How Much Power Can a Generic 500 W Power Supply Really Deliver?

Secondary Analysis

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.

Generic 500 W Power SupplyFigure 9: +5 V, +12 V and +3.3 V rectifiers.

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.

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Author: Gabriel Torres

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.

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