When you are shopping for a new power supply, take several factors into account. First, consider the power supply’s shape, or form factor. Power supply form factors can differ in their physical sizes, shapes, screw-hole positions, connector types, and fan locations. When ordering a replacement supply, you need to know which form factor your system requires.
Some systems use proprietary power supply designs, which makes replacement more difficult. If a system uses one of the industry-standard form factor power supplies, replacement units with a variety of output levels and performance are available from hundreds of vendors. An unfortunate user of a system with a nonstandard form factor supply does not have this kind of choice and must get a replacement from the original manufacturer of the system—and usually must pay a much higher price for the unit. PC buyers often overlook this and discover too late the consequences of having nonstandard components in a system.
Name-brand systems on both the low and high end of the price scale are notorious for using proprietary form factor power supplies. For example, Dell has used proprietary supplies in many of its systems. Be sure you consider this if you intend to own or use these types of systems out of warranty or plan significant upgrades during the life of the system. Where possible, I always insist on systems that use industry-standard power supplies, such as the ATX12V form factor supply found in most systems today.
With backward compatibility ensuring that the new 24-pin ATX power connector will plug into older 20-pin motherboard sockets, when purchasing a new power supply, I now recommend only those units that include 24-pin main power connectors, which are usually sold as ATX12V 2.x, EPS12V, or “PCI Express” models. For the most flexible and future-proof supply, also ensure that the power supply includes two or more PCI Express graphics connectors as well as multiple integrated SATA drive power connectors. Choosing a power supply with these features provides flexibility that allows it to work not only in newer systems, but also in virtually all older ATX systems—and with no adapters required.
As a guide, here are some of the features I recommend looking for in a PSU:
- Adequate power connectors (24-pin main, 4/8-pin +12 V CPU, 6/8-pin PCIe Graphics, SATA, and so on) for the intended system
- Adequate power output (watts) for the intended system
- 80 PLUS certification
- Active Power Factor Correction (required with 80 PLUS)
- SLI and/or Crossfire certification
- Single +12 V rail design
There are other variables to consider, depending on your specific needs or desires. One feature that many people like is modular cables, which minimize the clutter in a system. Another feature to consider is noise, which is mostly related to cooling. The type and arrangement of cooling fans has a great effect on how quiet (or noisy) the unit will be. There are even some fanless units that are completely silent, but these usually come at a premium price and with a lower overall power output capability.
When building systems with case windows, some people also like to look for PSUs with appearance-related features like colored cases.
One feature often discussed in relation to PSUs is the use of modular cables. This means cables with connectors at both ends that are detachable from the power supply. Modular cables allow you to attach only the cables you need—in some cases greatly reducing the congestion inside the system.
The main argument against modular cables is that additional resistance is introduced via another set of connector contacts. This is true, but how much resistance exactly, and is it enough that it really matters? Fortunately, this can easily be calculated.
The connectors used in modern power supplies are mostly Molex Mini-Fit Jr. types, which have a contact resistance of 10 milli-ohms (0.01 ohms). Most power supply cables use 18 AWG (American Wire Gauge) copper wire, which has a resistance of about 0.0064 ohms per foot. This means that adding an extra connector at the PSU end is equal to about 1.5 feet of wire in additional resistance.
To put it another way, in a maximum load situation, each terminal normally carries a maximum of about four amps, at which point the additional resistance equals about 0.16 watts of power loss. In an eight-pin power connector, this only adds up to around a watt, a loss I consider negligible.
Finally, when you consider that a typical PSU cable already consists of 1.5 feet of wire with a connector on the end, adding another connector to make the cable modular only adds about one-third more overall resistance to what is already there, which was negligible to begin with.
If modular cables aren’t much of a problem technically, why don’t more PSU manufacturers include them? Well, besides the (negligible in my opinion) extra resistance, they do add to the cost of making a power supply, and that is reflected in a higher final price. They can also create clearance issues with other components in the system, depending on exactly where the connectors attach to the PSU. In addition, modular cables can easily become lost or misplaced. Think of opening a system to add another internal drive or upgraded video card several years after it was initially built, finding that the PSU uses modular cables, and discovering the extra cables needed are nowhere to be found. One solution to this problem is to place any unused cables inside the case when building a system. For example, you could place them in a small plastic bag and tape them inside, so that if or when you need them in the future, they are easy to find. Besides these issues, perhaps the biggest drawback to modular cables is that modular PSU cables using standard connectors are patented (www.google.com/
patents/about?id=w0ehAAAAEBAJ), and the patent is owned by Systemax (aka TigerDirect and Ultra Products). There is another patent on modular PSUs that use nonstandard connectors at the PSU end (www.google.com/patents/about?id=iOGqAAAAEBAJ). If there was no legal “baggage” against using them, I suspect we would see more modular cable equipped PSUs on the market today.
- Power-Use Calculations
- Power Savings: 80 PLUS, Energy Star, Advanced Power Management
- Power Savings: Advanced Configuration And Power Interface
- Power Cycling
- Power Supply Troubleshooting: Basics, Overloading, Cooling
- Power Supply Troubleshooting: Test Equipment
- Power Supply Recommendations
- Power-Protection Systems: Surge Protectors And Line Conditioners
- Power-Protection Systems: Backup Power Options
- Real-Time Clock/Nonvolatile RAM (CMOS RAM) Batteries