I replaced a load of ceiling lights in the house yesterday with LED fitments. I noticed that one of them continued to glow for some time after being turned off. The others don't do this.
I swapped the light out, and the replacement doesn't glow.
Just to treble-check, I wired up the glowing light to a 13 amp plug, and it still glows after being unplugged.
Sounds like phosphors. LEDs certainly don't glow after removal of the current. Some 'white' LEDs are actually UV, some are blue, and both use phosphors to simulate white. Perhaps your odd one used different, longer-glowing phosphors than the others.
In the days of real oscilloscopes, with some high-end models the phosphor glow time could be chosen from two or three values.
It could be that there is supposed to be a resistor in place to discharge any capacitors in the PSU when it is powered down, and also reduce the possibility of inductive pickup causing flickering when turned off. If that were missing or faulty it might not be spotted during inspection and testing since it would still appear to work correctly.
That makes good sense. I'm not intending to open it up at any stage, so is it worth returning it for that?
Doesn't it depend on the circuitry in the base of the bulb? For example if there is a capacitor in there that holds charge for a few moments after the power is turned off, the bulb will continue to glow until that capacitor loses all its charge through the bulb. Other circuits may not have such a capacitor and the bulb extinguishes almost instantly.
Yes, but I wouldn't expect so. The power wasn't mentioned, but I'd guess around 4-5W. About a third to half an Amp of current, depending on how many LEDs. That's a hell of a capacitor.
If it is a SMPSU, then the capacitor is usually on the mains side after a rectifier - so it will be a fraction of a watt at ~330V. So that is only a few mA
(I certainly have some lamps that glow for a few secs on power off - it tends to be the more traditional LED ones rather than the "filament" versions)
But the capacitor could be at mains voltage and the circuitry may allow only a very low current during discharge after switch off.
But does it stay at full brightness after the power is turned off? An LED will stay lit at a very small proportion of the full current, but obviously not at full level.
C: "I wish to complain about this light bulb." S: "Of course, sir. Does it not produce light?"
Reminds me of the trick light bulbs that light up when you hold them in your hand. "Amaze your friends!" (Until they get so bored with your silly tricks that they aren't your friends any more.)
My one is just a faint glow.
Like CFLs?
LEDs can easily remain on purely by induced voltages along wires close to each other. Not much you can do but some of them have a small bleed resistor inside that makes them discharge anything that might otherwise make them glow eerily. Neons in pieces of gear back in the day used to glow purely by the pick up in the wiring loom, Once again a resistor of around 470k fixed it. Brian
Also most LED are actually very small solar panel like and can produce up to 2V in bright lighting conditions. There's a light flasher circuit in elekor mag using 2 x 10Meg resistors 2 x 100nf caps and a push switch. Under office lighting conditions it takes about a miniute for the caps to charge and then pressing the switch you get a faint flash of the LED. In reasonable sunlight it just takes a few seconds to charge.
Apparently you can do the same by holding a fluorescent tube under a high voltage distribution line.
Not when the mains have failed.
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