Power Supply Myths

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Power Supply Myths Exposed!


1. Does a power supply upgrade mean a higher electric bill?
2. Don't be misled by exaggerated wattage claims
3. Don't lose power with modular plugs
4. Marketing myth vs. warranty reality
5. An sli certified power supply will always power high-end graphics cards?
6. Should an ATX power supply be cooled with a 120mm fan?
7. Are two power supply fans better than one?
8. Are multiple 12-volt rails better than a single 12-volt rail?



1. DOES A POWER SUPPLY UPGRADE MEAN A HIGHER ELECTRIC BILL?


This is the biggest myth of all. First, it's important to understand that a power supply only delivers the power that's needed by the system, nothing more. If your PC currently has a 400W supply and the system needs 350W, it will still need and use only 350W - if the only change is upgrading to a 500W power supply (the upgrade makes sense since there are many advantages to running a power supply at a lower percentage of its rated capacity). Whether the electric bill goes up or down is solely determined by the efficiency of the new power supply. Greater efficiency means a lower electric bill because more of the AC power is converted into DC for the computer, rather than wasted as heat. The savings can really add up over time. For example, when the money saved in electricity over the course of its 3-year warranty is taken into account, the purchase price of the efficient Silencer 750 is less than $0.00. Here's the math:
Model Efficiency @ 550W Input Power @ 550W
Silencer 750 83% 662W
Antec True Power 550 73.8% 745W

For the same output power, the Silencer 750 uses 83W less input power.

3-year savings = .083KW x $0.10/KWH x 24 x 365 x 3 = $218.12

2. DON'T BE MISLED BY EXAGGERATED WATTAGE CLAIMS


To properly compare power supplies, wattage claims must state the maximum ambient temperature for continuous, full-load operation. Unfortunately for the consumer, this information is usually withheld, opening the door for manufacturers to exaggerate their wattage claims. They do so by assuming an unrealistic ambient temp of only 25°C (77°F), even though the actual internal power supply temp is at least 40°C (104°F). Since the proper full-load rating is 15°C higher for home use and 25°C higher for industrial use, these power supplies produce 33%-50% less power than their advertised ratings. See the derating chart on the right.

Elevated Temperature Testing
wattage comparison
Turbo-Cool 510 spec: full load @ 50°C.
"500W" Home PSU spec: full load @ 25°C, no load @ 70°C.

3. DON'T LOSE POWER WITH MODULAR PLUGS


Due to their look, convenience, and cost savings for manufacturers, modular plugs have become a popular power supply feature. Unfortunately, there has been little or no discussion of the impact of this feature on overall performance and reliability. The fact is, modular plugs limit power by adding to electrical resistance. The voltage drop can be as much as would occur in 2 feet of standard wire. Worse yet, modular plugs utilize delicate pins that can easily loosen, corrode, and burn, creating the potential for a major system failure. That's why professional system builders specify uninterrupted wire!

4. MARKETING MYTH vs. WARRANTY REALITY


As with all consumer products, the fine print in the warranty will often contradict a manufacturer's marketing hype. Unfortunately, this is especially true for computer power supplies. Take for example this quote direct from the product box of a leading manufacturer: "we've earned a stellar reputation for producing stable, reliable, industrial-grade PC power supplies". Now compare that statement to what is specifically excluded in the company's "legendary 3-year warranty":

Sample of What is NOT covered -

a. commercial and industrial use
b. wear and tear from moving parts
c. that the product will meet your requirements.

In conclusion, don't be mislead by marketing hype, avoid superficial gimmicks, and read the warranty carefully before purchasing your next power supply.

5. AN SLI CERTIFIED POWER SUPPLY WILL ALWAYS POWER HIGH-END GRAPHICS CARDS?


We went through three power supplies before we found one that consistently worked with these high-end graphics cards and an FX-60. We initially started with a Silverstone 600W SLI certified power supply, but running some games resulted in the system powering down under full load. We next moved onto a higher rated Thermaltake PurePower SLI certified 680W unit. But while most games ran most of the time, the system would still occasionally shut down. Finally, we ended up using PC Power and Cooling's massive TurboCool 850 SSI. This just goes to show that if you really want to build something that pushes the bleeding edge, make sure you have the right power supply.

ExtremeTech Review Feb 12, 2006

6. SHOULD AN ATX POWER SUPPLY BE COOLED WITH A 120MM FAN?


Most low-noise ATX power supplies today utilize a top-mounted 120mm fan rather than a rear-mounted 80mm fan. The 120's favorable reputation is based on the fact that under low to medium load conditions, the 120mm fan provides sufficient cooling at low RPM and low RPM fans are generally very quiet.

However, problems occur with this design when the load exceeds 50%-60%. Because the 120mm fan consumes about 1.5" of vertical space inside the PSU, heat sinks, capacitors, and other components are about 30% smaller in height compared to a PSU with a rear-mounted fan. The smaller parts can handle less current, so the maximum power available with the 120mm design is limited. And, because the heat sinks have less surface area, more air flow is needed with this design to keep the thermal situation under control. With 80%-100% load, the 120's fan speed can double and the noise level can jump by up to 20dB.

In conclusion, for systems that require more than 50% of the power supply's capacity, a well-engineered PSU with a rear-mounted 80mm fan will provide superior performance and reliability (due to larger components) at a noise level comparable to a PSU equipped with a 120mm fan.

7. ARE TWO POWER SUPPLY FANS BETTER THAN ONE?


No. A power supply with two fans doesn't exhaust any more air from the case than a power supply with one. That's because due to space limitations, only one fan can be used for exhaust, while the other is limited to spot cooling. Possible explanations for a 2-fan PSU include: thermal engineering problems; an attempt to exaggerate the wattage rating by spot cooling the transformer; or an attempt by the marketing department to create a new gimmick.

8. ARE MULTIPLE 12-VOLT RAILS BETTER THAN A SINGLE 12-VOLT RAIL?


With all the hype about multiple 12-volt rails (ads claim that two rails is better than one, five is better than four, etc.), you'd think it was a better design. Unfortunately, it's not!

Here are the facts: A large, single 12-volt rail (without a 240VA limit) can transfer 100% of the 12-volt output from the PSU to the computer, while a multi-rail 12-volt design has distribution losses of up to 30% of the power supply's rating. Those losses occur because power literally gets "trapped" on under-utilized rails. For example, if the 12-volt rail that powers the CPU is rated for 17 amps and the CPU only uses 7A, the remaining 10A is unusable, since it is isolated from the rest of the system.

Since the maximum current from any one 12-volt rail of a multiple-rail PSU is limited to 20 amps (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. With power requirements for multiple processors and graphics cards continuing to grow, the multiple-rail design, with its 240VA limit per rail, is basically obsolete.

PC Power and Cooling is once again leading the industry. All of our power supplies now feature a large, single 12-volt rail. The design is favored by major processor and graphics companies, complies with EPS12V specs (the 240VA limit is not a requirement) and is approved by all major safety agencies such as UL and TUV.3

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