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Cheap (Free?) 12 volt power supply from an old computer

Discussion in 'Radio Circuits, Repair & Performance' started by N3HNA, Nov 12, 2012.

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  1. N3HNA

    N3HNA Ham Member QRZ Page

    Just sharing this. This isn't anything new, but if someone has an old computer laying around, is tight on funds, and needs a relatively small 12 volt regulated power supply for a moderately low output transmitter, using an old computer power supply is pretty easy and quick.

    I put together a quick little YouTube video on the wiring. It's real simple. The old computers I have around here didn't have on/off switches on the actual unit. The main connection plug that goes to the computer motherboard has the on/off wires easily accessible.

    The green wire and any of the black wires can be tapped for a simple switch that is properly rated. For the power supply in the video the unit offers 300 watts, or say 15 amps at 12 volts. Again this isn't for massive high output stuff, but for smaller mobile rigs, or HTs it might be an easy option.

    Be safe. Have fun.

  2. K8ERV

    K8ERV Ham Member QRZ Page

    You mean like this one?

    ATX Supply K8ERV .JPG \

    TOM K8ERV Montrose Colo
  3. WA4OTD

    WA4OTD XML Subscriber QRZ Page

    Nice video, I use an old computer supply for my 10M beacon. Thanks!
  4. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    I might point out the 12V supply in these computer things is not a high current supply, and, as shown by the meter, was only 11.59V. This would be ok for a QRP rig, but don't even think of hooking a HF transciever up to it. Or a reasonable power 2M FM. HF rigs usually start to FM once the supply voltage gets down to 12V. That is why I have to use a Battery Booster with my gel Cel. The computer supplies are mainly for the high current 5V, and 12V was just there for some circuitry that needed 12V. I suppose in an emergency it would work. But I will hook my rigs to an Astron.
  5. N8WWM

    N8WWM Ham Member QRZ Page

    Sadly, many will not understand the idea of "start to FM". Maybe an anthology of all the QRZ posted info on power supplies, indexed and consuming a year of time to do so, would help. I am not so sure if it would, though.

    The sendoff of "Be safe, have fun" was never more true, however.
  6. K8ERV

    K8ERV Ham Member QRZ Page

    I think you mean not a high voltage supply. The current depends on the PS wattage rating, mine puts out
    10 amps easily. Some supplies have a trimpot that can boost the voltage a bit, but probably not to 13.8.

    But the price is right.

    TOM K8ERV Montrose Colo
  7. N3HNA

    N3HNA Ham Member QRZ Page

    Great comments. Many thanks. Exactly something like that, Tom - good job.

    And, yes, without any major modification this is for smaller rigs. The price is certainly right.

    Good idea for a beacon supply, Leroy. I hadn't thought about that.

  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    With many (maybe most) PC supplies, especially the older AT and ATX versions, the loading on the +5V bus allows the +12V bus to properly regulate and will also effect the output voltage on the +12V rail.

    I have some here that with a 5A load on the +5V rail, the output is actually about 12.6Vdc on the +12V rail. If I reduce the +5V load to only 1A the power supply still functions but the +12 drops down to about 11.9 or 12.0. It makes a rather large difference.

    Of course the 5A load on the +5 dissipates 25W which is just "thrown away," but might be useful for a bunch of LEDs or something.
  9. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Take a look at the article in the AntenneX preview files. There's a 15VDC 40A power supply and using the limits of plus or minus 15% of 13.8VDC for almost all the rigs out there the 15VDC is an acceptable level.
    The download is here at; Just below the page intro are the listings of things you are allowed access. A few are restricted and they'll tell you so when you try to view them but the others are for you to enjoy. The 15VDC 40A supply is on the left list just about the bottom.
    BTW Steve that's a lot of LED's. Perhaps you wish to light up the room?
    Have fun
    Last edited: Nov 14, 2012
  10. WA9SVD

    WA9SVD Ham Member QRZ Page

    I don't like the idea of using a "15 Volt" supply for my radios. That's right at the limit (or within one volt) specified for many of my radios, and a spike or even small overshoot can exceed that. Even 14 Volts is more than necessary for most radios, and will not accomplish anything other than generating a bit more internal heat.
    If the "15 Volt" supply could be adjusted down to 14 or 13.8 Volts, I'd feel a lot more comfortable about attaching one of my radios to it.

    One drawback to using old PC type computer supplies to power radios (aside from the possiblity of being on the low side of 12 Volts) is that many are poorly filtered, and can cause horrendous RFI. Just a caveat to consider. YMMV
  11. OA4AJP

    OA4AJP Ham Member QRZ Page

    The ATX power supply needs a minimun load to work properly around 1A on the 12v, 5v and 3.3v output.

    You must use a dummy load and that is a waste of power.

    I advice you to buy a power supply from Meanwell or a Linear Power suppy.

    Alfredo Mendiola Loyola
  12. KO6WB

    KO6WB Premium Subscriber QRZ Page

    There are some manufacturers that suggest the higher voltages and it actually reduces the IMD of the transmitter.
    If the 15VDC level gives you the willies then use a 50 amp diode in series with the power supply output lead. One diode will drop you down to 14.3VDC and two diodes will drop you to 13.6VDC. These are just suggestions on how you can get the power you need from junk supplies.
    I'm in agreement with OA4AJP that you would do well to purchase a nice linear power supply. The Astron series are very good.
    Hope this helps
  13. M6BPN

    M6BPN Ham Member QRZ Page

    I've heard about the rf noise from switch mode power supplies, yet I would have thought that for the sensitive stuff going on inside a pc, the power would need to be reasonably clean?

    Alternatively, maybe logic circuits running on square waves (which pc's really are) are not too fussed by noise input voltages?
  14. KF5LJW

    KF5LJW Ham Member QRZ Page

    Then you ought to be scared to death to operate a radio in your vehicle because the alternator will at times be at 15 volts. When you get right down to it a 12 volt system is 13.8 volts. That is just what most LA batteries recommend them to FLoat at using DC power supplies as the charger. A fully charged LA battery is 12.5 to 12.6 volts.
  15. K8ERV

    K8ERV Ham Member QRZ Page

    I think you are confusing noise on the dc line and radiated noise.

    The dc power to a computer is probably well filtered, and the radiated noise is much too weak to be a problem. A radio is much
    more sensitive to radiated noise.

    TOM K8ERV Montrose Colo
  16. N4EYZ

    N4EYZ Ham Member QRZ Page

    I'm curious as to how a supply like this would work for amateur purposes:

    I fly electric RC models and I've seen many threads on the RC forums where folks are using power supplies like the above to provide 12VDC to power their current hungry lipo battery chargers. Many of the guys are charging many battery packs at once hence the need for a high current supply. I fly the smaller stuff and I have two chargers which need a minimum 5 amp (12V) supply. I don't need anything more than I already have but I wonder if the above could be used by amateurs who need high current 12V power supplies but don't want to spend the big bucks.

    btw--these server supplies seem to be fairly prevalent on ebay
    Last edited: Nov 21, 2012
  17. OA4AJP

    OA4AJP Ham Member QRZ Page

    Last edited: Nov 21, 2012
  18. K8ERV

    K8ERV Ham Member QRZ Page

    I think the requirement was cheap.

    TOM K8ERV Montrose Colo
  19. WA9SVD

    WA9SVD Ham Member QRZ Page

    Most current Amateur equipment is designed for a 13.8 Volt supply voltage. Exceeding THAT voltage by any significant amount will NOT result in "enhanced performance," but WILL result in increased heat generation, for which the devices may be at the threshold of tolerances. I will agree that some equipment may well perform better with 13.8-14.0 Volts vs. a marginal 12 Volt supply, but pushing a radio rated for 13.8 Volts to 15 Volts at or near it's specified rating will NOT result in any performance increase. It DOES, however, subject the equipment to operation near it's "breaking point." Many circuits in the radios use 16 Volt rated capacitors, and the "15 Volt" supply is too close to the tolerances to withstand occasional spikes and transients that may degrade such devices over time.
    IMHO, it's better to design (or adapt) a power supply circuit designed for high current ,often transient current demands, such as SSB) to provide the proper voltage, rather than relying upon diode drops in the output; that sacrifices the dynamic range and dynamic response of such a supply.
    Last edited: Nov 21, 2012
  20. N4EYZ

    N4EYZ Ham Member QRZ Page

    As I mentioned I don't need a supply (I have an Astron 35 amp) but these supplies are cheap (less than $30). I'm curious as to the difference in radio performance at 12.1volts vs 13.8. I ran my 706 mobile for many years and a fair bit of that time was sitting with the engine off. So at that point I was at 12 volts. I understand the output power would be reduced somewhat (I never measured mine) but would he quality of the transmitted signal suffer as well? Just curious.
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