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Author Topic: computer power supply project  (Read 1125 times)
KF6OWC
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Posts: 5




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« on: April 21, 2004, 12:47:21 AM »

anyone know where i can find how i can convert a compuer power supply to a 13.8 20 amp out heard  there is a way to do that.
tnx jim kf6owc
kf6owc@csfa.net
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OBSERVER11
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Posts: 657




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« Reply #1 on: April 21, 2004, 01:56:23 AM »

here is one, there are many more...
http://www.qsl.net/yo5ofh/projects/13.8v_pc_ps/13_8%20v%20-%2015a%20from%20a%20pc%20power%20supply.htm

Good luck, I have never been impressed by the outcome... almost a waste of a perfectly good computer supply.
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KQ6EA
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Posts: 609


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« Reply #2 on: April 21, 2004, 07:15:01 AM »

There have been several articles in QST about why this is a bad thing to do. Most of these supplies do not meet their ratings, and have poor dynamic regulation.
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W3JJH
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« Reply #3 on: April 21, 2004, 10:49:08 AM »

A particular computer power supply may be an excellent power supply for a computer, but the odds are that it will be a poor 13.8-V supply for a radio.  Moreover, the cost of conversion could exceed the price of a new purpose-built supply!

Form follows function on any well-engineered product.   Just as a well-engineered light pickup truck is not a good basis for a stretch limo conversion, a multi-output computer supply is unsuitable for modification to a 13.8-V, 20-A supply for a radio.
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KT8K
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Posts: 1490




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« Reply #4 on: April 21, 2004, 10:56:10 AM »

Computer power supplies aren't designed to handle the big swings in current demand required by the average 100 watt transceiver.  Also, computer power supplies provide high current at 5 volts, not 12.  The 5 volt section can not be boosted to 12 volts, and the capacity of the 12 volt section is usually under 2 amps, so getting such circuitry to provide 20 amps at 12 volts is asking a lot.  It's a nice idea, and fine to power a QRP rig or your station accessories, but a poor choice for a 100w transceiver.
Good luck es 73 de kt8k - Tim
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KZ1X
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Posts: 3229




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« Reply #5 on: April 21, 2004, 11:12:54 AM »

taking one of the nice switchers from ramboy.com and putting it across a gel-cell is a MUCH better way to get a cheap and quiet 20A 13.8VDC supply

better get one while they still have some!
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WB2WIK
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Posts: 20632




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« Reply #6 on: April 21, 2004, 02:51:45 PM »

This works for some, but doesn't work for many.

The PS/2 power supplies had limited 12V power available, and normally not nearly 20A.  They are also adjustable over a 5% voltage range, which would mean 12.6Vdc maximum, not 13.8Vdc, without tearing the power supply apart and changing things about the original design.

Also, the OVP (overvoltage protection circuit) in PC power supplies will usually shut them "down" if the voltage exceeds about 12.8Vdc, so that would have to be modified, too.

The ATX standard and the newer ATX12V standard power supplies have a lot more 12V power available, but of course are also newer and more expensive and not many are laying around in junk piles yet.

I've never found a PC power supply to be an adequate general-purpose 13.8V source of power for ham gear, although others claim they've done okay, especially when using low power.

WB2WIK/6
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TOASTY
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Posts: 41




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« Reply #7 on: April 21, 2004, 04:21:39 PM »

i made one of these power supplies out of 3 computer psu's connected in series from the 5v lead.  In the end i had a little over 15vdc at over 20 amps, you can easily bring down the voltage by adding a high rated diode or two.  havent had a problem with it yet.
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M0CVA
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Posts: 14




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« Reply #8 on: April 21, 2004, 05:47:18 PM »

James,

Have a look at this URL:http://www.qrp4u.de/index_en.html

I've converted one already from broken 200Watt computer PSU. It's a great learning too as well

73's

Vivian Dewey M0CVA
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K0IPG
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Posts: 252




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« Reply #9 on: April 21, 2004, 09:54:30 PM »

I think KT8K meant to say that 12 amps are usually available from the +12v bus, not 2 amps.

Despite all the naysayers, I still contend that converting computer PSU's is a great learning experience, and will teach you quite a bit about switching power supply design. All the problems cited with doing this sort of conversion can easily be overcome- it just requires a little more time on the project.

There have been several good threads discussing the pros/cons of this sort of project on the 'Elmers' form - try searching for them.
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K0RFD
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Posts: 1368




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« Reply #10 on: April 21, 2004, 10:10:02 PM »

K0IPG wrote:
>Despite all the naysayers, I still contend that
>converting computer PSU's is a great learning
>experience

No argument here, although sometimes all some of us want is 13.8 volts of clean power, without the "learning experience".

Converting old computer power supplies seems to be sort of a roundabout way to achieve this goal, since they are pretty much all different, and since each and every one might represent a "learning experience".

The original poster should ask himself a simple question:  "What's worth more, your time or your money?" and then proceed from there.
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KE4DRN
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Posts: 3734




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« Reply #11 on: April 21, 2004, 11:04:23 PM »

Hi

Here is an article by w8mia on the topic

http://www.antennex.com/preview/archive3/powers.htm

Antennex is a subscription site but this and many other articles are available at no cost to you.

Please be sure to use extreme caution if working inside any switching power supply, there are lethal voltages inside !

73 james
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K0RFD
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Posts: 1368




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« Reply #12 on: April 22, 2004, 12:50:51 AM »

KE4DRN wrote:

> Please be sure to use extreme caution if working
> inside any switching power supply, there are lethal
> voltages inside !

i.e., you can die trying to make a silk purse out of a sow's ear.

That was my point.
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W3JJH
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« Reply #13 on: April 22, 2004, 11:39:24 AM »

I suppose that don't have a proper hobbyist's point of view with respect to power supplies.  For the past decade, part of my job has included designing them.  I've seen enough exploded electrolytic capacitors and charred resistors, magnetic components, and pc boards to want to avoid overstressing components.  Many ... no, most bargain basement computer supplies have several components that are operated with nearly zero margin.  Increasing the stress past the rated limits on these parts is asking for trouble.
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WA9SVD
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Posts: 2198




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« Reply #14 on: April 22, 2004, 02:37:00 AM »

While it may seem attractive, it's not a good idea.  A computer supply DOESN'T normally supply 13.8 VDC, but rather 12.0 VDC.  Even if you can raise the voltage to 13.8 V, the components will be stressed more, and the 12 Volt section of the supply is NOT designed to supply up to 20 A. of current, regardless of the "rating" of the power supply itself.  (The greatest power is supplied and expected to be drawn at the 5.0 V. terminals, and it's not just a matter of "use less 5.0V. and get more 12V.").  You WILL be overloading that section of the supply by trying to draw more current than for which it was designed, and it will probably fail, perhaps disastrously, in a relatively short time.
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