How Much Power Can a Generic 500 W Power Supply Really Deliver?
By Gabriel Torres on March 15, 2008
We decided to take a generic 500 W power supply and test it using the same methodology we use for “branded” units. The idea behind this review is to be as much educational as possible and answer a few very important questions: how much power a generic power supply can really deliver? What are the differences between a generic power supply and a “branded” unit? Is there any danger for my equipment to use a generic power supply? Why generic units cost so little? Read on and see our findings.
We call “generic” any cheap very low-end product that we can’t find out who the manufacturer is – normally because the manufacturer doesn’t want to be found! In the case of power supplies, generic units cost just a few dollars and usually come for free on low-end cases. No attention to the product finishing is given and they follow the traditional ATX layout, with an 80 mm fan on the rear and some venting holes on the front of the unit. They are smaller and far lighter than “branded” power supplies. If fact back in the days that good power supplies were hard to find one trick several technicians used to select a power supply among several was by selecting the heaviest one. They also have an AC outlet on the rear for you to connect your video monitor and this feature isn’t seen anymore on “branded” units.
Here we can see the first important difference between a good power supply and a generic one: cooling. Even when a good power supply uses just one 80 mm fan on the rear it has far more venting holes on the front (with several units completely replacing the front panel with a mesh), which improves airflow and prevents your computer from overheating. The power supply is a key element in the computer heat dissipation, as it is in charge of pulling hot air from inside the computer (through these holes) to the outside, through the power supply fan. In Figure 3 we illustrate this.
Of course to cut costs generic power supplies don’t have PFC circuit (read more about PFC on our Power Supply Tutorial) and also we assume that they have a low efficiency, below 70%, but of course we will measure efficiency during our tests with this power supply. The higher the efficiency the better – an 80% efficiency means that 80% of the power pulled from the power grid will be converted in power on the power supply outputs and only 20% will be wasted. This translates into less consumption from the power grid (as less power needs to be pulled in order to generate the same amount of power on its outputs), meaning lower electricity bills.
Another main external difference between a good power supply and a generic one are the cables used. Generic power supplies use wires thinner than necessary, usually 20 AWG. The minimum required for today’s standards is 18 AWG. Also generic power supplies come with fewer cables than “branded” units. This generic power supply we bought, for example, had only two cables, one with two standard peripheral power connectors and another also with two standard peripheral power connectors plus one floppy disk drive power connector, plus the main motherboard cable and an ATX12V cable – no SATA power connectors (even though there are generic units with these connectors around) and no auxiliary power connector for the video card.
Power distribution is also a very important difference between generic units and “branded” ones. Generic units are usually based on the very first ATX specification, which was written at a time when computer power consumption was concentrated on the +5 V line. Nowadays power consumption is concentrated on the +12 V outputs, as the CPU (through ATX12V and EPS12V connectors) and video cards are connected to the +12 V output, not +5 V. So usually generic units have a higher current limit on the +5 V line while current good “branded” power supplies have a higher current limit on the +12 V outputs. There is also a major difference on how the +3.3 V output is obtained, but we will discuss this later in details.
But the most traditional “feature” of generic power supplies is that they can deliver far less power from what is printed on their label. Our power supply was labeled as a 500 W unit and one of the goals of this review is to check what the real wattage of this unit is.
There are several ways power supply manufacturers can use to label their power supplies:
Now that you know what are the external differences between a generic power supply and a “branded” one, let’s see the differences inside.
On the pictures below you can have an overall look from our generic 500 W unit. One difference between this unit and “branded” units we could see right away was the gauge of the wires used on the AC connection (18 AWG) and on the 110/220 V switch (20 AWG), way thinner than the ones used on good power supplies. The thicker the wire, the more current it can transport.
Other visible differences include the size of the printed circuit board (smaller on generic units), the size of the main transformer (smaller on generic units, meaning less current/power it can deliver), and the number of available components (fewer components on generic units).
Power supply manufacturers reduce the cost of the power supply by using cheaper components and simply removing components. On the primary you can see that practically all components from the transient filtering stage are missing and on the secondary less capacitors and coils are used on the filtering stage.
The recommend components for the transient filtering stage are two ferrite coils, two ceramic capacitors (Y capacitors, usually blue), one metalized polyester capacitor (X capacitor) and one MOV (Metal-Oxide Varistor). This generic 500 W power supply has only the two Y capacitors, all other components were removed. If you pay attention on the printed circuit board you will notice that the places for these other components exist and they are probably used on a “upgraded” versions of this power supply (ZNR1 and ZNR2 for the MOV’s, CX1 for the X capacitor and LF1 for the ferrite coil, see Figure 7).
In the next page we will have a more detailed discussion about the components used in the our generic 500 W unit.
As we explained, one of the ways power supply manufacturers can cut costs on cheaper units is by using cheaper components. With semiconductor components (diodes and transistors) they accomplish this by using components with lower current (and thus power) limits.
On the primary side of the power supply, generic units usually use four discrete diodes instead of a rectifying bridge – which is a component that has four diodes inside. These diodes can be see in Figure 7, present in the previous page.
This generic 500 W unit uses four 1N5408 diodes, which can handle up to 3 A each, rated at 105° C. “Branded” power supplies use rectifying bridges that can handle at least the double from that. At 115 V this unit would be able to pull up only 345 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver only up to 276 W without any diode burning.
On the switching section generic power supplies use regular power BJT transistors instead of power MOSFET transistors, using the half-bridge configuration, which is the configuration traditionally used by power supplies without active PFC. On a generic unit it is expected that the amount of current each transistor can handle to be lower compared to “branded” units, as the manufacturer chooses to use cheaper components.
Our 500 W generic unit uses two 2SD13007K transistors. Unfortunately we couldn’t find its datasheet so we can’t comment on its maximum rated specs. The third transistor in Figure 8 is for the +5VSB power supply, which is independent from the rest of the power supply.
Now let’s take a look at the secondary of this power supply.
This power supply uses three Schottky rectifiers on its secondary, one for each main voltage (+12 V, +5 V and +3.3 V). We were surprised to see an independent rectifier for the +3.3 V output on this power supply – which is the same design used by current good power supplies –, as on very old power supplies the +3.3 V output is obtained by a voltage regulator connected to the +5 V output and we were expecting to see this happening on this unit.
Like we explained, one of the way of cutting costs is using cheaper components, which deliver less current.
To calculate the maximum theoretical current on power supplies based on half-bridge topology is easy: all we need to do is to add the maximum current each diode can handle.
The +12 V output is produced by one F12C20C power rectifier (not a Schottky device like in all other power supplies; this can be translated into a lower efficiency, since regular diodes have a higher voltage drop compared to Schottky devices; translation: higher waste, lower efficiency), which can deliver up to 12 A (measured at 125° C), which equals to 144 W. The maximum current this line can really deliver will depend on other components, especially the transformer, the coil and the wire gauge used. It is clear by the rectifier used that this power supply could never be a 500 W unit. The joke is that the label present on this power supply says that it can deliver up to 20 A on the +12 V output, which is a big fat lie, as the rectifier itself can only deliver 12 A – and as the current limit depends on other components rarely we can pull everything the rectifier can deliver.
The +5 V output is produced by one SBL2040 Schottky rectifier, which supports up to 20 A (measured at 95° C). So the maximum theoretical power the +5 V output can deliver is of 100 W. Of course the maximum current (and thus power) this line can really deliver will depend on other components, especially the transformer, the coil and the wire gauge used, as mentioned before. The joke is that the label present on this power supply says that it can deliver up to 40 A on the +5 V output, which is a big fat lie, as the rectifier itself can only deliver 20 A – and as the current limit depends on other components rarely we can pull everything the rectifier can deliver.
The +3.3 V output is produced by one SB1040CT Schottky rectifier, which supports up to 10 A (measured at 25° C), which equals to 33 W. As we explained the real limit depends on other factors. The joke, once again, is that the label present on this power supply says that it can deliver up to 28 A on the +3.3 V output, which is a big fat lie, as the rectifier itself can only deliver 10 A – and as the current limit depends on other components rarely we can pull everything the rectifier can deliver in theory.
From the numbers above we can clearly see that this power supply is, at best, a 290 W unit: 144 W (+12 V) + 100 W (+5 V) + 33 W (+3.3 V) + 10 W (typical value for the +5VSB output) + 6 W (typical value for the -12 V output). Keep in mind that we are adding here only the maximum theoretical power each rectifier can deliver, the real amount of power a power supply can deliver depends on other components.
On the secondary we could clearly see that the printed circuit board had places for the installation of more coils and capacitors on the filtering section, which were removed for cutting costs (the coils were replaced by wires).
As this is a very low-end power supply, it doesn’t have a thermal sensor, component found only on power supplies where the fan rotates according to the power supply internal temperature and/or implements over temperature protection (OTP).
Talking about protection this is also a way for the manufacturer to cut costs: simply not implement any kind of protection at all, especially over load protection (OLP, also known as OPP, over power protection), which is important for preventing the power supply from burning if you pull more power than it supports. This power supply, however, is based on a chip (Weltrend WT7514L) that provides under voltage protection (UVP) and over voltage protection (OVP).
On this power supply the big electrolytic capacitors from the voltage doubler are from Canicon (a Taiwanese company) and rated at 85° C, while the electrolytic capacitors from the secondary are from Canicon and Jun Fu and rated at 105° C.
We conducted several tests with this power supply with the equipment described in the article Hardware Secrets Power Supply Test Methodology.
Since we didn’t know beforehand what the real wattage of this power supply was we did something different from what we usually do when reviewing power supplies. We loaded this power supply with 50 W and then started increasing the load pattern in 25 W increments until we reached the maximum this power supply could deliver – i.e., until we burned it. We knew that we would burn this power supply for sure, we only didn’t know when.
Our generic 500 W power supply died when we tried pulling 275 W from it, so the maximum amount of power we could extract was 250 W – half the labeled amount! This value matches the components that were used. In the table below we summarize how tests with this power supply delivering 250 W. The value listed under “total” was the total amount of power the unit was actually pulling, as measured by our load tester
11 A (132 W)
15 A (75 W)
9 A (29.7 W)
1.5 A (7.5 W)
0.5 A (6 W)
Ripple and Noise
Power Supply Temperature
The power supply died silently, no explosion happened. After disassembling the power supply we measured all main components and what burned was the +5 V rectifier.
Room temperature was below what we usually use because we couldn’t increase temperature inside our “hot box” since the power supply wasn’t pulling too much power and thus not heating enough.
Voltage regulation during our tests was o.k., with all outputs within 3% of their nominal voltages – ATX specification defines that all outputs must be within 5% of their nominal voltages – except +5 V, which was at 4.81 V when we were pulling 250 W from the power supply. This value, however, is still inside the 5% tolerance set by the ATX standard. Of course we want to see all voltages as close as their nominal values as possible.
Efficiency was surprisingly high for a generic unit; we were expecting something below 70%. The best value was when we were pulling 100 W (78.7%) and the worst value was when we were pulling 50 W (73.2%).
But the main problem with this generic unit was noise and ripple. This is something regular users don’t even think about: most users choose a power supply solely based on its wattage, paying no attention on how clean the outputs are.
Outputs from the power supply are continuous voltages and when watching them on an oscilloscope they should be a straight line on the screen. This, however, doesn’t happen; outputs aren’t perfectly continuous. They can have a little oscillation (called ripple) and, on top of that oscillation, some little spikes (called noise). If the value of this oscillation and spikes are low enough they won’t offer any risk of damage to your equipment.
ATX specification says that ripple and noise should be within 120 mV for the 12 V outputs and 50 mV for the +5 V and +3.3 V outputs for the outputs to be considered safe for the electronic components used inside the PC.
The problem with this generic power supply is that its noise level was above those values all the time! When we started at 50 W noise level at +5 V output was already at 105 mV! When delivering 250 W noise level at +5 V output was at 220 mV and at +12 V output was at 180 mV!
So even if your equipment isn’t pulling a lot of power – for example, you have a very basic PC with a low-end video card or even on-board video –a generic power supply can cause you trouble because of the amazingly high noise level (caused by the removal of the coils and capacitors from the filtering stage in order to cut costs). Have you ever heard of instability problems solved by replacing a generic power supply with a “branded” one, even when the computer wasn’t pulling a lot of power? Well, this explains it. The bottom line is: wattage is not everything.
This also explains why we say that 99% of power supply reviews around the web are wrong: since the majority of websites don’t have an oscilloscope, they simply can’t see something horrible like this happening. A power supply being able to deliver voltages on their correct levels means nothing; we need to know how clean these voltages are.
Just a clarification before anyone asks. We said that noise at +5 V output was at 220 mV but on the chart below you are seeing a 160 mV pattern. What happens is that the power supply was producing a lot of fast high spikes that aren’t being shown on this captured screen.
For you to visualize how bad this is, we posted below the noise level measured on the +5 V output from a true 500 W power supply, Antec Earthwatts 500 W, with the exact same load pattern described on the table above (noise level here was below 20 mV). Both charts are in the same scale (2 ms T/div and 0.02 V/div).
In this review we proved what everybody already knew: that generic power supplies cannot deliver their labeled power. Worst than that: manufacturers deliberately lie about the power supply power rating, as there is no math in the world that explains how a 250 W power supply can be labeled as 500 W.
We also showed you the main differences between a generic power supply and a “branded” one and where the manufacturer cut costs. Generic power supplies use thinner wires outside and inside the power supply, they simply don’t have a transient filtering stage, they use cheaper components with lower current/power limits, they don’t have additional but important protections like overload protection and they simply remove components (electrolytic capacitors and coils) from the power supply filtering section, what increases the noise level on the power supply outputs.
Noise level is the main problem with generic units. On this generic power supply we reviewed noise level was outside specs at any wattage we pulled from the power supply. When pulling 250 W from this unit, noise level at +5 V was at 220 mV, more than four times above the limit.
This explains why some stability problems (i.e., computer freezing, computer rebooting by itself, etc) on computers using a generic power supply are solved by replacing the power supply with a “branded” one, even when your computer isn’t pulling a lot of power.
The bottom line: avoid generic power supplies. Even entry level computers should use a decent “branded” power supply. Most manufacturers provide cheap entry-level models below USD 50 that will provide clean outputs and make your computer to work fine and protect its components from damaging – so the price excuse is simply inadmissible. So moving from a generic power supply to a branded one isn’t only about allowing your computer to pull more power, but also providing cleaner voltages to it.
Even though with educational information provided by our website and several others on the web the computer power supply is still the most neglected component in the PC. Several users are very picky about all other components for their new PC but when it comes to the power supply they simply choose the cheapest one. Of course you don’t need to buy an expensive high-wattage power supply for a mainstream PC, but using a generic one can really hurt your PC – who likes to have a PC that crashes all the time?
This review also helps us to explain why 99% of power supply reviews posted around the web are wrong. Most “reviewers” simply test power supplies by installing them on a PC and measuring voltages with a multimeter. Not only a typical PC won’t pull the amount of power necessary to say whether a given power supply can really deliver its rated power or not, but by just using a multimeter these “reviewers” don’t have any idea of the noise level on the power supply outputs, several times saying that a power supply is good just because the manufacturer was nice enough to send them a free sample when in fact the power supply is flawed as it is producing too much noise that can make the computer to work unstable.