Coolmax Taurus CF-300 Fanless ATX PSU

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For a complete rundown of testing equipment and procedures, please refer to the article SPCR's Revised PSU Testing System. It is a close simulation of a moderate airflow mid-tower PC.

In the test rig, the ambient temperature of the PSU varies proportionately with its actual output load, which is exactly the way it is in a real PC environment. But there is the added benefit of a precise high power load tester which allows incremental load testing all the way to full power for any non-industrial PC power supply. Both fan noise and voltage are measured at various loads. It is, in general, a very demanding test, as the operating temperature of the PSU reaches 40°C or more at full power. This is impossible to achieve with an open test bench setup.

The testing was conducted in the "sound lab", a 20' x 10' x 8'(ceiling) carpeted den with heavy drapes on windows across one of the short walls. Acoustics are fairly well damped. Ambient conditions during testing were 21°C and 15 dBA, with input of 120VAC at 60 Hz.


Initially, the standard test setup was used. In SPCR's PSU Testing System, there are four fans at 5V that blow the heat from the loaded resistors in the PSU load tester into the thermal simulation box. There is a slight positive airflow pressure in the box which actually helps with PSU cooling. As shown in the photo above, an exhaust Panaflo 80M fan at 5V serves to reduce the positive pressure somewhat and to better model a typical low noise system.

  • Because side of the CF-300 is open to the outside in the test rig, the vent slots on that side would act as an exhaust rather than an intake. I decided to close off those vents with a piece of cardboard. In no PC case can hot air exit the PSU to the outside in this way, while internal PSU side vents could easily be blocked.

    At the same time, the exposure of the top PSU panel to the open air also seemed unrealistic. This large radiant surface would never be exposed in a normal system setup. So the top was blocked off as well. It's true that close proximity would allow some transfer of heat to the top panel of a typical case, but I wanted to err on the stringent or conservative side in testing.

    The end result is shown in the photo on the right. Note the tiny blue thermal sensor held down by masking tape near the top of the PSU. It was positioned to be 1~2 mm from the PSU back panel.

  • During initial testing, it seemed worthwhile to ask the question,
  • What impact does the exhaust fan have on the PSU exhaust and test box temperature?
Because the PSU has no fan, the amount of air that flows through it depends on convection and the positive pressure in the case. The exhaust fan was removing some of the heat, on the one hand, but also perhaps reducing the amount of positive pressure in the case. In an attempt to answer the question, the exhaust fan was removed, thus:

After an initial minute or two of temperatures dropping and rising, once things stabilized, there was no change in temperature with the fan blowing out at 5V versus the fan completely removed. This held true for all four test loads from 65W to 200W. I believe this means that the airflow produced by the fan is effectively no greater than that created by convection alone, although it does not feel like that -- when I place my hand in front of the hole. Perhaps some cool outside air enters while some hot air exits.


The testing procedure utilized was considerably more complex than usual. Please read the notes carefully and do not simply skim the data in the table.

Coolmax Taurus CF-300
DC Output (W)
AC Input (W)
Exhaust Fan
None - see text at bottom of previous page
*See Box Text: Higher Power Testing*
Case Temp (°C)
39 / 43
- / 45
PSU Exhaust (°C)
55 / 47
- / 49
Power Factor
0.62 ~ 0.66 (higher with higher load)

NOTE: The ambient temperature during testing was 21°C. It has a direct impact on all measured temperatures. Please take this into account when comparing test data from other SPCR PSU reviews.

* Higher Power Testing and Case Airflow*

I wanted to avoid burning out the PSU while testing it at 250W and 300W. These are loads I've put on other PSUs, but the reality is that the most power hungry P4-3.2 system I've tested could only draw ~180W maximum peak in DC voltage from the PSU. So these are artificial benchmarks that are highly unlikely to be duplicated in real use. After 15 minutes at the 200W load, the temperature seen by the external probe at the exhaust side of the PSU had risen to 50°C. I was not certain how much more heat the PSU could withstand before suffering damage.

At the 250W load, without the case exhaust fan, the exhaust side temperature reached 55°C within 15 minutes. It's not clear whether the temperature had stabilized yet, but the smell of "electronic burning" (most likely caused by evaporation of thermal interface material [TIM] or the epoxy used in coils and transformers) in the lab made me very uncomfortable. So I reinstalled the "case" fan, but blowing in at 5V instead of blowing out.

My thinking was simple: I wanted to save the PSU from potential heat damage, and I knew the back fan in exhaust mode had no effect on PSU temperature. By making the fan blow in, cooler outside air would be added to the mix, and the overall air pressure in the box would be increased. This is probably something an end user might try with this PSU.

The effect was immediate. Within a couple of minutes, the internal case temperature went up by several degrees, and within 10 minutes, the PSU exhaust temperature had dropped by 8°C, down to 47°C, where it stabilized.

The data in the table for Case and PSU temps at 250W and 300W shows the result with no case fan first, followed by the temp with case fan blowing in at 5V.

The 300W load test was done for 10 minutes, only with the Panaflo 80M fan blowing in. PSU exhaust temp only went up by two degrees over the 250W load.

A NOTE on Voltage Regulation

VR is a relatively easy test for any good PSU to pass as long as it is operating within its power output limits. Many are accurate within 1%. The variances commonly found in reports from motherboard voltage sensors (such as provided by Motherboard Monitor) are not at all useful to test power supplies. The problem is that those readings are the sum of interactions between the motherboard, its power circuitry, the connectors and cables between the board and the PSU, and the PSU itself. Hence, it is difficult to identify the source of any anomalies.

The output voltage of the PSU must be monitored in isolation from external influences while it is doing work (delivering current). The only practical way to do this is to use a voltmeter (multimeter) to check the voltage across the terminals to which the power is being delivered.

In SPCR's VR testing, a multimeter is connected to each of the voltage lines for several minutes. The voltage reading is monitored continuously while the loads on each and all the lines is varied, and the peaks and valleys recorded manually.

1. VOLTAGE REGULATION is very good, well within the required -/+5% on all lines in any nominal combination of loads; it is closer to -/+2%. The low and high voltage seen on each of the main lines is shown:
  • +12V: 11.78 to 12.33
  • +5V: 4.78 to 5.22
  • +3.3V: 3.19 to 3.39

2. EFFICIENCY is exceptional at >80% through the top half of the output range. In spite of the >60% efficiency spec, the CF-300 matches the current efficiency champs among SPCR reviewed PSUs, the Enermax Noisetaker 475 and the Seasonic Super Series, Rev.A3. For a fanless PSU, especially, such high efficiency is extremely important. It minimizes the amount of self-generated heat that must be dealt with.

3. POWER OUTPUT: The unit ran with good stability all the way to the full 300W output. However, within the test rig, the CF-300 reached 50°C and higher at outputs of 200W or more. With adequate positive pressure case cooling, the unit will be as stable with high loads as any 300W PSU. However, in a low airflow, quiet-optimized PC, it's prudent to limit long-term output to below 200W, especially if the ambient room temperature is much higher than the 21°C in the lab during testing.

4. POWER FACTOR is low, as expected for what is probably a non-PFC model. It is not specified; this usually means there is no power factor correction, and the low numbers confirm it.


HOT - At all loads above 150W, the PSU was HOT to the touch. It became hot enough that if run in a system that routinely draws >150W, you might have concerns about using it in a PC that small children or pets could touch the back of accidentally. This was in a cool room, just 21°C. Higher ambient temperature will cause hotter operation.

No buzzing or humming could be discerned even at high power loads, but it must be pointed out that rare is the PSU that buzzes with the pure resistive load of the power tester. This is no guarantee that the unit will not humm or buzz when faced with the more complex (though less power-hungry) loads of PC components. Coil buzz is often the result of interactions between components.

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