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Do globes care about AC or DC?

Discussion in 'Electronic Basics' started by RR, Oct 24, 2005.

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  1. RR

    RR Guest


    If I have a 12V globe (or any voltage for that matter), does the globe
    "care" whether it is run on AC or DC?

    "Care" in the sense of:
    - will it work?
    - will it last as long (or maybe longer)?

    In particular, we're thinking of moving out of town and building a house
    without connecting to the electricity grid.
    We like the 12V lights that have now become so popular. These run from
    mains via a transformer. The transformer converts mains AC to 12V AC.

    If we ran 12V batteries for the house, it would make sense to just run 12V
    DC to all our lighting rather than convert to mains AC via the inverter and
    then back to the 12V with the above transformer(s).

  2. Ralph Mowery

    Ralph Mowery Guest

    AC or DC is fine for the globes. Just use the same voltage.
  3. Don Bruder

    Don Bruder Guest

    If it's a standard incandescent type, AC versus DC means exactly zilch.
    If it's an LED type, or a flourescent, then it's going to be picky about
    what it eats. In theory, a flourescent COULD run from either AC or DC,
    but the use of transformers to play with the starting voltage/current
    and such means that AC must be used to power it unless you go with
    something that's built to deal with DC. (which will more than likely
    simply turn the DC into AC or pseudo-AC via some form of inverter, do
    the transformer work on it that's needed, and run the light from that)
    LED-style, of course, will need to run on DC, and will expect the proper
    polarity. If fed the wrong juice, or the wrong polarity, they tend to
    leak out all the magic smoke rather quickly.
    Again, if it's an incandescent, it should work fine, regardless of
    whether you feed it AC or DC of the proper voltage. It *MIGHT* last
    longer if run on DC, or it might not make any measurable/significant
    difference, depending on the exact filament technology it's built
    around. I would think it extremely unlikely that the lifespan would be
    any shorter due to being run on DC.
  4. RR

    RR Guest

  5. Don Bruder

    Don Bruder Guest

    Feed 'em 12 volts of either AC or DC, and they'll work just fine. Going
    back to your previous message, I would say these guys should last at
    least as long when run on DC as AC, and likely a bit more besides. But
    that's just my hunch, with nothing scientific to back it.
  6. John Larkin

    John Larkin Guest

    DC can cause electromigration, but the effect is minor here.

  7. quietguy

    quietguy Guest

    I wonder if the length of the cabling to your lighting will be an issue - long
    runs at low volts and 'high' amps mean thick(er) cabling to avoid excessive
    voltage drop.

    Worth thinking about as 32v systems were generally the go in the old days to
    reduce cable losses

  8. Your 12v wiring will have to be much heavier than the wiring for 230v.
    Perhaps you might give some thought to an inverter to convert 12v DC to 230v
    AC. This will make consumer electrical goods easier to operate. Try a
    ship/yacht chandler for a suitable inverter and a wind generator to mount on
    the roof.

  9. Tom Biasi

    Tom Biasi Guest

  10. David

    David Guest

    The main thing to worry about here is cabling and switching. If you are
    using 12V, 50W dichroics thats about 4A per bulb to switch. If you have four
    that's 16A to switch. Obviously switch will have to be rated higher than
    that. Or maybe use relays and then there's dimming to consider. None of
    these problems exist when used with mains transformers/SMPS's as the primary
    current is small and dimmers are easy to add.

    Anyway in just a few years LED's will start to take over and they are about
    35% efficient as against 7% for your halogen globes. I would think about
    using LED's now and just a few halogens where you need good light. More
    expensive yes, but you will save on cabling and switching costs. Oh there is
    one drawback with LED's at the moment and that is colour/color temperature
    and CRI but thats another story.

  11. Andy Baxter

    Andy Baxter Guest

    David said:
    Do you know how LEDs compare to fluorescents? From my own testing they
    seem about the same or a bit better (maybe 10-20% better?) but I've never
    seen this properly tested anywhere.
  12. David

    David Guest

    Depends on what you mean by "better". Fluoro's produce about 40 lumens/watt
    against 25 lumens/watt for LEDs. But LED output is constantly improving.
    Lifetime for fluoro is about 5000 hours compared to 50,000 for LED. Lifetime
    for LED is when the light output reaches between 70 - 50% of original. Light
    from LED is concentrated, whereas light from a fluoro is uniform.
    Colour/color temperatures and CRI vary widely.

  13. Jasen Betts

    Jasen Betts Guest

    Not really.
    AC may be slightly more stressful on the filament, so possibly slightly
    longer on DC.
    yes, IMO, that would work. check with the maker of the globes.

    but if you're making your own electricity you may find that LED or CCFL use
    less electricity, and so can run for longer.

  14. RR

    RR Guest

    Thanks to all who replied! Very helpful.

    Quietguy: yes, I'm just looking into the issue of cable thinkness.

    Roger Dewhurst: it just doesn't seem to make sense to have 12VDC in my
    batteries, invert it to 230VAC and then run transformers to bring it back
    down to 12VAC. That's a lot of conversion and surely a lot of loss?

    David said:
    Thanks, but aren't switches rated on power? So a 240V switch rated at 10A
    will be quite happy to switch 12V at up to 240*10/12 = 200A?

    Maybe I've still got a lot to learn?

    LEDs suggestion....I'll think about that!
  15. Randy Day

    Randy Day Guest

    RR wrote:

    Not exactly. The power needed to melt the contacts is
    calculated by I squared R, where R is the resistance
    of the switch contacts. If I is constant (16A), R is
    constant, then power dissipation across the contacts
    is constant.

    The 240V only comes in to play when worrying about
    Hope that made sense...
  16. Most white LEDs have color rendering index rated to be 70-85, and old
    tech cool white fluorescent rates usually 62, sometimes 66. In both cases
    the main color distortions are making most colors - especially reds and
    greens - darker and duller than "proper". White LEDs are not as bad as
    cool white fluorescent.

    Triphosphor fluorescents, which includes most compact fluorescents, are
    different. Most compacts, Philips "Ultralume" and the better of the two
    common grades for T8 (GE SPX, others with color code of 8 followed by a
    2-digit abbreviation of the color temperature) have rated color rendering
    index of 82-86 (82 for compacts). The main color distortion is making
    most colors brighter and more "vivid" than proper, although reds come up

    Most white LEDs have color temperature around 6000K (icy cold slightly
    bluish). Most higher power white ones appear to me to be around
    5000-5500K - not as bluish to sometimes not looking bluish at all but
    still icy cold.
    There are some "warm white" LEDs out there, with color temperature
    mostly around 3500K. However, I see a tendency for those to be a little
    less efficient than the higher color temperature ones.

    - Don Klipstein ()
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