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5v to 12v Circuit

Discussion in 'Electronic Basics' started by LED Man, Feb 23, 2005.

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  1. LED Man

    LED Man Guest

    Looking for a very simple (Newby) 5v to 12v circuit, or device, low
    cost, to run 120 ma 12v led off 5-6v

    TIA
     
  2. You need a boost circuit.

    You can buy DC-DC converters for this kind of application, but they may
    violate your 'low cost' requirement. You can probably get them surplus.
    I know I've seen them at halted specialties in san jose for a few bucks.

    Here is a web page that describes the basics, if you want to try to
    build your own:

    http://www.powerdesigners.com/InfoWeb/design_center/articles/DC-DC/converter.shtm

    You can also use switched capacitor circuits to do this, but it's hard
    to get 120mA from one of these.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  3. CWatters

    CWatters Guest

    Does it have to be a 12V LED lamp?

    I would probably be cheaper and easier to use ordinary LEDs and run them
    from 5V.
     
  4. As Colin said, you need a different LED. Most of them are
    intrinsically around 3 volts or so, so a 12V LED is just an LED in a
    package with an appropriate resistor. Building a power supply to
    overcome the resistance is a lot more complex than changing the
    resistor (or getting the same LED with a resistor sized for 5V
    supply).
     
  5. philo

    philo Guest

    just use a different dropping resistor...

    a "12v" led and a "6v" led are really the same...

    it's just a matter of the dropping resistor (which is in series)
     
  6. It helps if you tell us what kind and type of 12V LED you're talking about.
    I never saw 12V LEDs so your 12V type is an appliance of some kind. May be
    electronic, may be some LEDs in series or only a LED with a simple series
    resistor. An optimum solution depends on this information.

    petrus bitbyter
     
  7. It helps if you tell us what kind and type of 12V LED you're talking about.
    I never saw 12V LEDs so your 12V type is an appliance of some kind. May be
    electronic, may be some LEDs in series or only a LED with a simple series
    resistor. An optimum solution depends on this information.

    petrus bitbyter
     
  8. LED Man

    LED Man Guest

    It's not a single led, it's a 30 led package MR16, which is a good area
    light, in a nice neat package.

    Works well, 10 hours off 10xnimh AA, just looking to run off 4 AA
     
  9. Rich Grise

    Rich Grise Guest

  10. Terry

    Terry Guest

    Struck me too that AFIK most LEDs operate from 1.5 to 3.0 volts?
    So the 12 volt package must either have a) A dropping resistor which wastes
    some 8 or 9 volts, or b) LEDs in series (unlikely?).
    Simplest would be, if possible, to modify the package so as to waste only
    two or three volts of the 6 volts that four AAs will provide?
     

  11. I seem to recall hearing of 12 volt LEDs designed as
    drop-in replacements for automotive/truck lamps in
    tail-lights, etc.
     
  12. 12V visible light LEDs can't exist.
    They would be in the hard ultra violet.

    What you are describing are circuits of multiple
    LEDs and ballast resistors and such.

    Duane

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  13. Or a switch mode constant current power supply ?

    Why unlikely ?? Works for us !


    J/.
     
  14. Ecnerwal

    Ecnerwal Guest

    or b) LEDs in series (unlikely?).

    I believe it was already mentioned that the item this thread started
    about was a 10 or 12 LED in one package lighting unit, which is probably
    a few series strings in parallel, depending on the LED operating voltage.
    LEDs in series makes all kinds of sense. If you change from wasting
    power on a dropping resistor to running as many as you can (depends on
    voltage available) from a constant current supply (easily made from an
    LM317 voltage regulator [for less than a dollar], among other methods)
    you get more light out for the same power in. And you can more precisely
    control the current (which is what LEDs care about and die from too much
    of) so that you can run them at maximum rated current (for maximum light
    out) without paying too much attention to the supply voltage. With a
    dropping resistor, the resistor value must not permit more than rated
    current at maximum voltage in, and in most applications is taking more
    power than the LED is.

    Nothing is free, in that the regulator is eating some power (it
    basically acts like a smart resistor) but for a 12V supply, with a
    typical LED operating voltage of 2 or 3 volts, the same power that would
    be running one LED and a dropping resistor can run 3-5 LEDs. One can
    approximate this with a smaller dropping resistor, but the constant
    current supply does a better job.
     
  15. CWatters

    CWatters Guest

    I suspect that your LED lamp might be designed to replace a normal halogen
    lamp. Remember that normal 12V halogen downlights operate from 12V _AC_ not
    12V DC.
     
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