Advice on dial light bulbs?

Discussion in 'Electric Horology' started by Uncle Lem, Feb 13, 2017.

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  1. Uncle Lem

    Uncle Lem Registered User

    Jul 26, 2013
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    Working on an early 1970's General Time clock with the rolling film numbers. The backlight is burned out. The bulb appears to be a normal incandescent, although very small, and it taps directly off the 110v motor power. I'd like to replace it with an LED, but all of the 110v LEDs are too large. Any advice on where to source such a bulb?
     
  2. James McDermaid

    James McDermaid Registered User
    NAWCC Member

    Apr 29, 2011
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    LED's come in many sizes down to teeny tiny.

    you would need a resistor and a diode to change to DC and lower the voltage to about 2 volts for the LED.

    Personally I would replace the incandescent bulb with an original type.

    I work on radios of the 1930's and have no trouble finding bulbs.

    Jim
     
  3. mxfrank

    mxfrank Registered User

    Oct 27, 2011
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    You should be able to find something compatible from an electronics house like Digikey. It would help to know the base style and diameter, as well as the bulb style.
     
  4. Tinker Dwight

    Tinker Dwight Registered User

    Oct 11, 2010
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    LED can be used with a resistor and diode as mentioned but
    they are really intended to be run with a transformer or
    bucking power supply.
    A LED that is bright enough to equal an incandescent bulb
    will draw about 20-100ma. Will need a 2W resistor of about
    1200 ohms for 100ma.
    The diode should be placed in parallel with the LED, rather than
    in series.
    An alternate trick is to tap some power from the magnetic field
    of the motor. You can use some magnet wire and wrap it around
    one of the pole pieces. Make sure to put some tape so the wire doesn't
    short on the sharp corners.
    Use an AC volt meter and use enough turns to get about 5 or 6 volts.
    You can then use just a resistor without the diode because of the
    low voltage involved.
    One can even strip an LED from a light bulb if you can't find one you
    like on ebay.
    Tinker Dwight
     
  5. THTanner

    THTanner Registered User
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    Jul 3, 2016
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    Did you mean that you can then use just the diode without the resistor - not "you can then use just the resistor without the diode"?

     
  6. Tinker Dwight

    Tinker Dwight Registered User

    Oct 11, 2010
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    The LED produces light when forward biased. They can generally take 5 to 6 volts
    in reverse bias. At such a low voltage, you could get by with just the current
    limiting resistor and AC source.
    Using the diode is OK. At the lower voltage it shouldn't be put in series but put
    across the LED to reduce the reverse voltage. The current limiter resistor works
    better, the more voltage it drops.
    Tinker Dwight
     
  7. THTanner

    THTanner Registered User
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    Jul 3, 2016
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    Interesting :) good to know -

     
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