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Help with picking a power supply

Discussion in 'Electronic Basics' started by Rod, May 23, 2005.

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  1. Rod

    Rod Guest

    It has been a while since I was in school working with this stuff so maybe
    someone out there can help me in this area. I have a piece of equipment that
    has 12 VDC input, and is rated 170mA (max), and 2.25 Watts (max). What I'm
    trying to do is find a off-the-shelf power supply that will allow 1 unit to
    be powered or, up to 16 simultaneously to be powered. For the life of me, I
    can't remember how to calculate this out. Can someone help?

  2. kell

    kell Guest

    To run sixteen units, your power supply would have to supply nearly
    three amps (at twelve volts); but it would also have to supply twelve
    volts on the very light 170 mA load that a single unit presents. The
    voltage of an unregulated supply (wall wart) will vary quite a bit over
    such a wide range of loads. I would suggest you get a reasonably
    priced regulated switching power supply from one of the surplus supply
  3. Rod

    Rod Guest

    Thanks Kell, this helps a lot!
  4. If you decide to go this way, you'll likely have to put a load resistor on the
    +5 output to get the multi-output PS to regulate properly. Most PC supplies
    expect to have a load of 10% or more of the rated output current on the +5.
  5. There is nothing complicated about it, it's just standard
    multiplication and division.
    170mA * 16 = 2720mA. 1000mA = 1A so the total amperage is 2.72A, always
    round amps up not down. you need 3 Amps. To calculate wattage then you
    want volts * amps.... 12V * 2.72A = 32.64 watts. If you know only the
    wattage and volts it is you can use that to find the amps, vise versa.
    32.64W / 12V = 2.72A.... 32.64W / 2.72A = 12V.

    My first pick for a 12Vdc power supply (that can do 3+ Amps) would be a
    car battery charger. If you want you could add a 12V regulator stage
    and/or some large (10,000+ uF) filter capacitors for cleaner power.

    Also what james said about computer PSUs is true. (only the new ones,
    i.e. ATX???) They require a minimum load for it to work correctly. Also
    ATX and newer style PSUs do not have a power switch. You will have to
    short 2 pins on the 20 pin block to turn the unit on and off, I don't
    remember the pin numbers. If you are going to go with a computer PSU
    then you want to use the old AT style PSU. With computer PSUs the
    yellow wire is always +12, red is always +5 and black is always ground.
    Go to a local mom & pop computer shop an ask if they have an old AT
    style PSU, I bet they would give it to you for 5 bucks or less.
  6. I've seen the problem on older AT style PSUs, too. Wattage doesn't seem to
    matter as I've seen it on both 400W and 85W AT supplies.
  7. John Smith

    John Smith Guest

    Wouldn't a car battery charger perhaps also require a minimum current? I'd
    go for a standard lab bench PSU.
  8. kell

    kell Guest

    Doesn't a car battery charger allow the voltage to vary quite a bit? I
    thought they adjusted for current going into the battery. Present it
    with a variable 170 mA to 2.7 A load like the OP has -- what will a car
    battery charger do? Deliver a steady 12 volts? Maybe you can tell me.
    Plus the one in the link costs 45 bucks or so. And what about ripple?
    No need for filtering on a car battery charger. Might not have a
    single filter cap on the output -- I know my bike battery charger
  9. kell

    kell Guest

    Okay I just noticed Nikolas addressed the ripple issue in his post.
    But I still think the OP should go with a small, light, well regulated
    switching $20 power supply than a bulky battery charger that he would
    need big extra caps for, or something else that costs more than
    necessary or requires a lot of extra work. Unless he's an inveterate
    scrounger/serious hobbyist like some of us, with a big junkbox, and
    enjoys messing about with such stuff.
  10. The reason I picked the battery charger is because it can deliver high
    amps and most people already have one, I think I have 3 around here
    somewhere. If he had to buy a new battery charger then your solution
    would be a better option...

    Also here is a link on how to convert a computer PSU into a general
    purpose power supply:
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