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Old 05-09-2011, 12:10 PM   #1
SkyerSK
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Turning on PSU without motherboard


Hello,
short question:
(I assume this is the way it's done) which pins should I "connect", and what is the correct procedure to turn on PSU through 20/24 pin ATX connector?

longer question: I am trying to get better understanding of electricity, so I was searching for some power source for my "experiments". Got an idea immediately, computer PSU should be good with it's 12V cables. However, I came to problem, since I don't want to be running computer on the same power source unit as my electrical try outs. (I found dumped PSU ready for this purpose, and I don't want to damage any of my pcs.). So, the question is - what is the correct procedure and which pins should I connect to turn PSU on? Also, is there any possible damage to PSU for doing things like this?

Thanks much.
 
Old 05-09-2011, 01:12 PM   #2
cascade9
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Easier to link you to pics rather than give directions-

http://www.overclock.net/faqs/96712-...upply-psu.html

In theory ATX power supplies should run without load. Cheap and nasty power supplies do have the possiblity of blowing if turned on without a load though.
 
Old 05-09-2011, 01:49 PM   #3
SkyerSK
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Thanks,
and sorry for not finding it by myself, seems to be general question. I'll try it...
 
Old 05-09-2011, 03:19 PM   #4
Latios
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ATX psu can give you several voltages :

12 V - yellow
5 V - red
3.3 V - orange

5 V ("+5VS") even when PSU is off - violet
-12 V (when this wire is - and earth is +) - blue
-5 V (only in some power supplies) - white

All voltages are relative to earth - black

To power up connect green with black



Notice !

Trashed PSUs sometimes have burned components inside. If it will work at all, it may give wrong voltage or burn up after a while (it can sometimes give correct voltage for a while or untill you load it)

If there are damaged capacitors inside (they look balooned), or simply if the PSU is of low quality, the voltage you get out contains a lot of ripple. Ripple is when the voltage "shakes" around its correct value : for example going quickly (tens thousands times in a second) and repeatedly between 12.0 and 12.1 V instead of being just 12.1 V. Ripple can damage some types of electronic comonents connected to it, and is generally more destructive than over voltage (for example, ripple between 12.0 and 12.1 is worse than 12.7 constant for many components). This is often why it has been dumped in the first place - ripple in a computer causes crashes and freezes. Many low-tech electronics and electrical stuff won't mind ripple too much (but high ripple can still destroy the power supply itself). A ripple problem can be fixed by replacing the bad capacitors, or if the capacitors are ok then by connecting more capacitors (of high capacity) to the DC outputs in use

Most cheap PSUs DONT withstand the current or power ratings on them. For example if it says 12 V 15 A, it may only hold up to 12 V 5 A and burn up if you try to take more than that (instantly, or for more than few seconds). Same with watts. A cheap power supply should not be expected to give more than 1/4 - 1/3 of its rating continuously

Cheap power supplies often dont have overload protection. Overload or short circuit - get smoke or sparks, the power supply will continue going. Sometimes there is protection from absolute (high current) shorts and no protection from not-very-high-current shorts and overloads. Avoid shorting your power supply

Make sure that the fan is running. A power supply won't hold up long without a fan

Open it up and see if there is visible burn damage in th circuit board. Avoid using power supplies with damage to power stuff you care about

Ensure the power supply is connected to earth on the input side. It contains filtering capacitors between live 120 / 240 V parts and the case, and can give you / your electronic stuff a tingle or zap if not earthed properly


Have success !

Last edited by Latios; 05-09-2011 at 03:23 PM.
 
Old 05-12-2011, 07:33 AM   #5
SkyerSK
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Thanks for your answer Latios,
I'll keep that in mind when I'll be playing around with it.
 
Old 05-12-2011, 07:52 AM   #6
MTK358
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Quote:
Originally Posted by Latios View Post
Most cheap PSUs DONT withstand the current or power ratings on them. For example if it says 12 V 15 A, it may only hold up to 12 V 5 A and burn up if you try to take more than that (instantly, or for more than few seconds). Same with watts. A cheap power supply should not be expected to give more than 1/4 - 1/3 of its rating continuously

Cheap power supplies often dont have overload protection. Overload or short circuit - get smoke or sparks, the power supply will continue going. Sometimes there is protection from absolute (high current) shorts and no protection from not-very-high-current shorts and overloads. Avoid shorting your power supply
That is just scary. IMO, the companies that make such PSUs should be ashamed that those PSUs are being put into unsuspecting peoples' PCs.

As for me, I promised myself to only use good quality PSUs from trusted brands ever since I got a few mild electrical shocks from touching the back of the "iCute" PSU that came with the case I built my first homebrew PC in.

Last edited by MTK358; 05-12-2011 at 07:53 AM.
 
Old 05-13-2011, 02:19 PM   #7
Latios
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Quote:
Originally Posted by MTK358 View Post
IMO, the companies that make such PSUs should be ashamed that those PSUs are being put into unsuspecting peoples' PCs
This is the least bad part. Computers usually dont short circuit, and you might think that if you put a cheap 500 W power supply in a computer that won't ever take > 100 W you are ok. NO. Those cheap power supplies also give off ripple (unclean voltages) which will slowly damage capacitors on other hardware (mainboard etc) and eventually kill it completely

Sometimes the power supply is built co cheap that it will degrade and fail even when running with little or no load, just from its own load

Quote:
Originally Posted by MTK358 View Post
I got a few mild electrical shocks from touching the back of the "iCute" PSU that came with the case I built my first homebrew PC in.
Looks like your earth wire was broken. The problem may be in the home electric system / power strips / cables etc and not inside the power supply

If you have no earth connection, remove the small filetring capacitors between AC input and not-existent earth inside the power supply. This will stop it from shocking, as it disconnects the path from the AC to the case, but does not solve the safety issues of running wihout earth connection
 
Old 05-13-2011, 02:37 PM   #8
MTK358
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Quote:
Originally Posted by Latios View Post
Looks like your earth wire was broken. The problem may be in the home electric system / power strips / cables etc and not inside the power supply

If you have no earth connection, remove the small filetring capacitors between AC input and not-existent earth inside the power supply. This will stop it from shocking, as it disconnects the path from the AC to the case, but does not solve the safety issues of running wihout earth connection
There isn't anything wrong with the house wiring. It was just that something like 3 times I felt a momentary tingle when I touched it. Maybe it was just static, but I don't remember that happening when touching other parts of the case or on other computers.
 
Old 05-13-2011, 04:14 PM   #9
Latios
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Static is one-shot zap. AC (originating from the mains power) is tingle

The fact you felt it means that a path from mains to earth formed through you :
- The computer was not connected to earth, but you were
- The computer was connected to earth, and you touched it and something else that leaks current (e.g. cable from the cable modem) at the same time

The device that leaks the current (be it the computer or something else) is a result of broken earth connection somewhere, and path for the current to get to the case

The broken earth can be anywhere (home wiring, cables and plugs, inside PSU etc). It can be intermittent - a broken wire that eventualy makes contact or breaks it - maybe if you move the cable it will break again

The path to the case is usually through filtering capacitors which arer there to filter out noise. They are pretty small, thats why there is only enough current to tingle and not to shock (at least while everything else is isolated well)
 
  


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