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I've taken a PSU from an old Compaq desktop I had lying around and I wanted to turn it into a cheap 12V power supply to power my CB transceiver. However, on inspecting the PSU, I found that it had a 12.8V rail with 9A output. I have never heard of such a rail in my life and was wondering where this rail is located. The PSU doesn't seem to have any special connectors of any sort, so which wires are connected to the 12.8V rail and what is it used for?

I'd like to know, because 12.8V would be more than ideal for my purposes! Especially because it can output 9A. I found more questions and answers on the internet regarding this question but none seem conclusive.

enter image description here

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Hrm Compaq. Isn't that one of those companies well known for doing weird proprietary stuff? – Zoredache Apr 25 '13 at 18:48
@Zoredache You should have seen the components I found inside the actual case. – BloodPhilia Apr 25 '13 at 18:50
What are you using the PSU for? – Breakthrough Apr 25 '13 at 18:58
You are overly fixated by digital precision. 12.8V is only 6.7% more than 12V. The typical 12VDC input tolerance is +/-10% (e.g. disk drives). If your CB transceiver is intended to be powered by a car battery, then be aware that 12.6V is a standard battery voltage (i.e. 3x6 cells). Get a voltmeter, and measure the "12V" line in your car. The real problem as I see it, is this PSU output clean enough for the radio? – sawdust Apr 25 '13 at 19:54
@sawdust CB transceivers will typically use about 13.8V (about your average car voltage with the engine running), but will work with less (albeit with less transmitting power). Therefore, every 0.1V helps! ;) – BloodPhilia Apr 25 '13 at 20:08
up vote 6 down vote accepted

I'm fairly certain this PSU was for a P4 board and the +12.8 VDC rail was on the 20+4 or 20+6 connector as the Vcpu pins. Looking at some of the older manuals on Manual Libs, shows that this is likely correct:

enter image description here

These were very OEM power supplies which fell "in and around" the ATX12V v1.2/1.3 specs.

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"These were very OEM power supplies" -- makes no sense. There aren't degrees of "OEM"ness; it is or it isn't. You probably mean that these were "proprietary" or "specialized". – sawdust Apr 25 '13 at 23:00

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