Quieting the Enermax & other thermistor fan PSUs

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Diving In

Please read the Disclaimer Warning before you dive in.

When I opened up the Enermax, I first removed the fans to listen to them by themselves in open air. Fans not only move less air when near things that obstruct airflow, they are noisier. The 92 mm unit is surprisingly quiet. The 80 mm fan is not, despite the noise spec being virtually identical. It is considerably noisier and is of a higher pitch.

I found the thermistor hot-glued to a large coil. The coil reaches a high temperature very quickly when AC power is applied. Given the positioning of the thermistor directly on this coil, it is not at all surprising that the fan gets to full speed in 10 minutes.

At this point, one wonders about why the design engineers would go to the trouble of adding the thermistor and associated circuitry in this particular way. Surely, it is self evident that if fan speed is meant to rise in response to increasing heat within the PSU, putting the temperature sensor on a component that always stays hot defeats the purpose altogether. I sent emails to Enermax to explain this choice of thermistor positioning, and while they did answer many other questions, this one was never tackled satisfactorily. I will have to try again.

The issue, really, is whether repositioning the thermistor is dangerous, whether it amounts to meddling with a key element in a broad design function I do not understand. Or whether it’s some kind of internal fracas between marketing and engineering departments that resulted in a half-baked solution that pleased no one. In the end, I decided what I want is more important than what the Enermax folks want.

The Mods

I gently pulled the thermistor off the coil. It was a green-headed device at the end of a 2.5" piece of yellow-insulation wire. Only the tip was exposed, the rest being covered with a bit of heatshrink. I then repositioned it away from the coils, off to one side.

I removed the 92mm fan and left the opening for it completely unobstructed. This was simple because the fans in the Enermax are connected to the printed circuit board via 2-pin jacks. My reasoning is explained in my comments about the push-pull fan configuration.

The loud 80mm stock fan was replaced with a Panaflo FBA08A12L. It is a drop-in replacement. The 2-pin jack it came terminated with was modified slightly to fit on the pins of the PCB. This fan is popular among silent PC enthusiasts because it is quiet, widely available (at least in the U.S.), and very inexpensive. Its key specifications are 24 cfm, 0.1 amps and most importantly, just 21 dBA. Fan monitoring would be lost because the Panaflo (my version) did not have a 3rd speed sensing wire. I left the nice wire grill off, figuring that any reduction of turbulence would be a good thing.

After putting the PSU back together, I ran it with the a spare motherboard to make sure it was still functioning. The fan did not start spinning, even though it was clear from the lit LED on the motherboard that the PSU was on. I touched the fan with my fingers and it started spinning ever so quietly. After a dozen or so repeated startups, I found that while the fan usually started, there were a few times when it did not. The voltage fed to the fan was too low to get it started consistently. Often, a fan will run at a lower voltage than it needs to start, because inertia must be overcome to start.

I had a number of Panaflo fans. Using a variable DC power supply, I tested all of them and selected the one that consistently started at the lowest voltage, which was 4 volts. It went in place of the first Panaflo. Now it started consistently every time.

Later, I discovered another way to ensure consistent fan start: just move the thermistor a little closer to the coils. They heat up so fast that the fan voltage goes up high enough to kick it into action within a few seconds after power is applied.

Fini

After these modifications, the noise emitted by the PSU laid on a piece of cushioning foam is inaudible from 4-5 feet away. From 2-3 feet it is audible as a whisper - just as Enermax promises.

I left the modified PSU on for several hours, checking the air exhaust with my fingers periodically, At the end of 3 hours, the exhaust air temperature felt slightly warm to the fingers. I deemed this satisfactory and installed the PSU back into my system.

The PC is under my desk. In this position the PSU is almost always inaudible, as the exhaust fan is pointed the other way. I give it a little help with a 1’ x 2’ piece of egg-crate shaped foam placed on the wall behind the PC.

POSTSCRIPT -- April 23, 2002

Yesterday, while trolling for information in the forums at Amdmb.com, someone identified only as Divebrake had this to say in response to this very article:

If you have the thermal sensing fans you can snip off the thermistor and replace it with a 10k ohm pot

The insight of this simple statement hit me like a ton of bricks. Of course! The thermistor is nothing but a variable resistor that changes value according to heat; why not just replace it with a variable resistor whose value can be changed directly?

This morning, I dug through my odds-and-ends bins and found a 100K-ohm potentiometer. $1.45 Cdn, the plastic bag tag read. The thermistor dangling at the end of the yellow wires in the Enermax was snipped, and its resistance measured: 96K-ohms. Close enough; it is a 100K thermistor. I soldered the potentiometer in place, then powered the system up.

Lo and behold! I now have manual control of the main fan with a simple turn of the pot. The voltage adjustment range measures ~5V to 12VDC. It is a perfect solution that obviates any concern about what the effect of hot weather might be. If a heat wave comes along, just turn up the fan!

Thank you, Divebrake!

*****

POSTSCRIPT 2 -- Aug 3, 2002

TerryW, an electronic technician with many years experience, cautions against replacement of the thermistor with a pot , saying in our forums that it functions much like a fuse. TerryW provides a quick technical summary of the switching power supply, which I certainly appreciate. My counter to his caution is that in many thermistor-fan PSUs, to continue TerryW's analogy, the fuse size selected is much too small. The thread is definitely worth a read if you have come this far. My Disclaimer Warning about any PSU mods stands, BTW.
*****

For your reference:

Enermax: www.enermax.com.tw

Globe Fan: www.globefan.com

Comment on this article in our Forums.



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