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Simple wiring question, I'm 99 44/100ths % sure I'm right, but I have to ask

Discussion in 'Electronic Design' started by theblooms, May 23, 2007.

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  1. theblooms

    theblooms Guest

    OK, I have a situation at work where I have to light a rather large
    area, but the lights have to be very bright and extremely impact and
    shock proof. So that pretty much rules out conventional incandescent
    light bulbs and fluorescent tubes. Plus, I have to do this on kind of
    the cheap.

    So this is what I came up with: Wire 8 automotive clear fog lamps in
    series. The lenses are rated for rock strikes and road debris at
    70MPH, perfect. They are absolutely vibration proof, being off-road
    lights, and because of the lenses, they are bright at hell. As a
    bonus, I can get them for $12 each.

    That said, they are rated 55 watts at 12VDC, but of course they easily
    go to 14.4V: the output of a car alternator. And being light bulbs,
    they couldn't care less if they are seeing AC or DC current. So,
    according to Ohm's Law, 8 wired in series, assuming 14.4V, they should
    now want for 115.2V. Perfect for plugging into a standard wall

    Am I right, or am I going to electrocute myself?
  2. They'll be a bit happier off of AC.
    Both, perhaps.

    Best regards,
    Spehro Pefhany
  3. Guest

    You know what... regarding that 56/100% that's not so sure... does
    anyone know how the resistance of a light bulb filament varies from
    room temperature to operating temperature? (All within one second, of

    Resistances could theoretically vary from light bulb to light bulb...
    theoretically this could kill some light bulbs prematurely... ???

    When in doubt, run the experiment! (With full safety goggles...)

  4. I would use low or high pressure sodium in reflectors with guards.
  5. budgie

    budgie Guest

    Things that will probably bring you unstuck:

    1. Lamps in your series strings won't necessarily share the volt drop equally.
    Sooner or later - probably sooner - one will go O/C and the rest will

    2. Those lamps *may* go to 14v4, BUT in the automotive scenario the typical
    volt drop between source (that 14v4) and lamp is at least 1v. If you manage to
    get 14vanything at the lamps, their life will shorten - and see #1 above for the
    consequences of failure of one lamp in a series string. Careful selection of
    cabling will help drop the voltage at each lamp.
  6. theblooms

    theblooms Guest

    Sodium lights won't work because of three problems:
    1. quality of light. I need it to be as white as possible.
    2. physical size. Looking back, I didn't mention this as a
    requirement, but the bulbs and housings also need to be as compact as
    3. cost. Sodium lights ain't cheap. Not in the slightest.

    Thanks anyway for the reply. I REALLY DO appreciate it.
  7. John Fields

    John Fields Guest

  8. John Fields

    John Fields Guest

  9. GregS

    GregS Guest

    Fog lamps are suppose to have a shield in front of the lamp, so you don't see the
    lamp, and the coverage is limited. if it does not have this, then its not a true fog lamp.
    A driving or passing lamp does not have a shield in front of the lamp so coverage is bigger.
    A standard auto headlamp does have a shield over the low beam lamp, but not
    necessarily over the high beams. Small halogen lamps with sockets are prone to socket failure in the long run.
    Just try to get a lamp with a wide coverage preferably with a seald beam system. Think
    about total blackout when one fails.

  10. Fred Bartoli

    Fred Bartoli Guest

    Spehro Pefhany a écrit :
    How's that? Electronic wind?
    I remember an interesting report about aircraft incandescent lamps
    failure mode analysis used as some info source about aircraft crash...
    (I think it was you that posted this).
  11. kell

    kell Guest

    Maybe use nine. And you could rig up a second series that comes on
    automatically when the lights black out. Something as simple as a
    current transformer that holds a relay in the open circuit position:
    when the lights black out, current stops, the relay closes and the
    second series comes on.
  12. They are also "assumed" to be in relatively free air and/or have an induced
    breeze on them from the forward speed of the vehicle. Don't put them into a
    hot box and expect them to last very long.

    Bogey value for lead-acid "automotive" style power sources is 13.4 volts.
    14.4 for very long will boil the battery dry. I'd use 9 lamps in series.
    If I wanted them to last forever, or if I was going to be switching them on
    and off a lot, I'd use an input surge limiter that could handle your four
    amps of steady state current.

    Yes, probably.

  13. Rich Grise

    Rich Grise Guest

    Ah, the joys of the ambiguities of the English language!

    By "off of AC", do you mean they'll be happier being powered by AC,
    or they'll be happier if they get off the AC like getting off the sauce,
    i.e., don't let AC near them?

    I suspect you mean "running powered by AC", because the filament
    evaporation will be symmetrical.
    Personally, I'd use more like 9 or 10 in series - 14.4 is probably pushing
    it for an incandescent.

    And, of course, we assume that OP knows about double-insulation. :)

    Good Luck!
  14. Rich Grise

    Rich Grise Guest

    They've been running series strings of Xmas tree bulbs for almost a
    century now. (not continuously, of course, I mean at Xmas, you drag
    out your series string of bulbs & stuff.) I don't foresee voltage-sharing
    as a problem - the one with the least resistance will, of course, drop the
    lowest voltage; they will reach equilibrium, much like series LEDs do.

  15. Rich Grise

    Rich Grise Guest

    You could also use a low-voltage system, like those yard lights that
    you see all over the place.

    The stumbling block, of course, is the cost of the transformer, and the
    need for really fat wire. At the transformer output, it has to carry
    ALL of the current for every bulb - 10X 4A bulbs would be 40 amps -
    #6 or #4 wire should be happy with that amount of current. Or, you
    could run individual pairs to each lamp.

    Good Luck!
  16. The former.
    Less notching effects, so the filaments should last somewhat longer.
    Nature has a way of reminding us of such things every now and then.

    Best regards,
    Spehro Pefhany
  17. We used high pressure sodium for color matching with good results.
    You can buy 12 V 50 W tungsten halides with reflector at the dollar store.
    You could try matching them for use in series strings, however if one burns
    out you will apply 115 VAC across that lamp and they may arc over since they
    are not designed for this service.
  18. krw

    krw Guest

    I guess the fog lamps on any of my cars haven't been true. Fog lamps
    simply are mounted lower so they don't reflect off the fog directly
    back. They're usually pointed more down so they illuminate the road
    directly in front.
    High beams are simply pointed straight forward.
    Not here. Low beams are simply pointed down and to the right so they
    aren't in the opposing driver's eyes.
    Never had one fail.
    That's why I have two. ;-)

    <stuff snipped because top posting makes no sense>
  19. krw

    krw Guest

    The series Xmas bulb strings have an internal structure that will
    short the bulb when the filament burns out, allowing the string to
    stay alive.
  20. Marra

    Marra Guest

    What will probably happen is one light will be lower resistance than
    the rest and take a lot of current and blow itself up as it lights
    first and the others remain almost a short circuit.

    I know they do this with xmas tree lights but they are usually run
    well below 12 volts each.
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