What to Do With That Old PC Part II: Power Supply

Discussion in 'FAQs' started by kf4jqd, Aug 18, 2006.

  1. kf4jqd

    kf4jqd Active Member

    If you have a good working PC and the power supply works. Take it out! It will be useful for you lights. The older power supplies like the AT and ATX will have the switch attatched them them. The newer ones, you'll have to add your own switch. The advantage is the 2 differant voltages. You'll have both 12v and 5v to work with. The other great thing is the power supple has an eternal fan to keep it cool!:thumb: Make sure you keep the power cord!!!!:D

  2. sumpter250

    sumpter250 multiscale modelbuilder

    The caveat here is; a computer power supply is capable of high current output. A short in your lighting circuit could generate enough heat in the wires to start a fire! I would highly recommend a fuse in the output of the supply, to limit circuit current.
  3. ezdays

    ezdays Out AZ way

    Thank you Pete, that's entirely true. A computer power supply five volt output may be capable of delivering ten or more amps, meaning that if you're using 22 ga. wire, you could be asking for problems. Rate the fuse for the wire size you are using. Select the wire size to fit the current load you expect.

    Some standards as I remember them:
    • 22 ga. 1 amp
    • 18 ga. 3 amp
    • 14 ga. 15 amp
    • 12 ga. 20 amp.
    You can always use bigger wire than your aticipated usage. As Pete says, the wire resistance will cause the wire to heat up at higher currents and can cause a serious fire. Since the power supply can sustain these higher currents, nothing happens to it.
  4. kf4jqd

    kf4jqd Active Member

    I forgot that one. Yes, fuse those out puts!!

  5. pgandw

    pgandw Active Member

    Normal maximum current laods for wire sizes run pretty close to this:

    12 ga - 20 amps
    14 ga - 15 amps
    16 ga - 10 amps
    18 ga - 7.5
    20 ga - 5
    22 ga - 3.5
    24 ga - 2.5
    26 ga - 1.8
    28 ga - 1.2
    30 ga - 0.9 amps

    Another precaution is that switching power supplies (like PC power supplies) require a minimum load to start and work correctly.

    yours in wiring
  6. railohio

    railohio Active Member

    Oh, the irony. I have two old computers in the closet both with dead power supplies. They'll soon meet their fate in a southbound gondola since I've no use for them anymore and trying to sell them wouldn't be worth the trouble or price of shipping.
  7. ezdays

    ezdays Out AZ way


    In the interest of safety, I've got to mention that as I remember, #22 AWG is only rated for one amp, and #18 AWG is rated for 3 amps. I found many tables on the Internet that shows the maximum current you can put through different wire sizes and it seems to vary all over the map, some go as high as 7 amps for #22 ga. But I know this, as a design engineer, I'd never get away with using #22 ga wire for anything over one amp, and I'd never risk it either.
  8. pgandw

    pgandw Active Member

    No doubt, bigger is better, and your table is more cautious than mine. Mine came from the electrical codes and from the rule of thumb that going down 4 sizes halves the current rating. The codes specify the maximum size circuit breaker that is allowed on a circuit with wire that size. It does not address voltage drop, which would push one to larger wire sizes for longer runs.

    If one uses your table, then the DCC decoders that use 30 ga wire for connections are grossly under-wired, as are many devices that use very small wire for very short distances. I can't imagine wiring my locomotives and rolling stock with a minimum of 22 gauge. Yet, I will use nothing less than 18 ga for runs from control panel to near the track, and often bigger, depending on load. But I will use 26 ga for feeder connections between the rails (I feed every piece of rail because I don't use rail joiners) and the heavier bus wire.

    I would also note that 18 ga extension cords (12ft or less) are expected to run loads up to 500 watts (just under 5 amps). You are expected to use a 16 ga extension cord if the run is longer or the load greater, and sometimes 14 ga for the heavy (usually garden tools) loads.

    my thoughts, your choices
  9. RailRon

    RailRon Active Member

    Just for us poor metric measuring folks outside the USA:
    Does anybody know the conversion formula to calculate the wire diameter from its gauge number? All I understood so far is that the smaller the gauge number, the thicker the wire... :confused:

    E.g.: What diameter in millimeters has a #22 gauge wire? (Ok, the diameter in inches would be alright, too - at least from there I'll manage to calculate the millimeters... :D)

  10. hooknlad

    hooknlad Member

    Ron - the numbers we utilize in the states are AWG meaning American Wire Gauge measured in circular mils. Ill research a chart on the Net for you. If you had a copy of the National Electrical Code ( US ) has tables in the back of the book with both the CM and inches already converted. Ill get back with that info for you Ron. Their are charts in their as well. I know you are looking for A formula and i will locate that for you as well.
  11. hooknlad

    hooknlad Member

    Ron here is a link with conversions from AWG to Metric to US measure - Hope this helps. AWG to Metric Conversion Chart

    "This table gives closest equivalent size cross references between metric and American wire sizes. In Europe, wire sizes are expressed in cross sectional area in mm² and also as the number of strands of wires of a diameter expressed in mm. For example 7/0.2 means 7 strands of wire each 0.2mm diameter. This example has a cross sectional area of 0.22mm². In America, the commonest system is the AWG numbering scheme, where the numbers are applied not only to individual strands but also to equivalent size bunches of smaller strands. For example 24AWG could be made of 1 strands of 24AWG wire (1/24) or 7 strands of 32 AWG wire (7/32)."

    See also: AWG to metric Parameter Calculator.
  12. RailRon

    RailRon Active Member

    Wow, Michael, I really appreciate the prompt answer! :thumb: Thank you very much! :)

    I'll print the table out and take down to my workshop as a future reference. This discussion just came in time for me, since I have two old computers and I planned to take them apart a save the power supplies for lighting purposes. Wire thickness vs. heating by high currents surely is a serious issue. I am glad that Pete started this discussion.

    As I understand, you can't use the 12 Volt part of a computer power supply for running trains, since you can draw only only very weak currants (200-300 milliamps) from them. Is this generally so or do modern computers deliver more Amperes at 12 Volts?

  13. hooknlad

    hooknlad Member

    Ron - you are very welcome. If i remeber the Ohms law thingy, a pc power supply is roughly 200 watts? Power = current * voltage soooo , current = power / voltage, which in this instance 200watts / 12 volts = roughly 16.67 amps . That is a helluva lot of power for your trains. Remember if you want to do this, you will need a mini fused distribution center with the proper sized fuses and wire sizes. If you were to do this, split up your layout in several districts, limiting each district to now more than about 4 amperes . The actual train rail would prolly disintegrate at about 5 amps if a short were to occur, then accompanied by a fire on your layout - not good.....
    Now all this districting is all im my head and not tested. I do not know if your DCC controller warrantee would be null and void if you used anything other than the prescribed power supply.

    If it were possible and were to work, i will draw a distribution system for your districting for you... If you'd like ....... Like i said - a bit much power for your standard layout. Now a club where you were going to run about 20 - 30 choo choos, it would be more feasible.... I however am gullible and will build this system for my layout. Advantages = zero voltage drop. Disadvantages = if a short were to occur on your choo choo in an engine, the engine will disintegrate, instant flambe' ....

    Soooo reconsidering this thing, id fuse each district to about 1 or 2 amps...
  14. pgandw

    pgandw Active Member

    The power supply (at least US versions do) should have a plate that specifies how many amps each at each voltage output. The minimum load is generally placed across the 5 volt circuit, otherwise the newer supplies will not turn on.
  15. cidchase

    cidchase Active Member

    PC power supplies are notoriously unreliable, in my experience. :curse: In addition, messing
    'round with 110AC switches, etc., in the home is inherently dangerous for persons
    without electrical training. As has been pointed out, just using an unprotected 12VDC
    supply can be hazardous from a fire safety aspect. I'm not trying to put a wet blanket
    on Andy's idea, but folks need to be aware of the potential for injury or damage.announce1

    I would hate to think we caused a problem like this for any member
    with our advice on The-Gauge!!! :thumb:
  16. ezdays

    ezdays Out AZ way

    That's a very good point Cid. Let's face it, these power supplies are cheap, I mean you can get a full computer chassis with around a 250 watt supply for under $50. Yeah, they deliver a lot of power, but they have to cut corners even though they make them by the thousands. That's one reason they are "notoriously unreliable". Fooling around with them if you don't know what you're doing or if you don't take precautions (like adding fusing) could result in damage to equipment, fire and possibly worse.

    As far as wire sizing goes, I still stand by my chart. The rule of thumb is: one amp per 500 circular mills. Some values are:

    #28 AWG - 159.8 cm/500=0.31 amps
    #22 AWG - 642.4 cm/500=1.28 amps
    #18 AWG - 1624 cm/500=3..24 amps

    There are a lot of variables that can change that, such as the type of insulation and if the wire is in free air or bundled with other wires. So you can certainly exceed these numbers, but there are risks when you do. The determining factor is as others have said, wire resistance. This will cause a voltage drop across the wire which will cause it to heat up. When it gets too hot it will melt the wire and that's what they call a fuse. If you don't reach the melting point, that's what they call a fire...:rolleyes:

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