How does an LED mains bulb work?

[elfcurry]

How does an LED mains light bulb work?

I bought a cheap 3W LED light recently on ebay and it worked very nicely for a couple of weeks, then died. I opened it thinking I might be able to spot a problem but I don't know how they work.

There's a simple-ish circuit board to convert 240V AC to whatever voltage DC 42 LEDs need. It'll be something between 2.4V and 3V x42.

The circuit comprises
4x 4007 diodes (presumably as a bridge)
4.7uF 250V electrolytic
4 banded 'resistors' (possibly some are inductors?) of two sizes and different values
one mystery brown thing which looks like an old type of capacitor (some used to be 'dipped' in gunk) and seems to have two wires and marked "474J 400V"

What's the brown thing? Passive/active?

How does the circuit work? Surely not just rectify, smooth and drop the excess voltage across resistors!? I assumed these things were more sophisticated - but then it was under two pounds, including postage from China!

I'm now more interested in knowing how it works but might be nice to resurrect it or use the parts.

 

[ronv]

Its a .47 Ufd line rated capacitor. Its impedance limits the current to the leds.
See if you have any DC out of the bridge.

 

[Menticol]

Cheaper lights don't even include a rectifier bridge, just a resistor. The LED is only lit during half of the AC cycle and service life is shortened due the reverse current, but as I said is cheap as dirt.

 

[RCinFLA]

Many (cheap) AC power LED's use a simple capacitor ballast to limit the current. It will be the non-polarized mylar film capacitor. If your LED's in series needs 250 mA and its stacked voltage is 16 volts (4 stacked 1 watt LED's) and you are running it from 240 vac 50 Hz mains then the capacitor value will be about 3.6 uF. The capacitor is on the AC input side of a bridge rectifier. You can eliminate the bridge rectifier by have LED's configured in reverse/back to back parallel config with several of these back to back parallel combos put in series.

The problem is a ballast capacitor can maintain its charge for some time and can have a nasty surge current when the switch is turned on at the sinewave peak that stresses the LED's. The resistors and electrolytic caps are to moderate the surge currents. The problem then is the electrolytic can have a high ripple current that will shorten its life.

The deluxe way is to have a ferrite transformer based switching power supply that regulates output based on current to power the LED's. All flourescent lights (CFL's) have this kind of power supply because a ballast cap will not work for them.

 

[k7elp60]

I think the brown thing marked 474J is a .47uF capacitor. How the rest works is questionable. If all the LED's were in series the total voltage for them would be 42 x 3V or 126V as white LED's have a forward voltage of between 3 and 3.2V depending on the applied current. It is possible that the capacitor is used as a resistor as the reactance at 50Hz is 6.8K and 5.6K and 60Hz. If the 1N4007's are in a bridge and the 4.7uF capacitor is used as a filter it would be okey for mains of 120V but not okey if the mains is 220V. If you could draw a rought schematic of how allthe parts were connected together maybe we can figure out how it was susposed to work.

 

[KJ6EAD]

Transformerless power supply methods.

https://www.electro-tech-online.com/custompdfs/2012/05/00954A.pdf

 

[elfcurry]

Thanks for your info and ideas. I'll have a go at a circuit tomorrow as it's nearly 2am here.
(I'm only still awake as I heard someone stumbling about in the garden!)

 

[ronv]

It probably looks like this. Except we have a spare part.

PS,
I used zeners in place of LEDs so I didn't have to draw them.

 

[dr pepper]

I pulled one apart last winter, I made the led into a light for my pushbike.
The circuit board in the bacl was a proper switching psu, the controller chip was just a black blob on the board as is common with mass produced stuff.
You could replace the board with a reactive capacitor regulator and a bridge, you'd be better just buying a new one though.