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Perplexing Light Bulbs

Discussion in 'Electronic Design' started by Mike Monett, Sep 6, 2008.

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  1. Mike Monett

    Mike Monett Guest

    To All,

    I have a perplexing question on some plain incandescent light bulbs.

    I bought a package of four ordinary Sylvania 25 Watt 120 volt
    candelabra light bulbs to act as current limiters in a HVDC supply.
    The manufacturer's code is 25B10C/BL/4PK.

    Here's a picture:

    http://www.servicelighting.com/catpics/sylvania/13452.jpg

    After the purchase, I opened the package to check the filament
    resistance. All four bulbs were open!

    I applied 120VAC and each bulb lit normally. Afterwards, each bulb
    read about 46 ohms on a DVM.

    The next day, three of the four bulbs were open. After applying
    120VAC again, they all read about 46 ohms.

    I examined the bulbs carefully under a microscope. Everything seems
    normal. The crimps at the filament look normal. The welds on the
    lead wires seem fine.

    I removed the base from one of the bulbs to check if anything
    strange was inside. There was nothing. The wires go straight into
    the bulb.

    The bulbs all look and behave like any normal 120 Volt incandescent
    light bulb when 120VAC is applied.

    But when low voltage DC is applied, the bulbs measure open circuit
    until 120V is applied. Afterwards, they all measure close to 46
    ohms, but most go open circuit again after a day.

    It looks like the only place something could be going on is at the
    filament crimp. But that seems to be normal and the same as every
    other light bulb I have. The others act normally.

    Can anyone tell me what is going on?

    Best Regards,

    Mike Monett
     
  2. Jamie

    Jamie Guest

    hard to say, try using your diode mode next time or switch the leads
    around..
    could be galvanic.

    http://webpages.charter.net/jamie_5"
     
  3. legg

    legg Guest

    It's got to be oxidation in a contact. Worst connection is to the
    screw-form, which is often simply press-fit. Shouldn't be much
    oxidation going on inside the envelope, as it's evacuated.

    RL
     
  4. Ross Herbert

    Ross Herbert Guest

    : To All,
    :
    : I have a perplexing question on some plain incandescent light bulbs.
    :
    : I bought a package of four ordinary Sylvania 25 Watt 120 volt
    : candelabra light bulbs to act as current limiters in a HVDC supply.
    : The manufacturer's code is 25B10C/BL/4PK.
    :
    : Here's a picture:
    :
    : http://www.servicelighting.com/catpics/sylvania/13452.jpg
    :
    : After the purchase, I opened the package to check the filament
    : resistance. All four bulbs were open!
    :
    : I applied 120VAC and each bulb lit normally. Afterwards, each bulb
    : read about 46 ohms on a DVM.
    :
    : The next day, three of the four bulbs were open. After applying
    : 120VAC again, they all read about 46 ohms.
    :
    : I examined the bulbs carefully under a microscope. Everything seems
    : normal. The crimps at the filament look normal. The welds on the
    : lead wires seem fine.
    :
    : I removed the base from one of the bulbs to check if anything
    : strange was inside. There was nothing. The wires go straight into
    : the bulb.
    :
    : The bulbs all look and behave like any normal 120 Volt incandescent
    : light bulb when 120VAC is applied.
    :
    : But when low voltage DC is applied, the bulbs measure open circuit
    : until 120V is applied. Afterwards, they all measure close to 46
    : ohms, but most go open circuit again after a day.
    :
    : It looks like the only place something could be going on is at the
    : filament crimp. But that seems to be normal and the same as every
    : other light bulb I have. The others act normally.
    :
    : Can anyone tell me what is going on?
    :
    : Best Regards,
    :
    : Mike Monett


    Possibly oxidised lead wires to the insulated center contact or screw base
    contact points. The high voltage will break down the oxide layer resistance and
    lower the resistance to around normal. Suck the solder off these points and
    check, then clean and re-solder.
     
  5. I have no idea, but if you care you could measure resistance as a
    function of voltage and see when they turn on. is it at 2 volts or
    100 volts. Also try another DVM.
     
  6. Phil Allison

    Phil Allison Guest

    "George Herold"
    Mike Monett

    I have no idea, but if you care you could measure resistance as a
    function of voltage and see when they turn on. is it at 2 volts or
    100 volts. Also try another DVM.



    ** Oxidised and sulphated contacts in switches and relays behave exactly
    like this too.

    So you have never see that ?


    ...... Phil
     
  7. I am suspecting that these "bulbs"/lamps have aluminum base shells.
    Aluminum typicaly has a thin oxide coating that may sometimes end up
    being an insulator to voltages to a volt or a couple, maybe a few volts or
    whatever,

    especially when voltage is less than 2 volts and you make contact
    gently.
    You may get more realistic readings if you scratch/scrup/scrape the
    meter probe that contacts the "shell" part of the base.
    The "tip" of the base may have a slight need for "digging in" also when
    voltage is very low to make "full contact".

    - Don Klipstein ()
     
  8. Mike Monett

    Mike Monett Guest

    Thanks for the reply.

    As part of the original testing, I removed the base on one bulb and
    measured the resistance directly to the wire leads.

    The bulb was open. I applied 120VAC to the leads. It then measured
    46 ohms.

    In another test, the bulb was originally open. Applying 120VAC
    caused it to light normally. The bulb then measured 46 ohms.

    After melting the solder on the center contact, the bulb went open
    circuit. Applying 120VAC caused it to light normally, and it now
    measured 46 ohms.

    I did another test that behaved the opposite. I found a bulb where
    the solder did not wet the center contact lead properly. It was
    open.

    I cleaned and scraped the lead, then applied solder with plenty of
    rosin flux. Under the microscope, you could see the lead was now
    properly wetted with solder.

    The bulb was still open. I applied 120VAC, the bulb lit, and it now
    measures 46 ohms.

    The lead connecting to the base shell is copper. The center contact
    lead is a shiny metal with about the same flexibility as copper, but
    thinner than the other lead.

    The shiny center lead is welded to a slightly thicker copper lead
    about 0.675 inches up from the contact. The copper lead goes inside
    the glass.

    As I understand it, bulbs of 25 Watts or more require a fuse in the
    hot lead. The wiring codes make the screw shell contact neutral, so
    the center contact is hot.

    That is apparently the explanation for the shiny lead going to the
    center contact. It's the fuse. But it's not normal fuse wire, since
    you can solder it without melting.

    The copper wire connection to the aluminum screw-form is welded. I
    found an open bulb, and measured the resistance to the copper lead
    right at the weld. The bulb was still open. I applied 120VAC, the
    bulb lit, and now measures 46 ohms.

    So it looks like the connections outside the bulb are not the
    problem. It seems to be inside the bulb.

    If this phenomenon is part of some new manufacturing process, we may
    have problems finding suitable bulbs to measure oven temperature,
    stabilize Wein bridge oscillators, generate Gaussian thermal noise,
    etc.

    Any suggestions for more tests to run?

    Best Regards,

    Mike Monett
     
  9. Phil Allison

    Phil Allison Guest

    "Mike Monett"
    ** You got a variac ??

    See what happens when you try to bring them it up and dim them.

    IMO - the lamps are all faulty.

    See one lamp just like them recently myself.



    ...... Phil
     
  10. legg

    legg Guest

    The bond between the tungsten filament and the lead-frame may be the
    source of low-voltage effects. I know that both parts have a
    shelf-life prior to use, as oxides do form on them, though no-where
    near as readily as on other more common conductors. If the internal
    contacts aren't welded, this may cause some trouble.

    I'd run some bulbs for 24 hours, then see if the low-voltage
    intermittency is still measurable. The manufacturer may not consider
    this kind of performance important enough to alter fab processes in
    products destined for a specific high-voltage application.

    If anything, I think you've just discovered one reason 'why I can't do
    that". In a power limiting protective function, this may be benign,
    particularly if the part is 'tested' regularly. There are HV PTCs made
    to work reliably in electronic circuits.

    RL
     
  11. Phil Allison

    Phil Allison Guest

    "legg"

    ** Maybe the maker should advise consumers that such lamps with dodgy
    internal pressure connections are in fact just like most CFLs - ie

    " Non Dimmable"............



    ...... Phil
     
  12. Mike Monett

    Mike Monett Guest

    Yes, I suspected the crimp at the filament. But I examined some
    other bulbs under the microscope and couldn't find any difference.

    None of them were welded - the filament was held in place by folding
    the lead wire over the filament and closing the gap.

    Of course, I emailed Sylvania asking them about the problem. As you
    point out, they have little reason to worry about this problem. I
    don't even expect a reply:)

    There are other solutions, but the light bulb is useful since it
    lights up when there is a fault condition. So I used a small 25W
    bulb from a microwave oven instead. It works fine!

    Thanks for your help.

    Best Regards,

    Mike Monett
     
  13. Did you try cutting off the glass and measuring the resistance right on the
    filament, or is there something about a tungston filament that makes it hard
    to make good contact?
     
  14. Mike Monett

    Mike Monett Guest

    Hi Tom,

    I was considering that, but it would disturb the connection so much
    it would be hard to prove the results meant anything. Also, there
    would be no way to restore the original condition to prove we could
    re-create the original problem.

    As mentioned, I did examine the crimp under a microscope and
    compared it with normal bulbs. There wasn't any difference that I
    could see.

    The curious thing is all the Sylvania bulbs behaved exactly the
    same. They are normally open, and applying 120VAC causes them to
    read the same. They all read 46 ohms within 1/2 ohm. It is hard to
    see how a loose connection can be so consistent in all the bulbs.

    Another problem is there are two crimp connections in each bulb. If
    it is the crimp, which one is doing it? Then, if it is one, why that
    one and not the other?

    So we have to design an experiment that analyzes something we cannot
    reach, and tells us why the bulbs act this way.

    I was thinking of applying a fairly low frequency, low amplitude
    signal to the connections to see if a capacitive probe could pick it
    up. Then trace it until it stops. That would require a narrow-band
    receiver to track a low-level signal from the probe. So I have to
    get my HP 8566 cleaned and give that a try.

    Best Regards,

    Mike Monett
     
  15. Den

    Den Guest



    No - my vote is on oxidation at the crimp interface.


    This thread lead me to some interesting info on tungsten, funnily enough at:
    http://www.tungsten.com/
     
  16. Mike Monett

    Mike Monett Guest

    Hi Den,

    Thanks for the reply. Oxidation is a neat word, but it requires oxygen. If
    there were any inside the bulb, it would glow very bright and burn out. For
    example, when you break a bulb with power on.

    According to Don Klipstein, the gas inside the bulb is probably 93% argon
    and 7% nitrogen. That adds up to 100%, so there's not much room for oxygen.

    However, you are right. There is a problem at the crimp. I describe it in
    my reply to Tom Del Rosso.

    Best Regards,

    Mike Monett
     
  17. Mike Monett

    Mike Monett Guest

    Thanks, Don.

    You are right. The base is aluminum, but my probes go right through
    any oxide coating.

    I solder small sewing needles on the ends of the probes to make a
    sharp point. I can push this through wire insulation and measure
    continuity. It also digs through soldermask, and through any
    corrosion or oxidation on a contact.

    I also use test leads with small alligator clips to grab and hold
    onto a lead. When the bulb has continuity, the resistance is pretty
    uniform at 46 ohms, so I think it's making good contact.

    Best Regards,

    Mike Monett
     
  18. Den

    Den Guest



    Hi Mike - I was thinking surface oxidation pre-assembly on either the
    filament or the support conductors.


    I saw your reply to Tom, it's interesting the thought something as seemingly
    simple as a incandescent bulb can provoke.

    regards
    d.
     
  19. Mike Monett

    Mike Monett Guest

    Tom,

    I discovered that gently tapping the bulb on the table would make
    the connection open up. This gave me some courage to try your
    suggestion, and I cut the top off a bulb.

    It was extremely difficult to make measurements without disturbing
    the filament. Eventually the connection closed and I could not get
    it to open again.

    But without the glass, I was able to focus in on the crimp
    connection at max magnification. And I noticed something that
    escaped me before using low power.

    One of the crimps is very tight. The wire folds back and touches
    itself.

    The other crimp is quite open, with a gap the width of the wire.

    And if you zoom in real close, you can see a brown powder residue at
    the crimp around the filament.

    I read recently that an arc inside the bulb makes nitrogen or
    tungsten compounds, and some are brown. If this is correct, it
    probably explains the fragile connection.

    I checked the other bulbs. All had one tight and one loose crimp,
    and two bulbs had a trace of the same kind of brown residue at the
    crimp.

    So I think that solves the problem. The machine that made these
    bulbs is out of adjustment, and one crimp is loose.

    Probably never happen again in a million years:)

    Thanks for the suggestion. I think it led to the solution.

    And thanks to all the others for your help.

    Best Regards,

    Mike Monett
     
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