach, actually read/monitor the fan tach signal and bitbang an output to the PSU monitoring circuit that is ~3X Fan Hz?  Theoretically the fw would pass self tests but still detect hardware problems with the fan...

Perhaps a PIC, maybe 8-DIP or something easily solderable with hacked up passives, bundled up in-line as part of the fan cable?  It's been a couple decades since I've messed with anything like that.  Time to dig through Digikey me tinks.


Is this sensible, or is there a better approach I should look at?

[Bud]

Just recently there was a thread on thus forum about doubling fan tach frequency. You may want to search for it.

[pipe2null]

Been digging into forum...  There's been some interesting reads, but haven't found anything RE doubling tach freq.

Errrr....  Scratch that:
https://www.eevblog.com/forum/beginners/square-wave-multiplication-by-a-given-factor/msg4497322/#msg4497322

Spot on, pretty much exact same problem.  Thanks!

I'll ignore this thread and bandwagon onto the other one.