Failed LED light glowing after turn-off

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Diver300

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I just noticed a strange effect in an LED bulb. There are two bulbs in the fitting, and one failed a few days ago and I hadn't got round to changing it, but the fitting was being turned on each evening as the other light works.

When I turned off the power, the LED lamp glowed for a moment, maybe half a second. I tried a few times and it glowed each time the power was turned off.

The fitting of two bulbs has its own switch. I don't know if the other bulb is the same type or not. There is no dimmer switch, and it's on normal 240 V ac, 50 Hz mains.

Any idea what is causing this phenomenon?

I'll open it up tomorrow and see if I can work it out.
 
Diver300 said:
I think that must be the cause, but I can't see how that can happen on an ac-powered light.
Click to expand...

Because LED's don't work off AC - in an LED bulb there's usually a fair amount of electronics, including capacitors. Basically it's a switch-mode PSU, so the mains is rectified and smoothed.
 
Nigel Goodwin said:
Because LED's don't work off AC - in an LED bulb there's usually a fair amount of electronics, including capacitors. Basically it's a switch-mode PSU, so the mains is rectified and smoothed.
Click to expand...
The question is what makes the light come on only after the mains turns off.

On several other LED lights, I've added extra capacitors to improve smoothing, and the light takes time to fade as the capacitor discharges.
 
Nigel Goodwin said:
Strip one and draw it's schematic out - it obviously depends on it's exact design - and what does it matter anyway?.
Click to expand...
It's only one. I intend to strip it and find out why it's behaving like it is.

It's not important at all. It's just for interest.
 
Mystery solved, I think.

When I looked more closely, the bulb was glowing all the time, but the other bulb beside it meant that the glow wasn't visible until the other one turned off.

The circuit is based on this:- https://pdf1.alldatasheet.com/datasheet-pdf/view/1140947/BPS/BP5131HC.html

I think that the IC has gone intermittent. I was just measuring voltages when it went to full brightness, and it seems to be working now.

With the IC in its failed state, there was just a small current in the LEDs and the 4.7 uF capacitor could keep the LEDs glowing for half a second or so after the power was removed.
 
Big Clive on YouTube explains more about LED bulbs than you could ever want to know.

Switchmodecpower supplies tend to be a rarity in LED bulbs – capacitive-droppers are far more common.
 
Visitor said:
Big Clive on YouTube explains more about LED bulbs than you could ever want to know.

Switchmodecpower supplies tend to be a rarity in LED bulbs – capacitive-droppers are far more common.
Click to expand...

I was under the impression that non-dimmable ones were usually switch-mode (which is why they can't be dimmed, and why they don't alter brightness with mains fluctuations), and dimmable ones were simple droppers.
 
I have a programmable LED bulb that connects via wifi. On flipping the switch to the fixture off, the bulb lights for about a second before going off. I assume it's due to capacitance in the circuit holding about a second's worth of charge before going dark. These are used in the spare bedroom for my grandkids and are normally kept dim for use as a nightlight. I'll have to experiment to see if the ~1 second timeframe changes when they are at different brightness levels. I'm guessing that setting the light brighter will use up the charge faster and the light won't stay on for that ~1 second.

I'll also note that white LED's are often constructed by starting with a blue or UV LED and painting phosphorus over it. The shorter wavelength energy causes the phosphorus to glow in a broad spectrum of visible light, i.e. white. When you turn the lamp off, the phosphor coating will continue glowing.
 
From Big Clive, this is the state-of-the-art in LED bulbs. The bulbs use a capacitive dropper, a bridge rectifier and a current-regulator chip. Each LED package has multiple elements inside to make the Vf of the string close to the rectified line voltage.

Clive has examined numerous bulbs from different manufacturers and found similar circuits in most of them. I have looked at several US bulbs and a Mexican bulb and they also feature a similar circuit. In the picture below, the dropper cap is socketed at the center of the board to isolate it from the heat of the LEDs, and positioned on the back side of the board.

Clive explains the circuit in this and several other entertaining videos, and also explains a simple mod to this circuit to reduce power consumption (and light output of course) to reduce heating and extend bulb life. See link to video following these pictures.







 

This circuit doesn't have a capacitive dropper.

I've seen LED bulbs that have switch-mode supplies, linear limiters (like this one) and capacitive droppers.

When there's a capacitive dropper, the Vf of the LED string is often much lower than the supply voltage. The low voltage means the the power factor will be poor, but there won't be much power lost in the dropper.
 
Yes, some bulbs have switchmode power supplies. However, the state-of-the-art is the circuit shown here..


Since Nigel Goodwin didn't seem to believe this, I wanted to set the record straight. This is one of Clive's areas of interest and I dare say he is an expert on the topic. You might learn something from the linked video
 

Might have been more impressive if you'd put a link in the link?.
 
Nigel Goodwin said:
Might have been more impressive if you'd put a link in the link?.
Click to expand...

You mean like the giant link to the video?????

 
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