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Cisco 3550 fan mod

This fan mod is for the following:
Cisco 3550 12T
Cisco 3550 12G
Cisco 3550 48

Parts needed per fan:
18" fan extension cord
5v regulator 7805 RS# 276-1770
heat shrink tubing - 2 sizes 1/8" and 1/4"


These are standard computer fan wire colors, however cisco does not use a standard pinout.
Top wire is +12v in (black)
Middle wire is ground (yellow)
bottom wire is +5 out (black0
background wire is bonded (red)

I had to bend the wires coming off the 7805 just a bit to get them to fit.   I had to install them face down to be able to tighten the nut.  


I did some measurements.  It seems the way Ryan (research) mounted them would be incorrect.  There is a 5v differential.  Not sure why fan ground would be +5v over chassis ground... Don't understand... Granted this is on the 3550-48.   Yes.. on the 12G also there is no voltage differential.  This must mean the 48 uses a +5 on the ground to drop the voltage to 7v hence why it is quieter.  

Test is in, the 12G does infact show the fans working ok with this modification.  So I'm going to dangerously assume that the 12T is the same as the chassis is identical. 

Finished and reassembled!

Before modification noise levels were 55db@1m, now my meter can't register @1/2m ~<45db.  So if you are in the same room with these switches, you need to do this modification.


So far no temp warnings and no complaints in the log.  Seems fine!