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Electrical res.reading

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Servicewiz_3
Servicewiz_3 Member Posts: 56
in THE MAIN WALL
I am not sure why my 120 volt pilot light will operate with 120 volts applied to it but I cannot get an ohm reading on the bulb (I'm on the 40 meg scale)

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  • SWEI
    SWEI Member Posts: 7,356
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    Is it a neon bulb?
    ChrisJJUGHNE
  • ratio
    ratio Member Posts: 3,626
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    That would explain it. A neon lamp is a gas-discharge lamp, no continuity until the applied voltage is great enough to arc over.

    I wonder if a megger would light one up. Time for an experiment!

  • Jamie Hall
    Jamie Hall Member Posts: 23,282
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    ratio said:

    That would explain it. A neon lamp is a gas-discharge lamp, no continuity until the applied voltage is great enough to arc over.

    I wonder if a megger would light one up. Time for an experiment!

    No... sorry. You have to get the voltage high enough to start the arc. A megger doesn't do that (I hope!).
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • BobC
    BobC Member Posts: 5,478
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    Some meggers can supply thousands pof volts but very little current. If I recall a NE-2 needs about 90 v to fire and then it needs to be limited to under a milliamp or the bulb will fail in a short time.

    https://en.wikipedia.org/wiki/Neon_lamp

    Bob
    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • ChrisJ
    ChrisJ Member Posts: 15,672
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    Jamie Hall said:

    ratio said:

    That would explain it. A neon lamp is a gas-discharge lamp, no continuity until the applied voltage is great enough to arc over.

    I wonder if a megger would light one up. Time for an experiment!

    No... sorry. You have to get the voltage high enough to start the arc. A megger doesn't do that (I hope!).
    Depends on the megger and the neon lamp I suppose.
    If I recall the megohmmeter I used at a previous job did up to 2KV. That'll arc over a small neon lamp easily and then some.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • SWEI
    SWEI Member Posts: 7,356
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    Could also happen with an LED and a series resistor. Unless you get enough voltage through the resistor to overcome the forward voltage drop of the LED, it won't register -- plus, it's polarity sensitive.
  • Servicewiz_3
    Servicewiz_3 Member Posts: 56
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    How can I tell if my pilot light is neon or standard?
  • Jamie Hall
    Jamie Hall Member Posts: 23,282
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    ChrisJ said:

    Jamie Hall said:

    ratio said:

    That would explain it. A neon lamp is a gas-discharge lamp, no continuity until the applied voltage is great enough to arc over.

    I wonder if a megger would light one up. Time for an experiment!

    No... sorry. You have to get the voltage high enough to start the arc. A megger doesn't do that (I hope!).
    Depends on the megger and the neon lamp I suppose.
    If I recall the megohmmeter I used at a previous job did up to 2KV. That'll arc over a small neon lamp easily and then some.
    I do hope that that megger had some major current limiting devices in it...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ratio
    ratio Member Posts: 3,626
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    Does it have a colored lens? Can you see the bulb itself? Without any kind of colored lens the color of the light is orangish. The bulb is clear glass, generally about 3/16 diameter & less than a half-inch long, with two wires going into the bottom. Inside, the wires connect to two posts that run parallel to each other some small distance apart & don't actually touch. If it's in an assembly of some kind, it may be stamped "Ne" somewhere.
  • ChrisJ
    ChrisJ Member Posts: 15,672
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    Jamie Hall said:

    ChrisJ said:

    Jamie Hall said:

    ratio said:

    That would explain it. A neon lamp is a gas-discharge lamp, no continuity until the applied voltage is great enough to arc over.

    I wonder if a megger would light one up. Time for an experiment!

    No... sorry. You have to get the voltage high enough to start the arc. A megger doesn't do that (I hope!).
    Depends on the megger and the neon lamp I suppose.
    If I recall the megohmmeter I used at a previous job did up to 2KV. That'll arc over a small neon lamp easily and then some.
    I do hope that that megger had some major current limiting devices in it...
    I don't know.
    I never tested it on my self, it was used for hi pot testing power tools.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • BobC
    BobC Member Posts: 5,478
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    @ChrisJ This was a Danish model (Breul) that was tube based and incapable of supplying more than a milliamp of current.

    Bob
    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • ChrisJ
    ChrisJ Member Posts: 15,672
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    BobC said:

    @ChrisJ This was a Danish model (Breul) that was tube based and incapable of supplying more than a milliamp of current.

    Bob

    Those Danes make good stuff. ;)
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • SWEI
    SWEI Member Posts: 7,356
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    If it's an incandescent bulb and it works, you will read a resistance using a normal ohmmeter. A neon bulb would be used in older equipment, but there was some overlap in the 1980s where it could potentially have been either neon or LED. How old is the equipment it's in? How big is the lens, and what shape?
  • ratio
    ratio Member Posts: 3,626
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    A megger is an insulation tester - uses high voltage to check the leakage resistance of insulation. I've got the little Fluke, ranges from 50 to 1000 test voltage. Industrial models can go to 10k volt & higher. I recommend it, if you have a need. It doubles as a multimeter, so it's not necessarily an additional thing to carry. I've proven a number of intermittent shorts with it that were causing our service guys to pull their hair out.

    Now, to find a neon lamp...

  • ratio
    ratio Member Posts: 3,626
    edited October 2015
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    How odd. I posted that in response to Jamie Hall's first post above. Must have gotten stuck somewhere.

    Still haven't found a neon lamp though.

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