Run a Power Supply without a Fan?

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April 21, 2002 - by Mike Chin

It's no news that power supply units are one of the major noise sources in a typical PC. We've looked at modifying a PSU for quieter operation in another article, and we will soon be examining power supplies designed specifically for operation without forced air cooling.

One related question that comes up repeatedly among silent PC enthusiasts is: Can I run a normal PSU without a fan? Certainly most power supply fans seem to spin far faster and noisier than necessary, and they sure make a lot of noise. It is a valid question that many have asked and few have answered.

One answer comes from pioneering PC-silencer Cub Lea, who wrote in his web page, Shut That Damn Thing Up!:

Let's begin by dealing with what is likely the most significant noise producer in the average PC: the power supply fan. The sooner you can get rid of it, the better.

But he added immediately thereafter:

If that notion scares you, that's good. If it doesn't scare you, you probably never got a basic grounding in PC maintenance. There are some circumstances where it is a little too scary...

While Cub was able to run his PCs fanless for years, it must be noted that these systems were all low powered by today's standards, the hottest CPU being a Celeron 300A , rated at just 18W heat dissipation.

A few in the Yahoo! Silent-PC group have reported successfully running PCs in which the PSU fan is removed. Some have placed the PSU outside the PC case to ensure adequate cooling.

What Happens Without a Fan?

There is little in the way of repeatable documentation of what happens when a normal ATX power supply is run without any fans:

  1. How hot does it get?
  2. Is it safe?
  3. What happens if the PSU fails?
  4. What about long term reliability?

Terry Gray, the author of Building a Silent PC: My Quest for Quiet, suggested recently that perhaps one could look at heat sink temperatures in a decent PSU with / without (much) load, and with / without fan. This is pretty much what I have done here, albeit only with a load. This is by no means a conclusive test, as it examines only one power supply among the hundreds available, in one particular environment. It is merely an experiment.

Test Setup

  • The PSU: Enermax EG-365P-VE, one of the newer units with a manually adjustable fan plus the Enermax-trademarked second 92mm fan, with a thermistor control for this one as well. The brand is generally well-regarded and equipped with good sized heatsinks, making it a reasonable candidate for fanless operation.

    (Great thanks and appreciation go to Vancouver computer retailer Anitec Computer Technology, who contributed the Enermax review unit! Do check out Anitec's excellent pricing if you're in the Vancouver area. Their service is quite decent as well. I'm told that online ordering is coming in the near future.)

  • The PSU was placed on a desk, and run first with both fans, the adjustable one at full, then with both fans off and the cover removed completely.
  • With the ambient temperature at 22 degrees C. and the cover completely off, air convection cooling was probably close to the maximum possible. I could have lifted the unit up so that there was air beneath the unit as well, but in truth, I don't think it would have made much of a difference.
  • A highly accurate Veriteq Spectrum 1000 Temperature Data Logger with Veriteq's SPECTRUM V3.1 software was used with a thermal probe inserted between one of the hot coils and a heatsink in the PSU. The time period was approximately one hour.

The photo below shows the positioning of the thermal sensor in the PSU between heatsink and coil.

The load on the PSU comprised of the following system, running CPU Stability Test 6.0 on :

  • Chaintech 6VJD2 motherboard
  • VIA C3-933 MHz (Ezra core) CPU
  • ATI Radeon VE 32 Mb
  • 256 Mb PC133 SDRAM
  • Seagate Barracuda IV 40G hard drive
  • CDRW drive
  • Floppy drive

This system was setup open on a test bench for ease and flexibility of testing.

The measured AC power consumption under this load was a steady 53-55 watts, admittedly very low. Enermax specifies the efficiency of this PSU to be at least 70% at full load. Efficiency here is defined as the amount of AC electrical power input that is converted into DC output. The power that is not converted into DC output is lost as... heat. Whether efficiency is higher or lower at less that full load is not specified, so using the 30% figure, this means the PSU has to get rid of ~16.5W of heat generated at this load. It could well be higher, perhaps it's closer to 20W.

Whatever the heat output, run normally, the Enermax does an excellent job of keeping cool at high or low loads, whether the outside fan is set to its lowest or highest speed. The temperature remained at ~30C for a good 2 hours, during which time I used the system for a variety of tasks.

Results: Not so Cool

The temperature of the PSU was at 30.5C when I disconnected both fans and removed the cover. The graph below shows what happened.

The temperature reached 56C in just 20 minutes. It was still climbing slowly when I connected the fans back up to prevent any possible damage. The Enermax specification about its operating temperature states:

0' C ~25'C for full rating of load, decrease to zero watts O/P at 70'C

If the output capacity of the unit drops to ZERO at 70C, then what is its capacity at 56C? Or 60C, which would have come soon? I didn't want to find out. This Enermax still has lots of other work to do before it is allowed to melt down to that electronic parts nirvana in the sky!

All kidding aside, I stopped the test because I was beginning to smell that slightly acrid tinge of electronic burning in the air. Whether it's PCBs or something more benign, that smell always makes me a little nervous. (Come to think of it, maybe it was the hot glue used to attach the internal thermistor on the coils.) But if you examine the graph carefully, you'll see that the curve was not rising so quickly by the time the test was stopped. It might have stabilized a bit above 60C or so.

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