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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 22nd Oct 2005, 3:17 pm   #1
Royboy
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Default Ready made power supply unit

Having come into possession of an AT power unit from a scrap

computer, it occurred to me that it would form the basis of a

low-voltage power unit for the bench.

Outputs of +12 volt at 8 amp, +5 volt at 20 amp and –5 volt at 0.5 amp are available.

These voltages are regulated and the unit has a built-in cooling fan.

It must be an AT power unit and not an ATX unit as the latter do not have an on/off mains switch, being started by a circuit through the mother board.

Has anyone ever tried this?

As a test I connected a 12 volt 21w auto bulb across the 12 volt supply and it lit up with full brilliance with no sign of distress from the power unit.

Royboy

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Old 22nd Oct 2005, 4:03 pm   #2
paulsherwin
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Default Re: Ready made power supply unit

Hi Roy. What you suggest is perfectly feasible. A while ago I sent an old AT PSU to a forum member who wanted to use it to power an LCD monitor (can't remember who it was now). The only thing to be cautious about is the current ratings, which often aren't for continuous operation, but if you derate by 50% everything should be OK.

You can also use them to power old car stereos to build a compact workshop or kitchen stereo.

You can actually use ATX supplies too, but it's less straightforward.

Best regards, Paul

Last edited by paulsherwin; 22nd Oct 2005 at 4:13 pm.
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Old 22nd Oct 2005, 7:05 pm   #3
Sean Williams
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Default Re: Ready made power supply unit

It is important to have a load across at least one output at all times, otherwise the regulator circuits are a bit touchy....

Cheers
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Old 22nd Oct 2005, 10:55 pm   #4
Paul Stenning
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Default Re: Ready made power supply unit

Quote:
Originally Posted by paulsherwin
You can actually use ATX supplies too, but it's less straightforward.
Not much less straightforward. You just have to connect the PS ON pin on the output connector (normally green wire) to 0V (black) to make it switch on.

The pinouts are on this page of the useful Hardware Book website.
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Old 23rd Oct 2005, 12:30 am   #5
paulsherwin
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Default Re: Ready made power supply unit

Quote:
Originally Posted by m1ecy
It is important to have a load across at least one output at all times, otherwise the regulator circuits are a bit touchy....
This may be good pracitice Sean, but whenever I've checked a computer power supply with nothing connected all the voltages have been pretty close to where they should be

Putting a few K across each output won't do any harm though

Best regards, Paul
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Old 23rd Oct 2005, 10:00 am   #6
Paul Stenning
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Default Re: Ready made power supply unit

Quote:
Originally Posted by paulsherwin
This may be good pracitice Sean, but whenever I've checked a computer power supply with nothing connected all the voltages have been pretty close to where they should be
Exactly!

I used an AT power supply to power my ADSL router for a few months when the internal supply died. It needed something like 5V@2A, 12V@1A and -12V@0.5A which is a light load for a 300W supply, but it worked fine. The PSU was bigger than the router! I have now obtained a PSU that would fit inside during a clear-out at my old job.

The only point I would make using a PC supply as a bench supply is that being switch-mode and cheaply made you'll probably find it's fairly noisy. So it may not be a good choice for analalogue electronics. Also they often don't have much/any short circuit or over-current protection (due to being made down to a price again), so if you short the 3.3V, 5V or 12V rails it may blow its internal fuse or fail more dramatically.
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Old 23rd Oct 2005, 12:17 pm   #7
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Default Re: Ready made power supply unit

Quote:
Originally Posted by Paul Stenning
they often don't have much/any short circuit or over-current protection (due to being made down to a price again), so if you short the 3.3V, 5V or 12V rails it may blow its internal fuse or fail more dramatically.
Many years ago I misconnected a floppy disk drive in an old AT, shorting the 5V line. The wiring plastic insulation had literally melted by the time I noticed and switched off

Paul
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Old 23rd Oct 2005, 1:40 pm   #8
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Default Re: Ready made power supply unit

Quote:
Also they often don't have much/any short circuit or over-current protection (due to being made down to a price again), so if you short the 3.3V, 5V or 12V rails it may blow its internal fuse or fail more dramatically.
A switch mode supply, will/should shut down if a s/c is across o/p supply.

brunel

Last edited by Paul Stenning; 23rd Oct 2005 at 9:01 pm. Reason: fixed quote
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Old 24th Oct 2005, 11:09 am   #9
Royboy
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Default Re: Ready made power supply unit

Oops. I seemed to have started something. Many thanks to all who replied and gave valuable advice. I'm glad my idea has at least some merit.
I intended using the power supply to test car radios so I will give it a whirl as they say.
Once again, many thanks to you all.
Roy
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Old 24th Oct 2005, 12:24 pm   #10
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Default Re: Ready made power supply unit

Always check the voltages with your expected loading as the 12V supplies are not always stabilised and can vary widely. Also note that with light load or a load balance that it doesn't like it can shut down - basically because of over-voltage protection which crow-bars the supply. Some supplies must then be left several minutes before recovering.
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Old 24th Oct 2005, 1:00 pm   #11
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Default Re: Ready made power supply unit

Quote:
Originally Posted by GMB
Always check the voltages with your expected loading as the 12V supplies are not always stabilised and can vary widely. Also note that with light load or a load balance that it doesn't like it can shut down - basically because of over-voltage protection which crow-bars the supply. Some supplies must then be left several minutes before recovering.
Caution does no harm, and it's always sensible to check voltages with a new PSU, but I've used PC PSUs with a wide variety of nonstandard loads and not had any problems with protective shutdowns etc. If anything the problems related to the PSU *not* shutting down when you do something silly.

I'm pretty sure the 12V supply is always stabilised, since the load presented by the disk drive motors will vary greatly during normal operation.

Best regards, Paul
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Old 24th Oct 2005, 2:31 pm   #12
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Default Re: Ready made power supply unit

Quote:
I'm pretty sure the 12V supply is always stabilised, since the load presented by the disk drive motors will vary greatly during normal operation.
My actual experience is that there exist(ed) some which most definitely are not stabilised. The discs really don't mind as the electronics run on the 5V. Without the correct load I have found these rogue supplies delivering 18V.
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Old 24th Oct 2005, 3:31 pm   #13
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Default Re: Ready made power supply unit

If you take the cover off, no doubt you will see some very hefty stabiliser
I.C.'s.

They look like large power transistors, all usually bolted to same heat sink.
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