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bulb question - 12v/20W vs. 6v/5w

Discussion in 'Electronic Basics' started by Mike, Oct 29, 2004.

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  1. Mike

    Mike Guest

    Hi folks,

    Just a couple of days before halloween, it's important for your light-up
    pumpkins and displays to work properly. Unfortunately, one of our displays
    blew a bulb the other night and my wife is somewhat upset about it.

    So, after going to four stores (two home depots, one lowes, one wal-mart),
    nobody had the replacement build. Home depot had a spot for the original
    (6v/5w), but both stores were out! So, I picked up at lowes today the
    closest match I could find: 12v/20w. Thinking I can just apply some
    resistors in-line with the bulb, I figured I could have this working again.

    Here's my assertion:

    The original bulb is a MR11 type, 6v, 5W. The power supply output is 6VAC,
    1000ma. How can I make this bulb work? Sorry if this is a naive question.

    Thanks.
     
  2. peterken

    peterken Guest

    you don't.....
    the 12V bulb would only glow a bit, and overload the transformer since it's
    resistance is too low

    only "fast" solution would be to use a bicycle bulb, usually they are about
    6V


    Hi folks,

    Just a couple of days before halloween, it's important for your light-up
    pumpkins and displays to work properly. Unfortunately, one of our displays
    blew a bulb the other night and my wife is somewhat upset about it.

    So, after going to four stores (two home depots, one lowes, one wal-mart),
    nobody had the replacement build. Home depot had a spot for the original
    (6v/5w), but both stores were out! So, I picked up at lowes today the
    closest match I could find: 12v/20w. Thinking I can just apply some
    resistors in-line with the bulb, I figured I could have this working again.

    Here's my assertion:

    The original bulb is a MR11 type, 6v, 5W. The power supply output is 6VAC,
    1000ma. How can I make this bulb work? Sorry if this is a naive question.

    Thanks.
     
  3. Tom Biasi

    Tom Biasi Guest

    The original bulb pushes the power supply pretty close. The bulb that you
    bought will exceed it a little. If you can change power supplies it would be
    your best option.
     
  4. Mike

    Mike Guest

    I think I want to keep the same power supply since this unit also has a 6VAC
    motor to rotate a colored wheel. The halogen is used to light some
    fiberoptics -- pretty neat setup.

    I can understand overloading the transformer by using a 12V bulb. But,
    wouldn't putting a resistor in series increase the resistance? I guess it
    would also make the bulb be dimmer.

    Thanks.
     
  5. You can't, really.

    A 12 volt bulb will be very dim when run on 6 volts.

    You could build a voltage doubler to get enough voltage - however,
    since your power source is only good for 6 watts, it still won't run a
    20 watt lamp.
     
  6. Rich Grise

    Rich Grise Guest

    You really need to take an electronics fundamentals course before
    any answers to your questions would be meaningful to you.

    For your project, get any 6V flashlight bulb. That would be one for
    a four-cell flashlight, or any "lantern" that uses a 6V lantern
    battery.

    Or, stick a flashlight in the pumpkin.

    But a 12V bulb will not work from a 6V supply.

    Good Luck!
    Rich
     
  7. Mike

    Mike Guest

    Well, you might be right, but so far nobody has presented a solution to me
    that didn't make sense :)

    Due to the construction of this display, a flashlight bulb will not work w/o
    serious modification to the display.

    Thanks.
     
  8. Whether you believe it or not: You cannot power a 12V/20W lamp from a power
    suppy that has been made to serve a 6V/5W lamp. But nothing needs to hold
    you from trying. You will only end up with no light and a blown power
    supply. As you have no skills in electronics the only thing you can do is
    looking for lamps <=6V and <=5W. Two 3V lamps in series will do as long as
    they are <=2.5W each. Other lamps <6V will need a series resistor and give
    you a big problem as you don't know how to calculate its value. Nevertheless
    you're welcome to ask again when you found a lamp inside the boundaries I
    mentioned.

    petrus bitbyter
     
  9. Mike

    Mike Guest

    Thanks for explaining. If I wanted to be really naive and "do it anyways," I
    wouldn't have asked the group for their opinion. Anyways, you're wrong. I do
    have some skills in electronics -- this just happens to be an area I'm not
    too familiar with. Again, you're incorrect about calculating resistor
    values.

    Actually, I've found the right bulb, so I don't need any more dicatorial
    advice from you.
     
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