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Glowing LED Bulb - How Does Circuit Flow

Discussion in 'LEDs and Optoelectronics' started by dave9, Feb 15, 2018.

  1. dave9

    dave9

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    Mar 5, 2017
    I have what I'll call a random LED E26 bulb. It has a driver of unknown design, outputting 52VDC to 2 parallel groups of 8 LEDs in series (dual die LEDs, so really 2 parallel of 16 LED dies in series).

    It is plugged into a utility light fixture, two conductor wire, hot/neutral on a US 110AC supply.

    I have read in the past about people who had their LED bulbs glow dimly when turned off, due to the hot wire remaining live and the neutral being switched off instead of the other way around, and the cause is capacitive coupling between the wires.

    While this makes sense, I have a different situation. The LEDs are not dimly lit with the neutral switched off, but do dimly light up if I touch the plastic body of the light with my fingers. It is continuous light output not getting dimmer and dimmer still. It is definitely a plastic light body, measures infinite resistance to itself, to hot, neutral, and ground. My body is not in contact with any neutral or earth ground object, nor especially close to any wires. My other hand is touching nothing. It is not a grounded work surface.

    How is this circuit being completed enough to make the LEDs glow?
     
    Last edited: Feb 15, 2018
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Current flows through a capacitive connection between you/ground and the bulb/mains.

    It's a very small current, but it's enough to make the bulb flow very dimly.

    The same thing can happen with a small neon bulb, such as those in screwdrivers that flow when touching something live.
     
  3. dave9

    dave9

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    I don't understand how there is a capacitive connection between myself and ground, especially when only touching the plastic bulb casing. I'm sitting in a leather chair, on plastic wheels, on carpet, even with my feet off the carpet, touching nothing but the plastic LED bulb.

    If I touch an earth grounded power strip simultaneously, the LEDs do glow brighter, but still very dim.

    I just jumped up and down, touching nothing but air and the plastic LED bulb casing... still glowing while I was in the air, lol.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That's the whole thing about capacitors. No electrical connection to anything.
     
    hevans1944 likes this.
  5. AnalogKid

    AnalogKid

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    For reasons I've never quite understood, the human body presents a ground to AC. As above, the current is very small. It is larger when you touch a real ground because the human body is a better resistor to ground than it is an actual ground.

    ak
     
  6. dave9

    dave9

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    Mar 5, 2017
    Well I'll be darned, it's starting to make more sense now. I figured there must be the trapped soul of a tortured ghost inside the bulb, or a little bird on a wheel like on the Flintstones, so in attempting to open it and release the ghost, I discovered that what I claimed was a plastic body was instead a thick powder coated metal body that just looked very much like plastic... so my fingers were only a few dozen microns away from that instead of more than a few millimeters from anything metal.
     
  7. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Can you post a photo of the light?

    ak
     
  8. dave9

    dave9

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    Mar 5, 2017
    ^ Sure. The brand name is "Simply Conserve" and it looks like it shares some construction details with past lights IKEA has sold, except different wattage and LED series/parallel config.

    Upon looking at the light again, I realize I posted the LED config from a different, 2nd light I was also looking at, this one is two parallel, 6 series LEDs with 3 dies per LED so 2 x 18.

    There's not much more to show without destroying it, below the mPCB the LEDs are on, is a press-fit aluminum plate in very tight (won't budge and I broke a tool trying to budge it, lol), pressed into the white base which is coated aluminum and the driver board is permanently potted into the bottom in an inner plastic sleeve that extends down and forms a ring right above the threading.

    Heatsink grease and two screws are on the mPCB and silicone holds the dome on. Dome was not easy to get off, a tight fit until a soft jawed vice put some tension on it to pop one side of the silicone loose.
     

    Attached Files:

    Last edited: Feb 15, 2018
  9. Sunnysky

    Sunnysky

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    Jul 15, 2016
    There are 4 Vin vs Vout quadrants or trigger polarity conditions in Triac switches and the sensitivity is different depending on if you choose line or neutral switching. When Neutral switching leakage may be less but stray E field on line voltage on the wires to the bulb creates more electric field to trigger it

    With your 100pF finger capacitance and body antenna impedance being lower than air , the Efield goes right thru you without and voltage drop. ( a 10M probe on finger can get 1/4 of the line voltage easily with good contact.) Of course 10M impedance is ab out the impedance of your 100pF finger tip so you know the current is small ( lets say 50V/10M = 5uA max )

    Since SCR's are essentially a PNP and NPN back to back collector to base the overall current gain may be as high as 10k so you 5uA x 10k = 50mA which is enough to be more than dim. So I overestimated Hfe or how much E filed you were collecting. But a sensitive gate Triac is very sensitive. Snubbers add leakage and will not dim them. but a load R will. Triacs are two SCR's reversed back to back. hence the 4 quadrants . Our bodies would be > 1uf with large foil on either sides but only 10 to 1000pF on the finger tip depending on surface area. Air is also a dielectric capacitor just with a lower constant normallized to 1.
     
    Last edited: Mar 14, 2018
  10. Bluejets

    Bluejets

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    In days gone by, we used to have to tred lightly when confronted with fluro tubes that would glow or have spiral glow in the middle of the night while switched off.
    Some were convinced it was the return of Uncle Fred, but we had to explain it was simply the rise in neutral voltage, usually at premises spaced a long way apart such as farms etc.
     
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