Guide: What We Mean by "Low Quality" Power Supply


This is a guide to all those who have been told on this forum that their power supply is low quality, and are like, "huh?" This is meant to be informative as to what we mean when we say a power supply is low quality.

The first misconception that needs to be cleared is that rated wattage should be ignored. Everyone knows what rated wattage is - one may say, I have a "700W power supply". Power supplies included rated, or combined wattage, usually on the box and in the name, so everyone refers to the power supply by its rated wattage. It is more complex than that.

The power supply needs to provide direct current of different voltages to meet the needs of different components and parts of the components of a PC. We use the term "rail" to describe the different cables and the voltages they deliver. As a simple analogy, think of long tubes of water, and each tube carries a different liquid. The machinery the tubes lead to can only run on that liquid. It is similar in a power supply, with certain cables carrying currents with different voltages.

The primary rails are the 12V, 5V, 5VSB, and 3.3V. There are negative rails but they aren't used in modern day PCs anymore nor in every power supply anymore. The simple equation of voltage x amperage = wattage is important, because each rail has its own specified, or rated, amperage. Below is an example of a load table:

What you primarily want to look at is the 12V rail. Today, the 12V rail carries the majority of the weight and powers many components, including the hungry CPUs and GPUs. The minor rails also supply power to these, but only to electronics which draw less wattage. So when looking for a quality power supply, try to find one with a good amount of amperage on the 12V rail, particularly one that has the unit's overall rated wattage alone on that 12V rail, or on multiple 12V rails.

Voltage Regulation
You want your 12V rail to actually be delivering 12V. You don't want the 12V rail outputting 11V, do you? Of course not! Not only can that be dangerous to the computer, but it would be indicative of a poor quality power supply. This is where voltage regulation comes into play. Because the power from a wall outlet is an alternating current, its voltage is constantly fluctuating, so a power supply runs it through components to stabilize it into a proper voltage. But it's not easy to make it the exact number of volts required.

Line regulation is a test of how offset the rails' voltages are from what they should be. According to the ATX specification, the 3.3V rail is allowed to range from 3.14V-3.47V. The 5V rail is allowed 4.75V-5.25V. The 12V rail is allowed 11.4V-12.6V. This is +-5% in both directions that is allowed. The closer to the nominal value, the better.

Load regulation is more important than line regulation. When a power supply is under low load, voltage tends to be higher. As load increases, voltage decreases. Load regulation checks by how much the voltage actually drops from a low load to a high load. 5% of a drop is allowed, so going from 12.3V to 11.7V would be just on the brink of being out-of-spec. Having good voltage regulation is important for the health and lifespan of components.

Ripple Suppression
Remember how I said the alternating current from the wall has a fluctuating voltage? Unfortunately, some of these characteristics are present even in the output of the power supply. This is called ripple. Ripple is a small wave-like change in the voltage on the output of a rail. The smaller this wave is, or the less amplitude it has, the better. Ripple suppression is measured in millivolts. 1000 millivolts make up 1 volt, so it is very small, but can have a major affect. Noise is also important. Noise are the little spikes, whereas ripple is the actual fluctuation from top to bottom. Noise are even smaller spikes on that fluctuation.

Here is an example of a poor-performing power supply:

That is out-of-spec ripple on the 3.3V rail of an Apevia power supply. It is greater than 50mv, which is the limit for the 3.3V and 5V rails. The 12V rail limit is 120mv. Here is a good quality unit:

Now which one do you think ensures a healthy power delivery and running of your computer? Again, good ripple only help your components last longer than poor ripple.

Power supplies are supposed to have protections which shut them off in case of an emergency to prevent damage. Unfortunately, some companies don't implement many of these protections, or they make the threshold so high that they never shut off the unit on time, causing it to literally blow or catch fire. Two important protections are overcurrent protection and over power protection. Over current protection will shut off a power supply if it notices any one rail is delivering more current, or wattage, than it should. Sometimes overcurrent protection is only implemented for the primary 12V rail. This protection is good in case someone installs too power-hungry of a graphics card for the unit, for instance. It can shut itself off. Multiple 12V rails just means multiple overcurrent protections. Ideally, each wire should have its own protection, but it's far too expensive.

Over power protection monitors how much total power is being drawn from the wall, and will shut itself down if it detects too much. Over temperature protection is present on high quality units. You should be able to guess what it does. There is also over voltage and under voltage protection, which use an integrated circuitboard to monitor the outputs of the power supply and shut it down if voltage is unstable (remember I talked about it being out-of-spec?). Many poor quality units don't have all functioning protections.

There is always controversy stirring over capacitors. Below is an example of an electrolytic capacitor:

A capacitor's job is used to regulate voltage or ripple and bring it to a more stable or particular value. Point be told, they are an important part of a power supply. Poor quality capacitors will leak much earlier, or the sealant will break on it. When this happens, voltage gets out-of-whack. If voltage gets out of whack, and overvoltage or undervoltage protections are not working, guess what happens? Damaged computer perhaps.

The Japanese are notorious for their high quality capacitors. It is simply true that they have superb quality. The Chinese, Taiwanese, Koreans, and Americans even make capacitors, which all vary in quality. Many argue they're getting a lot better, but that's certainly not true for all brands, and cheap power supplies often use cheap capacitors that will die very quickly, especially under heavy load. Temperature also plays a role in decreasing capacitor lifespan.

I am just a simple person who likes power supplies. I do a lot of research on them and was hoping I could formalize it in a simple manner that some people could read to get a good idea on power supply quality. I also used some non-technical terminology such as a power supply "supplying" power just for the sake of understanding, as power supplies do not supply anything, the charges are already present. So please don't be on my back for that.

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