And on to Step 3: the power supply fans.
The power supply is behind the front panel. It's rated at 700 watts and is surprisingly compact.
So compact that looking through with the fans removed, it's hard to see any daylight:
This is what the fans blow into. The power supply is in two stages, the first making approx. 360VDC, then those six modules converting that to +5, -5.2, +/-12 and +3.3. Those three brown filter caps are 220uF/450V. Unfortunately, there aren't smaller caps with this rating that could be substituted to give less restriction to the airflow. At the power inlet on the right it looks like some of the airflow is supposed to go through one set of perforations of that cage and out the rear perforations.
The fans are 60 mm Nidec TA225DC M33455. There is a
data sheet available at that gives the fan curve for this model:
The static pressure is the pressure rise across the fan. The more restriction is placed on the airflow coming out of the fan, the higher the static pressure gets but at the expense of total airflow. That inflection point is where the blades of the fan aerodynamically stall and cause an efficiency drop. (And probably noise increase.) It's best to operate to the right of that point. The units are inches of water column, that is, how high a column of water will be lifted by this pressure. For this fan, best running is with a static pressure of less than about 0.08 inches or 2 mm. So not much.
The challenge is, there is no easy way to determine the flow resistance the fan will face. If the flow resistance is lower than the fan can work with, the volume of airflow increases, which is a good thing. More volume of air also means greater velocity which improves the convective heat transfer coefficient, which is also a good thing. So reducing restriction to the flow of air is a priority.
One option is to replace the current 60 mm fans with quieter models. Noctua fans get good reviews and their
60 mm model is rated at 29.2 m³/h at 2.18 mm H?O static pressure. (Often manufacturers only give flow at one static pressure, rather than providing the full fan curve, making comparisons between fans more difficult.) That translates to about 17.2 cubic feet/minute at 0.088 inches of water. That's about 50% more airflow than the original fans at this static pressure. And with a noise rating 10 dBA lower. Running this model fan with thermostatic control would also let it run more slowly, producing less noise.
In addition, if the restriction of the airflow path is reduced, the fans can be turned down even more and give the same airflow. Here's the inlet flow path to the fans:
Not very promising. I'm looking at this unit as a long-term replacement on my bench for both logic analyzer and scope, so I'd like to go for as little noise as I can. What about using one bigger fan turning more slowly? Here's the 120 mm fan from the chassis side hung in front of the PS.
It will take a little chassis surgery, but could work. A 120 mm fan will flow more air than two 60 mm fans at similar conditions. In the Noctua fans, the F12 (highest static pressure model) flows about triple the air as the A6 into about 20% higher static pressure at the cost of 3 more dB. It has a lot of room to be turned down.
So, going beyond what almost anyone would consider reasonable, time to start fabricating a transition piece for a big fan.
This is the aluminum chassis of the power supply (on the right) and a mounting flange for the 120 mm fan being held in place by a block of wood. Cereal box cardboard and masking tape are being used to make a template of a transition piece. Unnecessary bits of the power supply chassis have been cut away already. Here's the resulting template, ready to transfer to sheet metal:
Looking for suitable sheetmetal, I found something in an e-waste bin:
It just seemed right...
This view is the back of that bulkhead that holds the power supply. Some of it was cut away for the fan and transition piece. I went with the Noctua F12 PWM fan, which is the beige thing with the brown corners. The blue stuff is RTV engine gasket compound, which was on-hand It is a silicone sealant, used here to seal the gaps in the sheetmetal work.
The power supply was shifted about 35 mm to the right to make it fit. I also removed those three big caps and mounted them on the right (using 600V rated wire). One of the three mounting screws for the floppy drive was sacrificed, but it's still well attached (and I don't have anything else that uses floppies anyway). Here's the side view:
(The grey band to the left of the fan is the ribbon cable to the CD-ROM. It's not helping to promote good cooling, either...)
I took one of the thermistors that came with the fan speed controller and cut off its metal shroud. It then fit snugly between the 5V and 3.3V modules in the power supply, near their switching transistors. I figure this would be about the warmest place. This is set as fan 2 on the controller. With five cards in the unit, and now drawing about 690 W, the temperature reading from the power supply is 38.0C. This checks with how PS chassis feels, as well as the air coming out. The fan is running at 1080 RPM. Maximum speed of the fan is 1500 RPM, so there's room for the controller to run the fan faster.
And the noise? I've just got a cheap tablet with a noise meter app. Before any work, 30 cm directly in front of the unit it read:
Ambient level: 23 dBA average; with unit on: 37 dBA average (although it sounded like a lot more...)
After the steps described here:
Ambient level: 24 dBA average; with unit on: 25 dBA average
I'm sure it is considerably louder than this app says, but there is definite progress. Now it's closer to lawnmower than hovercraft.
The hard drive is now the biggest noise source. I've ordered a SCSI2SD board to take care of it.
There's more checking and tweaking to do. One thing is that the cards draw a lot power. If unused cards are replaced with filler panels, the fan speed controller can turn down power supply fan speed, saving some noise there.
Anyway, I hope this might give some ideas for options to quiet noisy test equipment fans.
Cheers,
Tim