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  1. #1

    Default Advice on dial light bulbs?

    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. #2

    Default Re: Advice on dial light bulbs? (By: Uncle Lem)

    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. #3

    Default Re: Advice on dial light bulbs? (By: James McDermaid)

    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. #4
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    Default Re: Advice on dial light bulbs? (By: mxfrank)

    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. #5
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    Default Re: Advice on dial light bulbs? (By: Tinker Dwight)

    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"?

    Quote Originally Posted by Tinker Dwight View Post
    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
    If you are not fired with enthusiasm, you will be fired with enthusiasm. Vince Lombardi

  6. #6
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    Default Re: Advice on dial light bulbs? (By: THTanner)

    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. #7
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    Default Re: Advice on dial light bulbs? (By: Tinker Dwight)

    Interesting good to know -

    Quote Originally Posted by Tinker Dwight View Post
    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
    If you are not fired with enthusiasm, you will be fired with enthusiasm. Vince Lombardi

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