Voltage

Discussion in 'The Projects Forum' started by Lightfire, Mar 23, 2011.

  1. Lightfire

    Thread Starter Well-Known Member

    Oct 5, 2010
    690
    21
    Hello,

    I have read somewhere that it is OK to put three 1.5 V batteries in a bulb rated 3.5 volt because if you put the bulb onto the batteries, the batteries' volt will fall to 3.5. Is that true? How it comes?
     
  2. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    That is a ridiculous statement (by whoever wrote it) because it makes no reference to what this bulb draws. Incandescent bulbs are rated in Volts and Watts which will give you 'I'. Your V drop will be Dependant on this Vs the capacity of the batteries.
     
  3. Lightfire

    Thread Starter Well-Known Member

    Oct 5, 2010
    690
    21
    http://www.kpsec.freeuk.com/components/lamp.htm
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    This implies that 3 AAA or 3 D cells would produce the same results...Not!
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,173
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    There are many LED flash lights using from one to 9 bright white LEDs in parallel running on 3 ea AAA, AA, or N . Some of mine are quite old and never lost a LED.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    Post #3 would seem to imply he's talking about incandescent lamps, not LEDs. This statement, extracted from the same post implies a specific voltage drop but ignores the large difference in mAH between battery families. The mAH-AH rating of a battery is the governing factor here.
    Code ( (Unknown Language)):
    1. [I]For example: a lamp rated 3.5V 0.3A is correct for a 4.5V battery (three 1.5V cells) because when the lamp is connected the voltage across the battery falls to about 3.5V[/I].
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    This is indeed standard practice (but soon may not be, as filament lamps grow obsolete). The nominal voltage of the filament lamp will be lower than that of the battery intended to power it. This leads to lamp ratings such as 1.2V, 2.4V, 3.5V, and 4.5V to about 5.5V, the latter range reflecting the fact that some portable lights used very much higher capacity "6V" batteries.

    This is not very precise, and clearly it will not apply unless there is some rough relationship between the current taken and the battery capacity. A considerable variety of lamp voltages and currents are (or were) available.
    http://www.lampco.co.uk/results.asp?page=2&section=Lamps&watt=0-2W&type=Torch&volt=0-15V


    Typically a torch (flash-light) using small cells might have a lamp using 300mA or less. A bigger torch or portable lantern might use more current, or else use a rather higher voltage rated bulb relative to the nominal battery voltage.

    The system would originally have been worked out for zinc-carbon batteries which had a pretty rapid decline in voltage with use. Modern alkaline types hold their voltage better, so the optimum lamp voltage would be higher. In practice, it may or may not be worthwhile to get exactly the ideal voltage, given that it is a compromise between brightness and lamp life.

    Of course, these days it might be better to forget about filament lamps and use LEDs instead!
     
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