The faint glow of LEDs, in reverse...

[Externet]

Hello.
It is not uncommon for LED light bulbs to emit a faint glow when turned off, be it the nature of their internal driver circuitry, capacitive induction of the dwelling wiring, secondary effect of dimmers/switches, opening neutral, whatever. Found the effect a few times at home and solved or ignored.

But if I want to create the effect on purpose; how is it recommended ? Like to have a ceiling light working as 'night light' when turned off, as to be able to walk/aim without turning on the 'blinding' light at midnight ?
Tried a 0.1uF/400V in parallel to the wall switch. Nope. Tried 1M resistor in parallel to the switch. Even a 10K. Nope. Does it depend much on the light bulb brand/construction ?

 

[dick_freebird]

The current from breakdown tends to be localized and not necessarily where it is benign, let alone useful. Forward current tends to distribute better and is designed-for. Some Leads have lower reverse than forward ratings for reliability.

Better to do something like a parallel path of low forward current, a high valued resistor could do.

 

[ShineLong]

Hello.
It is not uncommon for LED light bulbs to emit a faint glow when turned off, be it the nature of their internal driver circuitry, capacitive induction of the dwelling wiring, secondary effect of dimmers/switches, opening neutral, whatever. Found the effect a few times at home and solved or ignored.

But if I want to create the effect on purpose; how is it recommended ? Like to have a ceiling light working as 'night light' when turned off, as to be able to walk/aim without turning on the 'blinding' light at midnight ?
Tried a 0.1uF/400V in parallel to the wall switch. Nope. Tried 1M resistor in parallel to the switch. Even a 10K. Nope. Does it depend much on the light bulb brand/construction ?

Whether or not a bulb will light up slightly when you turn it off has a lot to do with the brand and the design of the driver circuit. Some bulbs are particularly sensitive to small currents, and even a little leakage current will make them light up, but others won't light up at all.
It could be that the leakage current isn't enough. You could switch to some bulbs that are more sensitive to leakage current, such as a cheaper model with a simple driver design might be easier to achieve the effect. Or use a small constant current source module to provide a stable low current and directly turn the LED light bulb into a night light.
Tossed, this soft light effect is quite practical, especially suitable for the middle of the night without having to turn on harsh lights!

 

[Easy peasy]

It is the phosphorus internal coating that glows for a time after turn off - this coating turns UV into visible light

 

[D.A.(Tony)Stewart]

It may be that the parasitic coupling of a long string of LEDs with a common mode voltage and asymmetric impedance converts the CM noise or e-field voltage into a differential current. If the string was PE grounded to 0Vdc, it should not radiate light or may radiate more or be shunted with the capacitance from your hand, using your body as an antenna.

It is on the order of< 1mA. If outdoors, it also exposes your DC supply to ingress from long wire pickup of lightning e-field with floating unclamped outputs exceeding the driver's reverse or forward rated voltage.

You need to steal about 10 mW of power from somewhere, and there's no free lunch if the radiated e-field or conducted CM noise comes from your premises.

Since the impedance for C coupling conducts more current with rising f, you can see how that works.

But it's much brighter with batteries and a solar panel.

 

[betwixt]

That's an awfully complicated explanation of a dimmed LED Tony

The underlying problem is that LED room lights are not simply a chain of LEDS across the AC so reducing the current will not reduce their brightness. Inside the base of a domestic light bulb is a tiny circuit similar to a switch mode supply that tries to maintain a constant current through the LEDs. If you try to reduce the AC to the bulb it stops works and you get no light at all or flickering. Even 'dimmable' LED lamps suffer from this, they still need high AC voltage to start them working so a fixed dropper such as a series capacitor or resistor will not work.

I live in a remote location, it actually has 'dark skies' status because it has no local street lighting or industry, at night it is totally dark except for starlight and moonlight. To be able to see in the house at night I wired normal warm white LEDs on a small PCB and mounted them inside the ceiling shades. A PV panel outside monitors light level and a small circuit measures its output voltage and the line AC voltage. After dark it feeds about 4mA through the LEDs to give a dim background light so we can see enough not to walk into things and if its dark and the AC fails it immediately switches to 500mA from a backup battery, giving very usable room illumination. When PV power is available it switches the LEDs off completely and recharges the battery. For instances where I want the lights off and they are in backup mode, for example trying to sleep at night when the AC is off, I have Bluetooth control from my phone to override normal operation. It does requires some extra house wiring so my solution may not be suitable for everyone.

Brian.

 

[D.A.(Tony)Stewart]

That's an awfully complicated explanation of a dimmed LED Tony

I noticed this effect with a LED string around my the fence on my backyard when the DC supply was off. My finger on an SMD LED could make the glow brighter.

I know that my "personal e-field" and finger capacitance must be strong enough to get rectified by the LED ( is a diode with some capacitance smaller than my stray connection to freespace or ground. Even my 10:1 scope probe can detect over 50V on my hand unless also touching the ground clip.

Heck, even when I recall long ago at a friend's place watching the Superbowl and for a few minutes losing cable TV, I used a string of wire plugged into the antenna port and held it and pointed my arms in different directions to pick up a broadcast signal faintly.

But now I have to install these cheap TEMU solar-powered LEDs outside and they are pretty dim but fancy with comet trails sequencing down from 8 daisy-chained clear rods. The other TEMU gifts were cheap Solar power motion-sensor LEDs. But I prefer the DC flood lamp I Illuminate all the trees in my front yard on a box I found someone left for garbage because the dead battery also tripped the DCDC current limit to power the converter for the 20W LED. The bushes reflect the light to reduce illuminating the sky. I miss the dark skies with Aurora Borealis where I used to live in Manitoba and the few trips at work going up to Churchill NRC rocket range. Very dynamic free night lights.