A question about using an Atx cpmputer power supply

SDHLaw1701 · Post by **SDHLaw1701** » Thu Jan 03, 2008 4:44 pm

Hey all,

I am taking the plunge and I have decided to use a converted atx computer psu to light my refit. I've done some research and think I can safely complete the conversion. I plan to use the instructions found at this link.
http://web2.murraystate.edu/andy.batts/ ... SUPPLY.HTM

My question is regarding the size of my power supply. I have an Antec Tru 430 (430 Watt Max) power supply with the following values: AC input 115V/10A, 230V/5A; 60Hz/50Hz. DC Output +5V 36Amax/0Amin, +12V 20Amax/0.8Amin, +3.3V 28Amax/0Amin, -5V 0.5Amax/0Amin, -12V 1.0Amax/0Amin, +5VSB 2.0Amax/0Amin. +5V,+12V and +3.3V max load 410W.

The power supply on the website I am using for reference is only 145W do I have to do something different to use my power supply? Is 430W too much to use? I plan on only using the +12V to power my refit. Thanks again for all te help.

Scott

Sparky · Post by **Sparky** » Mon Jan 07, 2008 4:25 pm

The only thing I see that is different is that your PSU is indicating the min load is on the +12volt line (that is where you would need the resistor/lamp/load). If you will hooking this up to something that can provide the min load, then avoid using the resistor, as it will generate ~10 watts (9.6 to be exact if you find a 15 ohm exact resistor) of heat (12 * 0.8Amps). The resistor should be 12/0.8 = 15 ohms.

macfrank · Post by **macfrank** » Mon Jan 07, 2008 5:03 pm

Poking around the insides of a switching power supplies can be deadly (or at least extremely painful) even if you know what you're doing.

I think it would be better if you step back, calculate your power requirements, increase it by 25% then go buy a wall wart or other self contained power supply that meets your needs. It's a lot less work and a lot less grief.

Frank

jwrjr · Post by **jwrjr** » Mon Jan 07, 2008 11:35 pm

There is a reason why switching power supplies are usually in a metal enclosure - there often is 300 volts running around inside.

Mr. Engineer · Post by **Mr. Engineer** » Tue Jan 08, 2008 6:59 pm

Correct me if I am wrong but the wattage on the PSU means the maximum wattage it can give. So, you might want to calculate the power and get the higher one. But personally, I only use the PC PSU for quick voltages in the office (to power up my own personal fan, for example) and because its switching, even if its off, its still dangerous. Moreover, it'll look out of place when you want to display your refit.

Anyhoo, the PC PSU gives out mainly two voltages, 12v and 5v. With a bit of recalculation, everyone's right that you can use a wall adaptor. I am going to start my refit half of this year if I have the space but right now, I have some basic circuits ready.

jwrjr · Post by **jwrjr** » Tue Jan 08, 2008 11:44 pm

Computer power supplies also put out heavy amperage at 3.3 volts as well. One thing to remember about switching power supplies in general - To get them to regulate the voltages very well, you have to load them down a bit. The rule of thumb is 10% of max capacity.

SDHLaw1701 · Post by **SDHLaw1701** » Wed Jan 09, 2008 11:31 am

I am a bit nervous about the death thing. but, the power supply has not been plugged in for over two years. Can the thing still keep a charge? Also what sort of load do I need for the switching supply? Is a resistor sufficient or shouls I hook up a light? If this is a bad idea can anybody recomend a good power supply that will do the job?
Thanks

en'til Zog · Post by **en'til Zog** » Wed Jan 09, 2008 4:23 pm

It's a bad idea.

Go to viewtopic.php?t=38441 and read up on "Wall Warts" or "single function, simple, one voltage, someone else carries lots of insurance so they make things safe" power supplies - a cube that plugs into the wall and emits the voltage you need. Just be sure to get a DC REGULATED wall wart.

Go. Read.

And what's with all the funky alien script in the thread now???

Sparky · Post by **Sparky** » Wed Jan 09, 2008 5:14 pm

jwrjr wrote:Computer power supplies also put out heavy amperage at 3.3 volts as well. One thing to remember about switching power supplies in general - To get them to regulate the voltages very well, you have to load them down a bit. The rule of thumb is 10% of max capacity.

I was recently looking for a new good PSU and read several reviews that talked about the new highend big Power PSU's don't have the load limits on certain rails. This is important when considering that the a high amperage rails may not see even 10% load when the pc is idling. Its only during heavy memory access or game play that certain rails get taped for power. Especially the PSus that taught separate multiple 12 volt rails. If you don't put a high end graphics card in one of those rails may never get loaded, certainly not to the level that a crossfire card setup would put on it.

For this PSU it looks like only one rail needs a load...
accroding to his specs there were max and min amperages listed, only one rial had a non-0 min amperage, that's the rail and its amperage that needs to be drawn to make that rail produce the rated voltage.

You can use either a resistor or a lamp, anything that is rated to draw that voltage at that current.

If you will me making something to take to shows, go with a regular power supply. But for the bench top a PSU can be a good source of raw power, it's best if you turn it off when you aren't around to watch it.