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HOW TO: Stop efficient LEDs from lighting up when off

  1. Apr 26, 2017 #1
    Saw an old thread, "Led Lights Burn With Power Off" and it was closed... so let me check out an issue with you all: in this same situation and I don't want the lights softly glowing, could I put a 100 ohm 1/4 watt resister in line to block the teeny tiny induced current?

    Would it behave well with 12 watts of LED behind it? Quantity = 3 of 4 watt each LEDs.

    If I replace one with a single incandescent 15 watt, nothing glows.

    Last query, say the resistor is in place and I forget and replace the three LEDs with three 40 watt incandescent bulbs - does it cause a fire?

    Thanks all,
  2. jcsd
  3. Apr 26, 2017 #2

    jim hardy

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    Try placing a small incandescent lamp anyplace on the LED string.. A 6 watt nightlight will do fine, just be sure it's incandescent.
    It will absorb the tiny current that's bypassing the switch, as was explained in that old thread you linked.

    I assume you're on a 120 volt system. If you find a "Long Life " 130 volt bulb it'l last much much longer than will a 120 volt one.
    a non-academic source but it conveys the point

    old jim
  4. Apr 26, 2017 #3


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    There may be another explanation for the LED idle current when the switch is off.

    In the previous thread, a capacitor was used to limit the rectified current to the LEDs.
    110 VAC; 60 Hz; 0.39 uF; then Xc = 6.8 k ohm, so ILED = 16.2 mA. That seems reasonable.

    But high frequency noise on the AC supply will not be limited in the same way.
    HF noise could come from power supply switching noise or from a local radio transmitter.
    Faulty transformers and insulators will also provide HF noise.

    HF noise will be coupled efficiently across the parallel wires to the open switch.
    If the noise amplitude is over a few volts it may well provide sufficient current to light a LED.

    A snubber, noise filter or a low-pass filter should identify and possibly solve that problem.
  5. Apr 26, 2017 #4


    Staff: Mentor

    I can confirm that. There is a place in Marathon, Florida where boats anchor within 200m of 3 Voice of America transmitters. All of them report that their LED lights glow even when turned off. They also have radio problems and galvanic corrosion problems.
  6. Apr 27, 2017 #5
    Thanks for the comments! Yes, north American household power. Thankfully, no VoA, TV, or other radio broadcast locations within 200m! There are a couple 6 miles away.

    I should have been more clear, currently, there is a 15 watt bulb in one socket and the other two are LEDs. Since nobody commented on the idea of a 1/4 watt resistor in line, how about 1/2 or even 1 watt?

    Also, since the 15 watt bulb is effectively in parallel with the LED lights what about the resistor in parallel?
  7. Apr 27, 2017 #6

    jim hardy

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    That was clear,
    and the reason it works is the resistance of that 15 watt light bulb is small compared to the capacitance between the switch wires. So it's almost a short circuit for the teeny current.
    And it's in parallel not series.

    It would take an unreasonably high number of ohms to block the teeny current. You are way ahead to just short circuit it with the lamp. Think of a dripping faucet - --- you let it trickle down the drain rather than stuff a rag in the spout.

    You already solved your problem with a resistance in parallel, the 15 watt lamp.
    Now fine tune your solution - try a nightlight bulb in your socket preferably a 130 volt one ..
    If you could find a 150 or 230 volt one so much the better, it'd last practically forever.

    I STRONGLY advise you AGAINST splicing a resistor in series , you are messing with line voltage here and will create the possibility of a home-made connection falling apart creating a hazard . .
    Here at PF we are VERY MUCH opposed to beginners modifying household wiring because Tiny Fingers are curious .
  8. Apr 27, 2017 #7
    Thank you, appreciate the facts! The original objective was a two parter: (a) to get the 3 LED lights to be off when turned off, and (b) have the lights provide identical illumination. Looks like those three will just stay incandescent for now.

    Many thanks for the nice welcome to the forum.:biggrin:
  9. Apr 27, 2017 #8


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    Assuming one switched 120V AC circuit, with three sockets in parallel for lights.
    A series resistor will have no effect because the AC supply voltage is high compared with the LED voltage and current. A series resistor would just dim the lights and waste power when turned on.

    When cold, the 15 watt filament globe will have a resistance of about 120 ohm. As Jim explained above, the resistive filament provides a path for the capacitively coupled AC current, or HF noise. Experiment with lower power filament globes. Use 3 LED globes, hide the filament lamp if you need even illumination.

    The fundamental problem is the capacitance between the two wires in the spur that goes to the light switch. Changing house wiring by replacing the two wire spur with two pairs of wires would eliminate the spur and so eliminate the problem. Alternatively manufacturers could build more electronics into the LED lights to better cut the off current. Both become expensive, but paying an electrician to rewire the spur only needs to be done once.
  10. Apr 27, 2017 #9

    jim hardy

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    If these are for decorating effect instead of illuminating a room,
    Look at industrial supply houses for 150 volt lamps. That S6 pictured is about the size of a night light bulb. A 150 volt one would last a VERY long time at 120 volts.
    (150/120)^12.5 = 16X longer than a 120 volt bulb. It'll be a warm light lacking in blue and green though, and not very bright for its wattage.

    If it's for room lighting then you're money ahead to buy 'energy saver' bulbs of lower wattage but higher lumen output and change them more often.

    Here's an introductory essay that covers a lot of basics.
    Search on the terms in it to familiarize yourself with lamp basics. It's a very interesting field - the humble lightbulb can teach us a lot.

    old jim
  11. Apr 28, 2017 #10
    Thank you all new (to me) old jim, Baluncore and anorlunda!
  12. Apr 28, 2017 #11

    jim hardy

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    Thanks for the feedback Mr Marble !
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