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Replace batteries with AC Adapter

Discussion in 'Electronic Basics' started by tylernt, Nov 28, 2003.

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  1. tylernt

    tylernt Guest

    I'm trying to use a Xenon flashlight bulb to light a micro-refugium
    for my reef aquarium ( The bulb is designed
    to be run at 3v off AA or AAA batteries.

    Obviously, I don't want to use batteries if the thing is going to be
    on 12 hours a day every day. But I do have a 9v, 300mA AC adapter
    laying around. Is there any way to use it as the power source instead
    of batteries? According to my multimeter, the AC adapter (obviously
    unregulated) puts out 14v with no load.

    I used some online LED resistor calculators, which seem to indicate a
    30ohm resistor would pull the 9v down to 3v. I tried the adapter with
    the smallest resistor I had on hand, 100ohm, but the bulb stays dark,
    not even a dim glow. So I think I'm lost in the jungle of ohms and
    milliAmps and milliWatts and voltage drops, and don't want to fry this
    $4 bulb I just bought...

    So, thanks in advance for any advice you can offer!
  2. Ian Stirling

    Ian Stirling Guest

    A resistor is certainly the wrong way to go.

    You do realise that most torch bulbs have a life of around 4 hours?

    If I was making a micro-tank, I might be looking at the cold-cathode
    bulbs widely available from the overclocking sites.
    These vary from 4-12" and come in a wide variety of colours.
    For example, I've just purchased from a 12"
    white CCFL tube (with 12V driver) for around $5(us) (it's a little more
    expensive if you'r in the US.)

    Add a 12VDC power supply (I see some on ebay for $5) and you'r off.

    Alternatively, you may want to obtain from any electronics retailer a
    9V 2.5W bulb.
  3. N. Thornton

    N. Thornton Guest

    No, get a 3v adaptor and use that. Your 14ver wont have the current capability.

    You paid 4 bucks for a torch bulb?? Hope its specs are good!

    Regards, NT
  4. David Wood

    David Wood Guest

    That bulb may want as much as 2 Watts of power. There are two problems
    with that for you: first, you'd need 3 Volts at 670 milliamps to
    supply it and second, the heat from such a bulb can deform and melt
    plastics used to house and mount it. While your existing adapter
    theoretically could supply enough power, there is no economical way to
    convert it to the voltage you need and still have enough current

    If you must use that bulb, find an adapter that is fused on the
    primary (input) side, outputs your needed voltage and supplies 1 Amp
    or more. Housing such a bulb underwater would probably require a glass
    dip tube with some air gap around the bulb. Tubes like that are used
    to house aquarium heaters as you probably know.

    Just out of curiosity, is there a particular reason why you've chosen
    that light source? Did you need the color, level of intensity and
    heat? If not, perhaps some white LED source could be used or a cold
    cathode fluorescent as mentioned in a prior post.
  5. tylernt

    tylernt Guest

    The $4 bulb is a special Xenon bulb, I got it because it was extra

    The light won't be underwater, it will sit on top of a small chamber
    and illuminate it so macro-algae can grow there. Spectrum can be
    anywhere from yellow to white. The object is to cram as much light
    into a 2" by 4" space as possible, without having so much light it
    spills over into the main aquarium (the two will be on reverse
    lighting periods).

    I did not realize Xenon flashlight bulbs last maybe 100 hours. That's
    not going to fly.

    Reading and using the
    calculator on,
    it looks like I can use my 9v AC mains adapter with a 270ohm resistor
    to light Radio Shack's 3.6v, 20mA white LED? Am I way off base here?
    Taking that a step further, I could use multiple LEDs off the same
    adapter, wired in parallel, as long as each LED has it's own resistor?
    How does LED voltage drop play into this?

    I'm not necessarily just looking for a pat answer, though that would
    be cool. I'd like to learn and understand this stuff too. :) Is there
    such a thing as a freeware circuit simulator that I could play with on
    my PC?

    Thanks again
  6. I find the student version of CircuitMaker quite adequate for the little
    circuits I play with.

    Bob Monsen
  7. David Wood

    David Wood Guest

    There's been much discussion of circuitry to drive LEDs on this group.
    Most has revolved around inexpensive but inelegant methods. The
    elegant method is to use a controlled current source for each LED to
    drive it at it's rated current.

    The design methodology you describe is valid but with a few caveats.
    First, remember that an LED is a diode and will only pass current in
    one direction normally. I must assume that the AC adapter you refer to
    is actually providing a nominal 9VDC, not 9VAC since your 270 Ohm
    resistor value is a correct design value for 9VDC but not for 9VAC.
    Second, given your supply voltage (9VDC) it makes sense to string two
    LEDs in series and use a 90 Ohm resistor. You don't have enough
    voltage for three but it's more efficient to drop voltage in another
    LED making light you want rather than heat you don't in a resistor.
    For example, in your original design you had a 270 Ohm resistor with a
    5.4 Volt drop wasting 0.108 Watt versus my design where a 90 Ohm
    resistor drops 1.8 Volts and only wastes 0.036 Watt (1/3 as much
    driving 2 LEDs for a total savings of 0.18 Watt for every 2 LEDs). For
    comparison, each of your LEDs is dissipating 0.072 Watts.

    So, the optimal design for your components is two LEDs in series with
    a 90 Ohm 1/8 or 1/4 Watt resistor. You may multiply this series in
    parallel across your 9 Volt source up to 15 times before exceeding
    your 300 milliAmp limit. That's 30 LEDs.

    End of part 1. I'll continue next post.
  8. tylernt

    tylernt Guest

    I find the student version of CircuitMaker quite adequate for the little

    CircuitMaker is perfect! Many thanks for the link! =)
  9. David Wood

    David Wood Guest

    There's been much discussion of circuitry to drive LEDs on this group.
    Since the voltage of your source, the effective resistance of your
    LEDs and the value of your resistors may vary from nominal values, you
    need to build the circuit on a breadboard to experimentally determine
    the best resistor value to drive your circuit at 20 milliAmps for each
    leg. Additionally, as you add more circuit legs in parallel, the
    loading on the power supply will decrease it's output making it
    necessary to adjust the resistor value downward after you've built the
    complete circuit.

    If you intend to build it, here are the steps:

    1 - Breadboard the complete circuit with 100 Ohm resistors on the
    circuit legs.

    2 - Substitute a variable resistance (250-500 Ohm potentiometer) on
    one of the legs for the resistor making sure that it's value is set at
    100 Ohms or more when you power the circuit.

    3 - Place an Amp meter in series with the adjustable leg.

    4 - Apply your 9 Volt DC source voltage to the circuit.

    5 - Adjust the potentiometer value until the meter reads 20 milliAmps.

    6 - Remove power from the circuit.

    7 - Measure the value of the adjusted potentiometer with an Ohm meter.

    8 - Remove and replace the potentiometer with a resistance value as
    close to but no less than the value measured from the potentiometer as

    9 - Repeat steps 2 through 9 for each leg of the circuit.

    10 - After verifying operation with final components and making sure
    that total circuit current doesn't exceed the load limit of the supply
    (less than 300 mA),
    build the components onto perf board or prototype board or etch a
    board for it.

    Before you go to the expense and difficulty of building such a
    circuit, I recommend that you do some more research into your lighting
    requirements. White LEDs don't provide significant amounts of
    ultraviolet light which your algae may need for growth. There are
    ultraviolet LED's available but I have no experience with them.

    If I were in your position, I would be checking with people
    experienced in micro reef culture to find out what solutions have been
    used previously and what light spectra and intensity are needed. I
    would also look to commercial sources of that light before endeavoring
    to build my own. Often, a hobbyist discovers late that the cost of
    components purchased at retail prices and the potential for failure
    that are inherent to experimentation can cause the costs of the
    experimental approach to exceed the cost of a commercial alternative
    and often without producing the intended result.
  10. David Wood

    David Wood Guest

    The values here are design values and if you want to build this thing
    and have maximum output you need to do some experimentation to get the
    resistances right.

    It's my understanding that your application requires a healthy amount
    of UV. You won't get it from white LEDs. There are some UV LEDs on the
    market but I've no experience with them. I noticed one of the recent
    "featured tank of the month" on nano reef was using a 13 watt lamp.
    Are you trying to go even smaller or just want to use a different
    lighting technology?

    The idea of using a combination of white and UV LEDs for your reef is
    intriguing since it would probably be more efficient and smaller than
    other lights but will certainly cost more to build than some off the
    shelf product using an older technology.

    By the way, what are you doing buying components from Radio Shack? I
    usually use Digikey, Jameco or Mouser. Sometimes HSC or All (surplus).
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