Why 99% of Power Supply Reviews Are Wrong
By Gabriel Torres on April 19, 2010


Introduction

With computers (and users) asking for better power supplies, nothing more natural than reviewing websites publishing power supply reviews. But contrary to other hardware parts like CPUs, motherboards and video cards, one must have deep electronics knowledge in order to test a power supply. Since most reviewers are simply users with a above-the-average knowledge in computers – but not in electronics – almost all PSU reviews posted on the web are completely wrong and they do more harm than good, as some websites recommend products that are really flawed. We updated this article where we explain in details why 99% of power supply reviews posted on the web are wrong and we hope that reviewers learn more about the subject by reading this article and also that users learn how to identify a bad review.

Most hardware-reviewing websites do power supply reviews by taking several pictures of the product, installing it on the “reviewer” personal computer and, if the unit works (and it probably will), they say nice things about the product (just some examples: herehere, here, here and here).

Calling this kind of article a “review,” an “analysis” or a “test” is insulting to websites that do real power supply reviews. Websites can publish this kind of article but, please, call them what they are: “article,” “first look,” “first impressions,” etc. The problem is that some websites go one step further and even give “reviewed” products awards, and by giving awards without testing the product they are doing a lot of harm, as they may be recommending a flawed product. We think there is no better example of what we are saying than this review here done using the above “methodology” where the “reviewer” gave the product a “Gold Award” to a 750 W power supply that burns if you try to pull more than 450 W from it.

The ugly truth? Most hardware-reviewing websites are ran by amateurs that think they should get as much hardware parts they can get their hands on – either to upgrade their personal computers or to sell them on eBay –, even if they don’t have a clue on how to review that particular part. They simply can’t say no to manufacturers offering products. Worse than that, there are some editors that think that only because a manufacturer sent them a product “for free” they should only say nice things about the product. This is obviously a veiled form of payola and not only unethical but illegal (at least in the United States). First, getting a product for reviewing is not “for free.” The exposure the manufacturer will get on a particular website is worth thousands of dollars, since they are way more effective than traditional advertising – and reviews are posted free of change (we know of some websites that charge manufacturers to post reviews, either in cash or in advertising – DailyTech has published an excellent article on this a while ago). Plus the time and money that are spent of producing the review. Second, when sending a product for review, manufacturers are expected to get an unbiased review – i.e., the truth to be told, not mattering if the manufacturer is a personal friend of the reviewer and/or an advertiser of the website.

Now let’s talk about the next step on power supply reviews: using a multimeter.

Using a Multimeter

The methodology most used to review power supplies is just adding a multimeter on the power supply outputs and measuring if there are any fluctuations on the voltages found there. Some websites even compare the voltages found with voltage levels found on competing products (some examples here, here, here and here). The problem is, this procedure is wrong and tells us nothing about the power supply.

Reviewers that do reviews like this probably think that at least they can see if there is any fluctuation on the power supply outputs, however in reality they simply won’t be able to measure this.

The idea of measuring a power supply with a multimeter comes from linear power supplies, where the power supply has a separated voltage regulator circuit (normally done by an integrated circuit or by a zener diode, sometimes with the aid of a power transistor). In this kind of power supply it makes sense to use the multimeter to check whether the regulator circuit is working fine or not. Even in this case, simply attaching the multimeter won’t let you know if the power supply is being able to provide its labeled current/power. You will need to add a load to the power supply outputs.

On linear power supplies, as they are an open-loop system (more about this in a moment), the output voltage can increase or decrease according to the applied load – so the idea of attaching a multimeter in parallel with the load to check if there is any voltage fluctuation depending on the load makes sense.

Power supplies used on the PC use switching-mode technology, which works in a very different way. They are closed-loop systems, meaning that the power supply measures its output voltages and corrects them if there is any fluctuation. This is done by the PWM circuit, which is in charge of switching the primary transistors. In other words, if there is any fluctuation on the output voltages, the PWM circuit will know it right away, increasing or decreasing the duty cycle of the signal applied to the switching transistors in order to correct this. Since the frequency of the signal applied to the transistors are in the range of kHz, it would take only a few microseconds to the power supply to correct any fluctuation found on its outputs. And no multimeter would be capable of measuring the power supply fluctuation, if any.

Also, since the power supply found on the PC have five different outputs (+12 V, +5 V, + 5 VSB, +3.3 V and –12 V) you would need to connect five multimeters to the power supply at the same time, and publications using this methodology usually use only one, measuring the outputs in different moments in time, making the results worthless, since they were taken in different moments, with different conditions (load, temperature, etc). Even if you connected five multimeters, you would need to read them at the same time. We don’t know any human being able to read five instruments at the same time and write down the values at the same time. Even if you are really fast, you will take some seconds to make this measurement. As we already mentioned, things inside the PC power supply happens in microseconds, so seconds make a huge difference.

One way to use the above methodology correctly is by using a device to grab the value of all five outputs at the same time, like a digital data collector. The problem is that we would measure the voltages, which, once again, mean nothing. One way to make a correct power supply test using this approach is by measuring the current (and not the voltage) of the five outputs at the same time using a data collector, if you add a correct load to the power supply.

Another problem regarding the use of regular multimeters is precision. We cannot guarantee the precision of low-cost multimeters. If you add five multimeters, we cannot guarantee if the multimeters are calibrated among them, showing the exact same results when measuring the same thing.

In our experience, only very low-end power supplies fail voltage regulation tests. Since manufacturers do not send this kind of power supply for reviewing and most websites don’t buy products to be reviewed on the market (they only review what manufacturers send them), most websites will never see a single power supply failing voltage regulation.

Using an Oscilloscope

Another flaw from most power supply reviews posted around the web is not using an oscilloscope to evaluate the noise and ripple levels at the power supply outputs. Funny thing is, this equipment is not expensive (you can buy a Stingray DS1M12 PC-based oscilloscope for around USD 220). Therefore we have only two explanations for why websites do not evaluate power supplies using an oscilloscope: either they don’t have a clue on how to use one and are not willing to take the time to learn or are cheap and don’t want to spend a dime on their own business. Or both.

In theory the outputs of a power supply should present a perfect line when seen on an oscilloscope screen, but when you zoom in, you can see some “imperfections” on this line, in the order of some milivolts. ATX12V specification is very clear on the amount of noise and ripple a power supply may present: 120 mV for the +12 V and -12 V outputs and 50 mV for the +5 V and +3.3 V outputs (all values are peak-to-peak). So power supplies must not surpass these values and in fact we usually want to see outputs presenting half of these values or less to consider a power supply “excellent.”

In Figure 1, you can see a power supply with a low noise and ripple levels, while in Figure 2 you can see a flawed power supply, presenting very high levels of noise and ripple. Even a layman can see that the power supply from Figure 2 must do some harm to your computer. And it does. Units that present noise and ripple levels above specifications overload components from your computer (especially electrolytic capacitors from the motherboard and video cards) and may lead your PC to present random errors (random reset, crash, Blue Screen of Death, etc).

Power supply with low noise and ripple levels
click to enlarge
Figure 1: Power supply with low noise and ripple levels.

Power supply with high noise and ripple levels
click to enlarge
Figure 2: Power supply with high noise and ripple levels.

The problem, of course, is there are several websites recommending power supplies that have huge noise and ripple levels without knowing – because they simply don’t test this. Here is an excellent example of what we are talking about: Thermaltake TR2 750 W presents an outrageous noise/ripple level as you can see here, but got a “Top Rank Award” from this website. And like this website, there are several others like this around, recommending flawed products that can damage your computer.

Load Testing

Another major problem with almost all PSU reviews around is the use of an inadequate load.

Some websites will use a regular PC on their PSU review. The problem is that high-end power supplies nowadays can deliver at least 600 W and a regular PC is not able to pull all this power. Even if you use a very high-end PC with two CPUs, several hard disk drives and even four video cards, you won’t be able to say how much power you are pulling at a given moment and you also won’t be able to say up to how much power you were able to pull from the power supply, since you are not using any measurement device.

The best example we can give on how this methodology is flawed is that website that gave a “Gold Award” to a 750 W power supply that burns if you try to pull more than 450 W from it.

Even if you are able to build a PC that can pull a significant amount of power from the power supply, you still have a few flaws. First, you don’t know how much power you are pulling from your system. Thus you can’t measure efficiency (efficiency is the relation between the amount of DC power that is being delivered to the computer and AC power that is being pulled from the power grid), even if you have a power meter connected to the power supply. Second, the amount of power a PC pulls from the power supply varies, even if you let the computer running the same program over and over. Third, you will evaluate a power supply under only one scenario: the amount your PC is pulling from it. With only one computer you can’t evaluate how the power supply will perform under several different scenarios. And this is imperative especially if you are willing to measure efficiency, because efficiency varies according to load.

Therefore the only correct way to evaluate power supplies is by connecting them to active load testers. You can see a list of websites that have this kind of equipment here.

Conclusions

We have a motto here on Hardware Secrets: if we can’t do something correctly, we prefer not doing it at all. Between publishing a lousy review and not publishing it at all, we prefer not publishing it.

We hope you have learned how to detect a good power supply review, and you will be amazed in noting that almost all reviewing websites are using wrong methodologies that really tell us nothing about the power supply real power capabilities. Even worse, some websites are recommending bad power supplies based on flawed methodologies.

If you are a reviewer, don’t get us wrong. Our purpose here is to educate both users and reviewers, so you now have a better knowledge on power supplies and have learned what you should not do. One suggestion? Instead of calling your articles as “reviews” call them something else, like “First Impressions,” “First Look,” etc, if you are not using a real load tester.

We should point out that there are some websites doing a terrific job on power supply reviews. A list of these websites can be found here.

If you want to learn more about the subject, don’t miss our Anatomy of Switching Power Supplies tutorial and a detailed description of our power supply testing methodology.

Originally at http://www.hardwaresecrets.com/article/Why-99-Percent-of-Power-Supply-Reviews-Are-Wrong/410


2004-14, Hardware Secrets, LLC. All Rights Reserved.

Total or partial reproduction of the contents of this site, as well as that of the texts available for downloading, be this in the electronic media, in print, or any other form of distribution, is expressly forbidden. Those who do not comply with these copyright laws will be indicted and punished according to the International Copyrights Law.

We do not take responsibility for material damage of any kind caused by the use of information contained in Hardware Secrets.