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help - incandescent pilot light - AC, DC, or does it matter?

Discussion in 'Electronic Basics' started by wahzoo, Dec 26, 2003.

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  1. wahzoo

    wahzoo Guest

    I have a mechanical latching relay that provides 24VAC for pilot
    indication when it is turned to the "on" state. What is the best type
    of lamp to get for the pilot? The wires carrying the pilot 24VAC will
    be 18AWG and run for not more than 80 feet.

    For this app, the smaller and simpler, the better. I'd like to mount
    the pilot thru a switch plate about 1/8" thick. (This is for a house
    low-voltage lighting system by the way.)

    I've looked into using LED's but don't have space for the extra
    electronics involved.

    I've looked into neon, but don't think any work with less than 65VAC.

    I see small T-1 incandescents in the 2003 Mouser catalog and see a
    voltage rating of 28.0, but they don't say whether that is AC or DC.
    (?)

    Also, 24VAC has a widely used in US HVAC systems. Where does 28V?C
    come from? Is it a different standard or are they just covering
    24-28VAC?

    Any advice appreciated.

    --wahzoo
     
  2. bg

    bg Guest

    An led needs only a resistor in series with it, and if you look around you
    can find 24 volt leds packaged with the resistor, with the intent of being
    used as a panel mount pilot lamp. Leds last longer than an incandescant
    light bulb but the bulb types can generally be brighter. The voltage rating
    is AC or DC, either one works, but an led will only light for 1/2 cycle of
    the AC. Neon on the other hand will need higher voltage.You will lose some
    of the voltage due to the 80 feet of wire. How much depends on the current
    that the pilot draws.
    bg
     
  3. Louis Bybee

    Louis Bybee Guest

    Some of the telephone systems I've interacted with use a nominal voltage of
    28v, but in this instance I suspect it's situation of applying a slightly
    higher voltage rated lamp for longevity.

    If the lamp will be in the "on state" at a high duty cycle, or the
    indication is a critical element of an important system, you might consider
    one of the pilot light indicators with integral LEDs. These assemblies are
    available with all necessary elements contained within the shell of the
    device, and the burn time is very long.

    If you have difficulty finding a suitable unit they are available at any of
    the electrical wholesale houses.
    Louis

    Since this
     
  4. John Fields

    John Fields Guest

    ---
    Since an LED is a diode, on the AC half-cycle when it's not conducting
    only leaksge current will flow through it and the series resistor,
    allowing the voltage across the LED to rise high enough to puncture its
    junction and destroy the LED. In order to keep this from happening a
    diode can be placed in parallel opposition across the LED. Another
    diode can be placed in series with the combination, facing in the same
    direction as the LED, in order to keep from having to increase the
    wattage rating of the series resistor.
     
  5. The bulbs will work fine on AC or DC. A bulb rated at 28 volts will
    have a life about 10 times longer than one rated for 24 volts when
    powered from 24 volts. By the way, if you want to use an LED, get one
    that is two color (one color for each polarity, with two leads) and
    all you need is a 2000 ohm 1/2 watt series resistor to run it off 24
    volts AC.

    E.G. T1 red green
    http://rocky.digikey.com/WebLib/Lite-on/Web Data/LTL-14CHJ.pdf
    T1-3/4 red green
    http://rocky.digikey.com/WebLib/Lite-on/Web Data/LTL-293SJW.pdf
     
  6. John Fields

    John Fields Guest

     
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