How doe a dual-filament bulb work in a car?

Discussion in 'Automotive Electronics' started by Roderick Young, Nov 12, 2015.

  1. Roderick Young

    Thread Starter Member

    Feb 22, 2015
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    I've noticed that in type 1157 or 3157 incandescent automotive lamps, there are two filaments with a common pin. The lamps are nominally 12 volts, but I wanted to use one as a load in a 24-volt application, and was wondering whether I could just use the two filaments in series. Or is that how they work?

    I've been considering several possible scenarios for how the lights actually work in service, and can't seem to find the info on a quick google search. Possibilities:

    1. One filament is energized for high beam, the other filament is energized for low beam.

    2. One filament is energized for high beam, both filaments are energized in series for low beam.

    3. One filament is energized for low beam, both filaments are energized in parallel for high beam.

    4. Something else.

    Anyone know?

    p.s. sorry about the grammatical error in the title. I can't edit it.
     
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  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    The dual filament 1157 was the tail lights and the brake(turn/emergency flasher) light.

    The full specifications on the 1157 can be found here;
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    The bulbs normally have one side of the filaments tied together and are common with the screw base.
    The other end of the filaments go to two separate contacts at the bottom center of the bulb.
    Normally I think the are lit separately (one or the other).
    I believe the bulb would get too hot if they were both energized in parallel at the same time.

    You can't connect them in series to 24V since the two filaments are of different size and the small one (higher resistance) would take most of the voltage and burn out.

    If you need a 24V load then just connect two 12V single filament bulbs of the same wattage in series.
     
  4. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    the 1157 can be thought of as a polarized bayonet based lamp.

    The parking/running filament is a lower power than the turn signal/brake.

    All sorts of scenarios are possible depending on the vintage of the car.

    At one point, the park lights went off when the headlights were on.
    Turn signals over-rode brakes at one time too.

    So, the filaments can be operated separately or together, but not in series.

    High and low beam for headlights also refers to the aiming of the lights. Usually, they are independently operated. On one vehicle, they were independently fused. In another, when one shorted the lights blinked. There was a single thermal self-resetting fuse in the switch.
     
  5. Roderick Young

    Thread Starter Member

    Feb 22, 2015
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    I'm thinking, the low power filament will take the bulk of the power, but normally runs cooler, so might only reach the temperature that is normal for the high power filament? Using two bulbs is not practical as that means two sockets on a relatively small device. What I'm doing is connecting a 24V lead-acid battery to a supercapacitor bank, and want to avoid a massive surge current while the voltages equalize. It will take possibly a few minutes for this equalization to occur. After that happens, I will flip a switch so that there is a low impedance path between the battery and supercap bank. I could use a power resistor, but using a bulb will give me a visual indication at no extra charge.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Both filaments tend to run near the same temperature at their rated voltage.
    It's the length (and thickness) of the filament that determines the light output. (Look at the two filaments and you will see the physical difference).
    If you put them in series, the high resistance, low power filament will take most of the voltage (much more than 12V for 24V applied to the two), and so will likely burn out in very short order, perhaps instantly.
    If you could find a double-filament bulb that has two equal power filaments then you could put them in series, but I doubt that such a bulb is available.
     
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  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
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  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    The turning signal, flasher signal, and brake signal is the higher power (brighter) filament. Think safety.

    The lower power filament is the running light (parking light).

    On edit, removed the word share in the first sentence and added a bulb pic

    [​IMG]
     
    Last edited: Nov 13, 2015
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    The filaments almost never have the same rating, such as a 6/23W indicator bulb or a 55/60W H4. The lower wattage filament will likely blow if you put the 2 in series across 24V.

    The exceptions I can think of; a WW2 era bus headlamp bulb that somehow found its way into my collection, and several small motorcycle headlamp bulbs on machines that had AC lighting - the bulbs run from a generator winding with no rectifier or battery, differences in dip and high beam would need the extra cost of switching in a load resistor to regulate the lower rated filament, instead they just make both filaments the same wattage.
     
  10. Roderick Young

    Thread Starter Member

    Feb 22, 2015
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    Yes, looks like that's the way to go. I ordered some #311 bulbs off eBay. rated 28V, 1.2A. Apparently, they fit in an ordinary 15s bayonet socket, which is readily available.

    Thanks, everyone, for the education.
     
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  11. Nepenthes

    New Member

    Dec 18, 2015
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    Thanks for the clarification on dual-filament bulbs. Ya learn more than one thing everyday as one should.
     
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