Cheap power supplies

[mjc]

There are many on this board who go on and on about cheap power supplies, especially those that are included with cases. Often times we don't supply reasons why we don't like the things...

This article should help to spell it out...

http://www.hardwaresecrets.com/article/534/1

The interesting thing that I noticed, was unlike many/most PSU review articles, this one puts the thing on an oscilloscope and shows, that even at low levels of power consumption, the cheap PSU is pure crap.

[Ajmukon]

ouch- why i recommend thermaltake, OCZ, Antec, Zantec or Coolermaster...
to all posters...

[mjc]

I've seen techno-pop and heavy metal that has cleaner waveforms than that PSU...

[Ajmukon]

it took me about.. 10 minutes to get the joke, but it was very funny!

[Mia]

Quality and reasonable pricing is the trick...
<a href="http://www.12vadapters.com">12V Power Supply</a>

There are many on this board who go on and on about cheap power supplies, especially those that are included with cases. Often times we don't supply reasons why we don't like the things...

This article should help to spell it out...

http://www.hardwaresecrets.com/article/534/1

The interesting thing that I noticed, was unlike many/most PSU review articles, this one puts the thing on an oscilloscope and shows, that even at low levels of power consumption, the cheap PSU is pure crap.

[SufferWell1396]

lololololol
fail attempt at spam.
reported.

[Ajmukon]

which reminds me, this should be made a sticky as a warning to those who buy cheap PSUs...

[Mini-Me]

From the article:

Label the power supply with peak wattage, which can only be achieved during some seconds and, in some cases, in less than one second.

This is the equivalent of the VERY NASTY PMPO way of labeling stereo equipment these days.

It is very common to see in electronic stores, stereo's rated at 777W PMPO or even far up into the thousands of watts.

PMPO - Peak Music Power Output is measured like with the PSU's we are talking about here, at the moment just before the amplifier self-destructs, and consists of normally more then 40% THD(total harmonic distortion) - you could not listen it it like that - it would sound apalling - and would also most likely destroy the speakers in the process...

777W PMPO = approx 45W RMS(root-mean-squared), which is the "Real" power from the amplifier.

IT IS NOT FALSE ADVERTISING to put 777W PMPO, as the amp really can do that, but the unit of measure is totally useless.

However, I think this is certainly MISLEADING advertising, but in NZ at least, there is no law against that - yet.

Any USA members here: Is it legal over there, to advertise stereo amp power in PMPO, or do you guys have to label using RMS?

Measure the power supply maximum wattage with an unrealistic room temperature, normally 25&#186; C (77&#186; F), while the temperature inside the PC will always be higher than that – at least 35&#186; C (95&#186; F). Semiconductors have a physical effect calling de-rating where they lose their ability to deliver current (and thus power) with temperature. So a maximum power measured at a lower temperature may not be achieved when temperature is increased.

Agreed.

Simply lying, this is probably the case with generic units.

Agreed!!!!

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.

While this is true, it has to be pointed out, that the mains current on any SMP(switchmode PSU) "Hot" side(mains voltage), is no-where NEAR the secondary current on the "Cold" side(low voltage side), where thick wire is most definitely needed for both current handling ability, and also to reduce voltage-drop. Generally speaking, there can be an in-rush current on the hot-side of up to 30-amps or more, for up to half a second, but once the PSU is running, the primary current is normally only an amp or two, usually less, and therefore, thin wire is perfectly acceptable for connections to the hot-side, provided the insulation is rated for the voltage. 18AWG is good for 7.5-amps continuous - this is equal to 1725-watts @ 230v or 862.5-watts @ 115v, and no PSU would EVER suck this much from the mains once started, so I consider this to be a false statement in this article. The article states that the AC Power consumption was 339-watts(page 5): This equals 1.47-amps @ 230v, or 2.94-amps @ 115v, therefore the 18AWG wire they talked about on the hot-side is perfectly adequate, as the maximum load is basically 3-amps, and 18AWG is good for 7.5-amps continuous.

Thin wires on the cold-side, can actually heat up quite a bit on these cheap bits of crap PSU's, as if you load the rails up heavily, the wires coming from the PSU actually get hot to the touch, as a thin wire has a higher resistance then a thicker one(and also a corresponding difference in voltage-drop figures: thicker has less voltage-drop, and is always better for low-voltage high current applications)

Wires on a PSU cold-side should always be at least 1mm-squared(18AWG, good for 10-amps) on the low-power outputs such as hard-drive connectors, and at least 2.5mm-squared(10AWG, good for 25-amps) on the mainboard/CPU connectors.

This generic 500 W unit uses four 1N5408 diodes, which can handle up to 3 A each, rated at 105&#186; C. “Branded” power supplies use rectifying bridges that can handle at least the double.

These are fine, and do the exact same job as a rectifier bridge, EXCEPT that if the primary current was in the order of 2-amps or so, then yes - they would run hot, and I would not design them in, if it was me...

I would be concerned about the 3-amp discrete-diode rectifier bridge circuit when running the supply on 115v - they would be running at 99% load, and would heat up close to 100'C - not good...

I agree with the comments on MOSFET vs Bi-Polar transistors - MOSFETS can pass the same current with less heat dissipation - Bi-polar transistors are a bad-call.

I agree with everything else stated in the article.


Yes, the noise-figures of that cheap PSU are apalling.

There is more noise then volts.

...not good...