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A 25W Light Bulb Married a GE Clock/Radio

  1. May 16, 2012 #1
    I'm wiring a 25W Edison bulb to an old GE clock/radio.

    The radio is 120V/60hz 8.5W input.

    Basic calculation I = P/E would mean the current is 0.07A

    If I wire the bulb in parallel between source and the clock/radio, there's now 0.2A for the bulb branch and 0.07A for the clock/radio branch making Current Total about 0.3A (300mA) correct?

    In doing so, however, I haven't actually made any change to the current flowing through the radio, right? It's still 0.07A?

    So if the power cord which goes from the source to the bulb branch (first) and then to the clock/radio branch is the only thing carrying 0.3A and is rated well above that, I'm not in any danger of burning up the clock/radio, yes?

    My concern is safety (obviously) so any help is appreciated.
     
  2. jcsd
  3. May 16, 2012 #2
    nathangriffin,
    Yes, you are correct. If the line cord can deliver the 300mA than the radio will continue to see the 120V it is expecting and will draw its normal load current.
    I can appreciate your concern for safety. Tell me a little more about your scheme and I can give you some tips on safety.
    Where will your lamp wiring be located, inside of the radio? Where is the lamp itself mounted?
    Does the clock radio have a safety ground (third prong on line cord)?
     
  4. May 16, 2012 #3
    Thank you for your response.

    It's a pretty simple project, really... I think. I have an old clock radio. The cord is being replaced (re-soldered) using a new SPT-1 polarized plug/cord (two prong, one hot and one wide neutral) and I'm wiring the lamp socket in parallel on the source side of line, making my connection about 3" in from the step-down transformer in the clock/radio using orange wire nuts.

    I'm connecting the line cord to the socket with 18AWG, remembering black to brass (to save my @ss) and, hopefully calling it a day? I've actually drilled a 1 3/8" hole in the top of the clock/radio and will secure the socket there. The only other thing I can think of for safety is, securing any extra wire away from things which heat up (eg. transformer)

    Hmm. Can't think of anything else.

    Again, if you can add anything, I'd love to know! I tried covering all my bases but had been out in the workshop these last two days debating whether I still remembered my principles, circuit basics, cause & effect etc!
     
  5. May 16, 2012 #4

    berkeman

    User Avatar

    Staff: Mentor

    Welcome to the PF.

    It sounds like you are thinking of the right safety issues. Does "black to brass" equate to wiring Hot to the center/tip connector for the light bulb? That's important. Also, are you mechanically strain relieving the cord where it goes into the device?
     
  6. May 17, 2012 #5
    nathangriffin
    No safety ground and use of AC mains puts this into IEC safety class II. Safety class II products are designed so that no single internal failure can pass hazardous voltage to an exposed conductor that a person can touch. This is generally done by either:
    1 - Not having any exposed conductors (think hair dryer)
    2 - Providing a double barrier between hazardous voltage and exposed conductors. The double barrier is designed so that it cannot be bridged by any single fault. (safety specs call this "double insulation", where insulation refers to any means of separation).

    If your radio does have exposed metal, such as knobs, antenna, fasteners, you will want to make sure you don't compromise the barrier.
    The idea is to assume a single failure occurs in your work, say your wire nut comes loose exposing your wires. Now look and see if that failure can cause AC mains to contact an exposed conductor. If so, add a second barrier. This could be as simple as a zip tie to hold the wires in a specific location. If both a zip tie and a wire nut have to fail to create a hazard then you are ok.
     
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