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continuity test pass, but lamp does not go on!

Discussion in 'Electronic Basics' started by [email protected], Apr 22, 2004.

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  1. Guest

    A quite simple question to you guys, but it beats me:

    I have a lamp in which I left the 2 AA batteries for quite some time.
    As you know, the green stuff accumulated on the metal parts of the
    lamp. By metal parts I mean the metals which come in contact to the
    batteries in order to close the circuit. I scraped the green stuff off
    and replaced the batteries but the lamp does not go on. I tested the
    light bulb on another lamp and it works. I tested the metal parts for
    continuity using a continuity tester and there is connectivity from
    any metal part to any other when the lamp is in and the switch is on.
    The batteries are new. However when the batteries are in and the
    switch is on, the lamp does not go on. What is wrong? Did leaving the
    old batteries for so long affected the connectivity of the metals
    somehow ? Maybe increased their resistance?

    Of course I could simply buy a new lamp, but my question here is
    mostly educational.

    Thanx
     
  2. Bill

    Bill Guest

    Obviously, I can't be sure but two things I would look at are 1. If there is a
    back plate where the batteries connect to each other check that. 2 Whether
    putting the batteries in causes more pressure against the metal plates, thus
    moving them, causing a loss of connection. You could also double check the new
    batteries in numerous ways.

    Incidently, I sometimes use a metal file to clean things off in similar
    situations - i.e. where you placed your continuity checker and where the
    batteries touch may not be the same.

    Bill
     
  3. John Fields

    John Fields Guest

     
  4. You would want to be careful with this. Battery terminals are usually coated
    with nickel. filing this off would expose the copper (or whatever they chose
    to use) underneath, allowing tarnish and further corrosion.

    The "file" repair doesn't last very long.

    Best to _try_ to clean off only the corrosion, but even sometimes this is not
    possible, because the corrosion has gone through the nickel. I usually replace
    in this instance (where possible).

    I've tinned with solder, which tarnishes easily, but also cleans easily.
    Sometimes just rolling the batteries in their case will scratch enough of the
    tarnish off to reduce the resistance enough to make it work.
    Not reliable enough for a customer repair though.
     
  5. Ross Herbert

    Ross Herbert Guest


    Put the batteries in and leave the lamp out. Switch on and see if you
    can measure the full battery voltage at the contacts which normally
    connect to the lamp. If you can see the full voltage with the switch
    on then make sure it disappears when you switch off. This proves the
    switch works. If this checks ok then the only possibility is that the
    torch contacts are not making firmly enough with the lamp contacts or
    the lamp filament is intermittent open circuit. Try gently tapping the
    torch with the switch on to check this out. Also you might try this
    tap test in the torch which showed the lamp to be ok.
     
  6. Stan

    Stan Guest

    () wrote:

    }A quite simple question to you guys, but it beats me:
    }
    }I have a lamp in which I left the 2 AA batteries for quite some time.
    }As you know, the green stuff accumulated on the metal parts of the
    }lamp. By metal parts I mean the metals which come in contact to the
    }batteries in order to close the circuit. I scraped the green stuff off
    }and replaced the batteries but the lamp does not go on. I tested the
    }light bulb on another lamp and it works.

    I would be suspicious of the switch (having first checked the batteries
    to see if I installed them correctly, + to -).

    Describe the switch as best you can.

    Stan.
     
  7. JR North

    JR North Guest

    There are sometimes riveted connections in the strips that
    complete the power path. You may have overlooked these in
    cleaning the unit. They may pass voltage, but little or no
    current.
    JR
     
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