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wall wart power supplies

Discussion in 'Electronic Basics' started by Jorgie Jorgenson, Dec 30, 2003.

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  1. hi -

    I've built a few projects using Rabbit RCM2200's and various ICs,
    LEDs, etc, and have used a variety of power supplies:

    - old PC supplies (old Mac laptops have nice compact ones)
    - open frame switching supplies from Jameco
    - 9v wall warts stepped down with 7805's

    I'd like to know if I can just use a 5V wall wart for most projects.

    - I only need 5V, about 1A max
    - Offboard power is desireable for reduced project size (but not
    - Reliability or ease of replacement (ie if its a wall wart someone
    can buy another one, but they couldn't be expected to replace an
    internal supply)
    - Simplest design feasible
    - safety (I don't want to burn anyone's house down)
    - I'll likely give some of these away, and perhaps sell a few in small
    quantities, so they'll be out of my hands.

    I think a PC supply is overkill for most projects. I like the open
    frame switching supplies but I had one die very prematurely so I'm
    concerned about lifespan. And stepping down a wall wart with a 7805
    produces heat and added complexity. I was only using the 7805 for a
    steady 5V signal, but if a 5V wall wart works well enough I'd just as
    soon use that.

    The Rabbit needs 5V +/- 0.25VDC. Any recommendations out there?

    Also, if I was using a wall wart I assume I'd need a diode or
    something to prevent someone plugging one in with backwards polarity.
    Anything else I'd need? How do the millions of electronics out there
    that use these things handle this question?

  2. cpemma

    cpemma Guest

    A *regulated* 5v wallwart (not all that common) would be OK, but I'd expect
    an unregulated one to blow 5v logic.

    The more common regulated 6v wart feeding the circuit via a 1N4001
    protection diode will leave around 5.2V at >400mA,
    but the usual reg spec is loose enough to leave a bit more than your 5.5v
    ceiling, so YMMV.

    For the price they are, use a basic 9v wart with a 7805 and sod energy
    conservation. If you want on-board diode protection, tweak the 7805 with
    another diode on the ground pin to compensate.
  3. JeB

    JeB Guest

    if the wart puts out something over 5v (or use a 6v wart) then a low
    drop out regulator might handle the cleanup. I wouldn't trust an
    unregulated wart. Polarity protection, etc is up to you.
  4. CFoley1064

    CFoley1064 Guest

    hi -
    You need a well-regulated +5V for the Rabbit -- I'd like to get to within +/-5%
    for all line and load conditions, although you can get to +/- 10%. An
    unregulated supply of any kind won't cut it -- your Rabbit will fry.

    Since you mentioned Mouser, you might want to take a look at their "brick"
    power supplies. They are switchers, but are usually pretty reliable. Also,
    they're pretty much guaranteed not to go overvoltage when they fail, so all
    you'll have to do is send your friend another brick, they plug it in, and
    they're good to go. I was looking at the Mouser 552-PSA-15W-050 ($20.68 in qty
    1, $18.80 per 5 ea.). It specs +/-2% and +/- 5% regulation for line and load
    from 0.0A to 2.5A, and has a standard DC power plug output. Remember to get a
    line cord separately.

    A 9VDC unregulated wall wart with a 7805 regulator isn't a good idea. Your
    minimum output voltage at low line/high load will be well below the 7805
    dropout voltage. If you'd like to do this, use a low dropout linear regulator.

    I usually use DC bricks for single board computer projects if I can. They're
    reliable and replaceable.

    By the way, if it's going on the factory floor or you don't trust the customer
    to RTFM (read the manual) and follow instructions, clip the plug at the end of
    the brick and put some kind of unusual connector there that can only be plugged
    in one way. I had a customer try plugging a 9V wall wart in where a brick was
    supposed to go, with predictable results for the SBC and everything else.

    Good luck.
  5. Thanks for the ideas everyone. The regulated brick seems like a great
    option for my needs. I'm begining to understand why every electronic
    product ever made has a different sized DC power plug.

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