LED lightbulb - blown LEDs

[Agent24]

Had this LED 'bulb' in a beside lamp because incandescent and CFLs were too hot in there. Cost $15 and claims to last 20 years (Yeah, right!)

Worked great and ran cool for a couple of months, then started flickering sometimes, and eventually died. Funny how a 90 cent incandescent bulb lasts longer than CFLs and LEDs both costing $15.

I found that the light diffuser can just pop straight off, which reveals a small PCB with 20 SMD LEDs on it underneath. The PCB has a metal backing and is screwed to a metal heatsink underneath that. Below the heatsink is the driver circuitry, which outputs 120v DC unloaded. I didn't bother trying to get into that since it works and I didn't want to cause damage (at this point).

It's clear from just looking what the problem is! Two of the SMD LEDs have burnt out.

What's interesting is that assuming a working voltage of 120v DC for the 20-LED string, they have a 6v forward voltage. I haven't heard of any this high but I suppose they have two LEDs in series on the same die\in the same package. Makes sense going by the two little bumps I can see though the yellow coating, anyway.

I removed the dead LEDs and measured them at about 3x2.5mm which doesn't correspond to any known LED package I know of.
The closest I found were these: http://lumis-led.en.made-in-china.co...-LED-0-5W.html but it's not really worth it.


If anyone knows a good source for new LEDs, I might give repair a try for a laugh. Otherwise, I'll probably just cut the end off and see how they built the LED driver, maybe re-use it and the rest of the working LEDs for something else.

[stj]

Re: LED lightbulb - blown LEDs

desolder a bad one and see if there is anyhing printed / embossed on it.
good luck btw, the leds are soldered on an aluminium plate coated in an insulating layer followed by the copper tracks usually.
so you need a lot of heat


--------
that pcb says 2835

are the leds 2.8 x 3.5mm?
because if they are:
http://www.rapidonline.com/Electroni...ount-LEDs/3528

[SteveNielsen]

Re: LED lightbulb - blown LEDs

What is the measured fwd V on a good led?

[R_J]

Re: LED lightbulb - blown LEDs

I would have returned it, It did come with a warranty did'nt it?

[Agent24]

Re: LED lightbulb - blown LEDs

desolder a bad one and see if there is anyhing printed / embossed on it.
--------
that pcb says 2835
are the leds 2.8 x 3.5mm?

Nothing printed on them but it looks like you're right about the size. I removed the bad ones (Hot air tool at 450C made short work of them, also I unscrewed the PCB from the lower heatsink first). With vernier callipers they measure 3.5x2.9mm.

The lamp has 10 Watt printed on it. I guess each LED is 0.5 Watt.

What is the measured fwd V on a good led?

I don't know. How would I measure that? Remove a good one and hook it up to an adjustable supply and measure Vf when it lights up?

Or could I short the pads of the two I removed and measure voltage across one of the others? I assume the supply is current limited as there are no obvious resistors. Or would having two removed and their pads shorted cause a problem?

I would have returned it, It did come with a warranty did'nt it?

Yes it wasn't old enough to be out of warranty, so I probably could have returned it, but where's the fun in that? Besides, I couldn't find the receipt so they would have been unlikely to accept it...

[SteveNielsen]

Re: LED lightbulb - blown LEDs

If the leds are wired in series you don't have to remove one from the board to test it. The easiest way to do it would be with a transistor/diode tester.

[mariushm]

Re: LED lightbulb - blown LEDs

A cheap fix would be to replace the leds with a couple of 5.6v zener diodes (or some other voltage that's common)... see this video for a fix using zener diode : https://www.youtube.com/watch?v=8otPjsN8jD8

But you could also just short the pads and let the driver pump more current in the remaining leds. They should still live for a few hundred hours.

[Agent24]

Re: LED lightbulb - blown LEDs

I rechecked the layout and it's actually 2 strings of 10 LEDs in parallel, which is interesting.

I jumped the pads of one bad one and put an ammeter across the other.

I got 6.18v across a good LED, and 68.11mA through one string. Didn't leave it plugged in very long in case they were overloaded but the readings seem sensible enough, I think? Works out about 0.42 Watts per LED.

Diode test on meter showed nothing even for a good one. Which makes sense if they are 6v, since my meter's diode test function only outputs about 4v maximum.

[Agent24]

Re: LED lightbulb - blown LEDs

A cheap fix would be to replace the leds with a couple of 5.6v zener diodes (or some other voltage that's common)... see this video for a fix using zener diode : https://www.youtube.com/watch?v=8otPjsN8jD8

But you could also just short the pads and let the driver pump more current in the remaining leds. They should still live for a few hundred hours.

I'll try that Zener idea if I can't find any suitable LEDs cheap enough. Wouldn't really want to increase the current further, as it is they're probably already overdriven.

[momaka]

Re: LED lightbulb - blown LEDs

If anyone knows a good source for new LEDs, I might give repair a try for a laugh. Otherwise, I'll probably just cut the end off and see how they built the LED driver, maybe re-use it and the rest of the working LEDs for something else.

Maybe try some reputable LEDs from Digikey or Mouser?
I put a violet SMD LED in my AN5252 headphone amp (Kingbright AA3528VRVFS/A for thouse interested) about 3 years ago. Been using the amp almost daily, most of the time 3+ hours a day, and many days with 8+ hours. LED is still good and bright. That said, I think I do only have about 8 mA running though it, so it is not really stressed by any means (max rated is 100 mA, and average DC forward is 30 mA).

Funny how a 90 cent incandescent bulb lasts longer than CFLs and LEDs both costing $15.

The only problem is they are banned in many countries now. I still keep a small stash in the garage, though.

If you want your incandescents to last even longer, buy only bulbs rated for 60W or less. And if possible, go with more bulbs with lower wattage than less bulbs with higher wattage. The light output may be lower and more yellowish, but the bulbs will last a very long time. If that bothers you, then halogen bulbs may be a good alternative for you - the light is a lot more white. But again, go with 50W (or 40W for GU10 sockets) or less to make the bulbs last longer.

I've switched many of the bulbs in my house with 40W incandescents. Quite a few are over 10 years old now. I seriously can't remember when was the last time I had to change a light bulb. I think it was in my room. Had a single 100 W bulb that went bang. Now I have 2x 40W. Not as bright, but it is good enough for my needs. When I need to do work, I just turn on my oldschool linear ballast CFL desk lamp. *blink*... *blibk* *blibk*... *hummmmm* .

[Agent24]

Re: LED lightbulb - blown LEDs

Maybe try some reputable LEDs from Digikey or Mouser?

Perhaps, if I can think of enough stuff to make the order over $125(?) or so for free shipping.

The only potentially suitable ones available locally are these: http://nz.rs-online.com/web/p/visible-leds/7965349/
3x3mm but they might fit.

[fzabkar]

Re: LED lightbulb - blown LEDs

I'm wondering about the failure mechanism of this lamp. AISI, if one of the LED strings goes open, then the remaining string will carry twice its normal current, resulting in its early death. In this second string, one LED would fail while the surviving LEDs would be stressed. ISTM that you would need to ensure that each string remains balanced after any repair or modification.

BTW I'm presuming that the lamp failed with both strings open, and that the array is powered from a regulated current source.

[Agent24]

Re: LED lightbulb - blown LEDs

I assume it's a constant current source, but I haven't looked at the driver circuit.

It was 120vDC unloaded, which would be 6v per LED if they were all in series, but they're not.. so that would be 12v each but they're not, from what I measured. I forgot to measure the loaded voltage, but I assume it's around 60v.

The lamp was flickering dim\bright occasionally before it died completely. Both strings must have blown an LED about the same time though. I don't know for certain, it wasn't in my own bedroom.

Since 1 LED blew in each string, I assume replacing with a new LED will keep it balanced even if the new LEDs are not the same as the old ones. I don't know how I could make sure the remaining string isn't overloaded if another LED blew though.

[Agent24]

Re: LED lightbulb - blown LEDs

Well, I got the LEDs I linked to from RS and installed 2 new ones. It lives again! Ran it for about 3 hours the other day in my desk light and it worked well.
Yeah, the old LEDs got a bit melted on the sides... that was during my removal of the bad ones originally, I must have used too much heat.

Now I just have to find something to glue that diffuser\cover back on that won't be too hard to get off again if needed. Any ideas?

[momaka]

Re: LED lightbulb - blown LEDs

Wow, nice you got it working again. How do the old LEDs compare to the new ones? Are they less bright? I just wonder if they are being overdriven, thus the reason for the original LEDs blowing.

As far as glue, perhaps hot melt glue (provided the bulb doesn't run too hot). I like hot glue since it is not too hard to remove and can be re-used again when heated up to proper temperature.

[Agent24]

Re: LED lightbulb - blown LEDs

Looking at them with naked eye they all look blinding!
With the camera you can see the new ones have a slightly 'cooler' colour... aside from that they don't seem noticeably brighter\dimmer.

Hot glue sounds like a good idea. I don't think it would get hot enough to melt it.

[stj]

Re: LED lightbulb - blown LEDs

the way to look at those leds is to put a sheet of paper over them as a defuser.

[Agent24]

Re: LED lightbulb - blown LEDs

the way to look at those leds is to put a sheet of paper over them as a defuser.

Now why didn"t I think of that...?

[retiredcaps]

Re: LED lightbulb - blown LEDs

Had this LED 'bulb' in a beside lamp because incandescent and CFLs were too hot in there. Cost $15 and claims to last 20 years (Yeah, right!)

A couple of years ago, I saw a teardown of the LED bulb and knew right away that they wouldn't last anywhere near the claimed longevity. At the current $10 to $15CDN each for good name LED brands, I will stick to whatever Costco CCFL bulbs are available for $1.50CDN each. Maybe once LEDs get into the $2 to $3CDN range, I will consider trying some out.

PS. +1 for the duct tape holding the Fluke 73 together!!

[kc8adu]

Re: LED lightbulb - blown LEDs

cheap leds went open then burned due to the high voltage.common on cheap bulbs.
those were some whoflungdung rather than cree,lumileds,nichia,ect