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  • desktop power supply from recycled computer power supply

    Please be very careful when handling computer (or any powerful) power supplies. The power supply in my very strong workstation is capable of a sustained 80A on the 12 volt rail. This much current is capable of starting a small car, welding light sheet steel together and starting a serious fire. While better made computer power supplies have very good protection against short circuits, some of the cheaper ones do not. In 25 years of building and repairing computers I have seen at least 10 power supplies that literally burst into flames upon an internal short. Also, as already pointed out, there are some very capable capacitors in computer power supplies and great respect should be shown to them. Cutting a number of wires at the same time risks a short (even unplugged) that could hurt you...see more »Please be very careful when handling computer (or any powerful) power supplies. The power supply in my very strong workstation is capable of a sustained 80A on the 12 volt rail. This much current is capable of starting a small car, welding light sheet steel together and starting a serious fire. While better made computer power supplies have very good protection against short circuits, some of the cheaper ones do not. In 25 years of building and repairing computers I have seen at least 10 power supplies that literally burst into flames upon an internal short. Also, as already pointed out, there are some very capable capacitors in computer power supplies and great respect should be shown to them. Cutting a number of wires at the same time risks a short (even unplugged) that could hurt you and/or the power supply. I NEVER cut more than one wire at a time. While the 3.3 Volt rail is only accessible from the 24 pin connector, 5 and 12 volts can be obtained from the 4 pin molex connectors. Female molex connectors are not hard to obtain and then you can wire up your project to the connector and just plug it in when you are ready to power it up.Lastly, the rated voltages can be out significantly on old and especially cheap computer power supplies; enough to fry your pi so to speak, so I always put a volt meter on it before I power up something delicate.As far as combining rails to get different voltages again care should be taken as not all computer power supplies can actually do this properly. Again, better quality ones almost always do and some cheaper/older ones don't.My rule of thumb is this: If it is a no-name cheap power power supply then I don't try to draw more than half of the rated current per rail on the label. Also remember that the wattage rating of computer power supplies is for the combined total of all rails. ie: a 500 Watt supply cannot safely provide 500 Watts on the 12 Volt rail. Usually it would be more like 375-400W. Others have two 12 Volt rails and are only capable of half of this power rating on each rail. As froKo pointed out, the -12 Volt rail is capable of only a fraction of the current the +12 rail can provide, so it is of limited use for 24 Volts. My platinum 1200 Watt supply only provides 1A on the -12 rail.Doom and gloom aside ;) they do make very good and inexpensive prototyping power supplies. You can frequently find them at the local recycle/reuse areas for free. If you want to use one, you do not really need to chop and short wires as you can use a paper clip to short the 24 pin ATX plug and turn it on. This is what I usually do for temporary use. See http://www.instructables.com/id/How-to-power-up-an...for instructions and pictures on how to find the correct pins. Using a paper clip lets me put enough "spring" on the contacts that they will stay there until I want them removed. I find copper wire too flexible to be reliable. There are pre-wired shorted connectors available for under $10 available to buy if you are uncomfortable sticking a paper clip in a plug, but why would you be afraid of that if you are a regular of this site!

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