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12 V Lamp On 240 V

Discussion in 'Electronic Basics' started by Dave.H, Nov 24, 2007.

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  1. Dave.H

    Dave.H Guest

    I need to operate a 12 volt AC/DC filament indicator lamp on 230/250
    volt mains. I'm guessing a resistor would do the trick. If so, what
  2. Greg Neill

    Greg Neill Guest

    What's the current requirement for the lamp?
    (alternatively, what's its wattage rating?)
  3. Dave.H

    Dave.H Guest

    Website doesn't say. It's a 12 V LES style lamp. I can probably get a
    5 watt wirewound resistor for it.
  4. Dave.H

    Dave.H Guest

    I looked on the page for the replacement lamps. 50 mA
  5. I make that to be 4560W. Be advised though that the resistor will
    dissipate about 11.4W.

    The calcs, like an LED.

    R = (240-12)/0.05 gives 4560R.

    Power rating is P = 0.05^2*4560.

    I would round it up to a 4k7 resistor.

    May your luck be in. Don't blame me if you blow yourself up.

  6. Greg Neill

    Greg Neill Guest

    So worst case is you'd want to drop 250 - 12 = 238 Volts
    at 50 mA across the resistor. Right away you can see
    that it'll have to dissipate 238V*50mA = 12 Watts.
    That's a lot of heat to shed just to support an indicator

    The resistance would be 238V/50mA = 4700 Ohms near enough.

    Maybe you should look at finding a 240V indicator lamp?
  7. Dave.H

    Dave.H Guest

    Thanks. Found a 4.7k 5 watt, wirewound, ceramic cased resistor on the
    Dick Smith website.
  8. Dave.H

    Dave.H Guest

    I have a neon indicator lamp installed at the moment, it's just about
    dead, and there's no way to hold it in place. I don't have a hot glue

  9. Well, according to Ohms law:

    R=(240-12)/50=4,56k so take 4k7.
    Wattage (240-12)*50=11400mW so you will need at least a 12W resistor.
    The thing also will become pretty hot. Generally speaking, this is not the
    best way to use a 12V lamp.

    A capacitor of about 68uF might do a better job. No need to say it should be
    able to handle at least 250Vac.

    Electronics may do even better but are more complicated. Not the thing I'd
    advise to a beginner.

    BTW. Why do you need this extraordinary application? Did you try to find
    other solutions?

    petrus biybyter
  10. Dave.H

    Dave.H Guest

    Perhaps an LED instead? A regular red, 20 mA one will do.
  11. You can buy one. Small types are dead cheap overhere. Guess the resistor you
    need may be more expensive. Especially as it will frie your equipment.
    Otherwise, you can use silicone kit or construction kit. They're available
    in small tubes.

    Neon indicator lamps are for sale in many styles. A lot of them with build
    in resistors. Whats the current lamps type? Has it a thread or a bayonet
    fitting? Has it a resistor build in? I should not be surprised if the type
    is available if only you know where to look for it.

    petrus bitbyter

  12. Why don't you use a 12.6 volt "Filament" transformer?

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  13. Phil Allison

    Phil Allison Guest


    ** Only if you intend to kill yourself or the next person who tries to
    change that bulb.

    What an extremely stupid question.

    ........ Phil
  14. ian field

    ian field Guest

    A LED lends itself to the "wattless dropper" trick, at the mains frequency
    the impedance of a capacitor is much higher than the load so it acts similar
    to a constant current source. The capacitor must be rated for mains - at
    least 400V, preferably 600V, and you need a resistor in series, about 1k2 to
    limit current surge and the LED must have an inverse parallel diode to stop
    its PIV being exceeded, try a 0.047uF to start with and work up to the
    required brightness with successive higher preferred values.
  15. Baron

    Baron Guest

    Are you aware that "Omron" do a 240v/12v panel lamp. Its basically a
    low voltage transformer and lamp in a single hole fitting. Not a cheap
    as putting a new neon lamp in though. You can buy those for a few
  16. Meat Plow

    Meat Plow Guest

    Use a transformer.
  17. Meat Plow

    Meat Plow Guest

    What an extremely stupid answer.

  18. ian field

    ian field Guest

    Well he was raised by dingoes!
  19. Why not rely on the impedance of a capacitor? A good textbook will give
    the the formula relating impedance to frequency and capacitance.

  20. Bob Monsen

    Bob Monsen Guest

    He can do this with his 12V incandescent. Use a 0.68uF cap and a 10 ohm 5W
    resistor in series with the 240V and the bulb. As you pointed out, the cap
    must be rated for 400VAC, which means it won't be cheap.

    If he uses an LED, he'll need another diode in parallel, backwards, and
    probably a zener in parallel to prevent overvoltage. The incandescent
    doesn't care about this, so it may be a better choice.

    As others have pointed out, the best and cheapest choice may be a glue gun
    and a neon indicator bulb which is rated for 240VAC.
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