Introduction: Repair Dead COB LED Light Bulbs

Picture of Repair Dead COB LED Light Bulbs

I have bought a bunch of Chinese light bulbs from eBay which are COB (chip-on-board) units which means a board with a bunch of LED chips soldered on it. They usually run or a month or two and end up dying there about for unknown reasons. Here is my repair technique and maybe why they die and how to repair if you are brave enough to do it. The repair costs about $0.02 each investment if you have the equipment to resolve the problems.

I have repaired nearly 50 of these little things.

These bulbs cost from $1 to $4 on eBay and other places.

Although this process is not that complicated, it does requires a little knowledge of electronics as well as some soldering experience and some safety to follow as well because this is a 110V AC device.

I hope this gives bravery and an idea of how to resolve dying light bulbs to everyone.

Step 1: Introduction and Background

Picture of Introduction and Background

These bulbs are basically a bunch of little LED chips placed in series on a board and driven by a rectified AC circuit. As with an series circuit, if a component breaks in the "chain", the unit will not work anymore (think Christmas lights here). As in any series circuit, the LEDs draw a current for each unit in the circuit and drop a voltage across each one as it emits light, and hopefully not heat.

In the bulb is a rectifier, a load resister and some filtering capacitors that give the LEDs a DC voltage to live on.

We will check these components out (power supply), and then look at the actual LEDs (parts that are usually broken on this kind of light bulb).

The second picture is a list of the parts and tools you need to accomplish this project.

The list includes:

  • Soldering iron and solder
  • Multi-meter that has the "Diode check modality"
  • tweezers to handle the tiny LED chips
  • Pick to move stuff around and serve as tiny fingers
  • Replacement LED chips (sell on eBay for like $4 for 1000 chips (like in this picture)

This is a good place to state "do not do this with the bulb screwed into the light socket, because you can get shocked! Do not plug in the bulb until it is put back together and you are sure nothing is shorting out...

Step 2: Disassemble and Checkout the Power Supply

Picture of Disassemble and Checkout the Power Supply

Open the bulb by snapping off the clear cover and pushing out the electronics boards using a small pick so the boards "pop out".

Once it is all out, look it over to see if you see any physical damage (not likely because it is enclosed).

When you look at the board you will see a rectifier chip and a couple of bigger capacitors (one looks like a "can", and one looks like a roundish piece of candy). The round "can" one is polarized, and as you can see is on the DC side of the rectifier. The candy-looking one is bipolar and is on the AC side. The candy one is to keep "power spikes" out of the light bulb. The "can" one is to make the DC voltage flat and so is a filter to that purpose.

LEDs are diodes that make light, and only use power in one direction (DC). The other direction does not make light and can damage them. The LEDs are linked end to end (think of a chain with each link being one of these LED chips).

Check the candy-looking capacitor with the ohm-meter to make sure it is not shorted out (not likely at all, but just check to be sure). Do the same with the "can" one. Check the rectifier which is four diodes connected in a circlt which makes the voltage go in one direction (DC) from the AC (back and forth) with the diode checker (see picture of setting on meter above): You will see it passing power in only one direction when you measure between the four legs of this chip (Again not normally the problem, but check it anyway).

Once you have these checked out, you are ready to jump into LED land... its all on the DC side of the power supply.

Step 3: Finding the BAD LED

Picture of Finding the BAD LED

When you look at the LEDs on the board, the number relates to the power they consume as a whole, and you see this is terms of the number of WATTS they consume (5watt, 7 watt, 9 watt, etc).

Look at each LED to see if you see a burnt one. These burn out mostly because of a defect in their manufacturing process I assume since they always burn out in the middle of the chains of LEDs. You can tell easily a bad LED usually be looking at it and observing a black spot in the middle of the chip (see picture with burnt spot circled in red). This chip needs to be replaced. Check this LED with the Diode checking function of the multi-meter.

Since the LEDs are diodes, you use the DIODE modality on the meter which will exert a little electricity to drive diodes for testing. In the case of LEDs though, it will light them up as the diode is powered slightly, and you can easily tell a bad LED from a good one. When you test the LED in the backward direction, it will remain dark. When tested in the forward direction, it will light (see pictures).

Test in both directions with the meter, taking note as to which direction drives the LEDs. These LEDs are in series, so the LEDs on each panel (board) are all lined up in the same direction (+ & - are all on the same sides of the board in other words). This is important to note when you go to install the new LED in the bad one's place.

Step 4: Remove Old Bad Chip

Picture of Remove Old Bad Chip

Use your soldering iron to remove the old chip by heating both ends and using the pick to remove the bad chip. Do not worry about damaging the chip since it is trash, but be careful not to burn or damage the connection pads that it was connected to on the board... You need them to connect the new LED to.

This is a picture of a bulb with the bad LED removed for an example.

I suggest putting a small ball of solder on the pad before installing the new LED so it has something to grab when you solder it in.

Step 5: Looking Over the New LED to Replace the Bad One

Picture of Looking Over the New LED to Replace the Bad One

Looking at the roll of LED chips, open it to take one out, and lay one on the table so you can test it.

Remembering how to use the Diode modality on the multi-meter, you can test the new LED by touching both sides of it with the leads: Remember one direction will cause it to light and the other will not.

Step 6: Install the New LED in Place of the Bad One.

Picture of Install the New LED in Place of the Bad One.

Take you tweezers and place the new LED where the bad one was paying attention to the polarity that drives it.

The new LED needs to go in the same direction as the other ones so your meter will make each one light up when the leads are placed along side them one at a time.

When you are sure they are all the right direction, tack the sides of the new LED down by using the soldering iron and lightly connecting the solder balls you places earlier and the ends of the new chip will tack into the solder. Practice makes perfect...

test your work by using the meter to light the chips again and make sure you have them all in the same direction (see second picture here).

Step 7: Back Together!!!

Picture of Back Together!!!

Carefully put the bulb back together and into the clear capsule like it was, being careful that nothing is shorting out or crimped anywhere.

Keep everything and organized when you stuff everything back in the base too.

Now that you have tested all the LEDs and all, screw the bulb in and see if it lights up.

When you see the light, it means you succeeded. Congratulations!

I think the bulbs break because of poor LEDs put in them as I am sure the cheapest and lowest quality chips were used to put them together in China.

Hope this helps you keep from having to replace your LED light investment for the COB bulbs if you have these..

I have to say that I am very intrigued that we can repair a light bulb.. that is something that I would not have thought of just a year ago when we had CFL or incandescent bulbs.

Step 8: Just in Case You Can Not Find the LED Chips and Still Want to Fix (Addendum)

Picture of Just in Case You Can Not Find the LED Chips and Still Want to Fix (Addendum)

I was messing aroung with one of my bulbs that I was repairing, and I was thinking about what another person suggested to me about needing one fixed, but not having an LED chip available with him... This started making me think about his situation..

If you do not have one of the LED chips to replace with, you can simply short across the place where the LED chip was at so that you are completing the circuit loop. There will just be one less LED chip in the chain which should not make a very big difference as long as it is just a couple of the chips at most. The result will be less lumens (amount of light produced in total) because there is some LEDs missing (makes sense, yeah?) This will make a burden on the other LEDs in that they take up the slack of the missing one(s). This can be a work around, but suggest you have the missing LED replaced.


FranzB10 (author)2017-10-16

a few days ago I came across this site in search of a way to repair
one of my LED bulbs. I guess it's chinese as well, gave up 6 month after

I used the method you described and found 3 burnt
LEDs. Problem is there is no hint to identify the correct replacement
LED. I tried to find some on the web but w/o success.

here's the
configuration of my LED bulb: 3000k, 110/220V, 7W, 110V DC after the
AC/DC converter, 12 x 2 parallel LEDs in series. as there are 3 bad LEDs
in 24 I could put any replacement LED in even one that isn't that
bright but I have no idea it this would work. a simple calculation to
divide 110V DC / 12 = 9.1V of DC voltage over each parallel pair of
LEDs. I wonder.... the nominal voltage should be 3V for white LED,
correct? so what replacement LED would I need?

mirdamail (author)2017-08-22

Hello. Can you write me the part number of rectifier in bulb? The one in my bulb gave it up and I would like to buy a replacement part, not new bulb.

mrstan (author)mirdamail2017-08-22

I think the link I attached is a color LED controller in stead of an LED driver.. Do a search for "LED driver" and I think you will find it. I had some bulbs which had these drivers for the LEDs to save physical space in the bulb.

Digikey has a few here:

here is another place that I have not heard of before.. must be a chinese place:

Those are what your are looking for though..

mrstan (author)mirdamail2017-08-22

hi there,

I do not think that chip is a rectifier. I think it is an actual LED driver chip.

I see one on ebay that might be this one.. Look here on ebay and see if this one lines up with how yours is wired on your board...

I am not sure if you are where the AC is 120 or 220 either. The driver chips are a bunch of filters and waveform shapers to help the LEDs live longer and operate better with less distorted power going to them.. That ebay item has a pretty good diagram of the pins and circuit setup to look over too.

Hope this helps a little..

apofview made it! (author)2017-04-07

Opened led bulb, but every led inside is burnt out, simple design. Have three more of these bulb instaled, probably won't last long. Installed leds last year.

mrstan (author)apofview2017-04-07

LEDS do not light up if you put the diode checker on each of them? Do you get DC out of the power supply? Your power supply looks easy to hookup and measure with a meter...

apofview (author)mrstan2017-04-07

Testing with meter shows that cicruit is whole, led are measured as break in both directions... When desoldering led board from supply board (+&- points), two of the leds were blinking wery fainted. Supply board shows output 400 VDC and around 30VAC but ac value is varying from 11 up to 70VAC...

mrstan (author)apofview2017-04-07

When I look at your pictures, I think I see a rectifier in the power supply there.. Test it with the diode checker as this is basically four diodes all pointing in one direction. Each diode should read like .590 in one direction and open in the other. You should not be seeing any AC on the LED side.. What you are saying seems to indicate your recfifier has failed and AC is getting into the DC side of the bulb. Check that out...

apofview (author)mrstan2017-04-09

Hi, thanks for replaying. Ok. Just opened working one, dc voltage is 190V ! So this are not ordinery leds, there are 19 leds, so voltage across each is 19V, never heard of these. This high forward voltage probably is reason why my multimeter can't check these ...

apofview (author)apofview2017-04-09

Hi, thanks for replaying. Ok. Just opened working one, dc voltage is 190V ! So this are not ordinery leds, there are 10 leds, so voltage across each is 19V, never heard of these. This high forward voltage probably is reason why my multimeter can't check these ...
Just searched net, there are some sort of led series arays with 20V DC firward voltage...

33gabor (author)apofview2017-04-10


Please note that a single LED normally consumes about 20-30 mA and this just happens when you feed it at 3 to 3.2 Volts.DC.

This applies to the series fed 2 to xx pieces also. For instance 10 LEDs consume 20-30 mAs at about 30 Volts. If the converter/rectifier modul as well as all ten LEDs are good you will get these values when you insert the Ammeter into the circuit and he Voltmeter actoss the 10 LEDs. If one of te series LEDs is broken there is no current and the modul runs off, you may measure the maximum possible Voltage of some 190 Volts.

It sounds funny but the converter/rectifier moduls are designed and manufactured not for controling Voltage but for reasonably precise control of the delivered current at an amazingly vide range of Voltage. This way the same modul is usable for a series of LEDs between 3 to 50 or more pieces.. One to six steps more current is needed when a compound chain of paralel and series LEDs and are to be fed. So the various moduls are made and stronger in steps for delivering say 20-30 - 50-60 - 110-130 - etc mA current. And of course, all between the same 6 to 190Vots. This way the variety is shortened, only 5-6 designs are needed depending only on the number of paralel rows and not depending on the number of LEDs within a seria in a particular row.

For me it has also taken some time to realise. Dont' worry :-)

apofview (author)33gabor2017-04-10

This is not the case. Search on net, this leds are complex and are driven by Vf 20V each.
For example:

33gabor (author)2017-04-08


Yeah. But hereabout for 220V mains we mostly get cornbulbs fed throuh a nonisolated converter/rectifier modul, which itself unfortunately often causes the brakedown. These are hardly repairable economically.

Then, if the converter modul is still good, comes in the metod discussed in this article. In these bulbs there are 3 to 6 parallel connected "cob" strips. So the problem of tidy removal from an aluminium based stip of a bad LED still remais acute because a single LED is soldered to the strip surface not only at the ends but at its bottom also, to achieve faster heat transfer. So just this makes it difficult to unsolder.

What I wish to contribute to this article is that I "slighly" modified my wife's discarded household favorite, the electric iron. So that instead of 215 (for linen) now it heats up (adjustably to max 350 centigrade) and melts the solder of the strip all along. Now I just carefully remove the bad LED and place the new one. Also carefully take off the strip so that no other LEDs would get disturbed.

Good luck!

mrstan (author)33gabor2017-04-08

That's awesome! It's also very innovative too. It's good that we can overcome problems that we encounter this way, and that just shows how someone can find a solution to any problem. Good work my friend!

karawin (author)2017-04-08

Repaired: It was the 50R CMS resistor burned.

All leds was lightning but very low.

mrstan (author)karawin2017-04-08

That's completely awesome! I think it's interesting that we can fix a lightbulb now.

karawin (author)karawin2017-04-08

The 510 resistor...

cnn2017 (author)2017-04-08

very nice

I have question can use it with this product in my yard?

link of the prodcut

offtherails2010 (author)2017-04-08

nice 'ible !

ive repaired alot of these too, an on-going practice for less than 100th less than the price of a brand new one !

for me, its *always* heat that kills my SMD LED's.. if not the LED then the resistor or IC, its always something but its always the heat killing the parts.

As an experiment, ive got a 80x80mm fan blowing onto one of these, its not blown over a year but another keeps blowing a different part every 2-3 months lol

atillakocsis (author)2017-04-07


So i had a few old burned out LEDs around so after seeing this i popped the top off and put my mulitmeter to it.

But my LEDs appear to be different then yours. (This is a BR40 flood light)I discovered that there are 4 connectors on each of my LEDs. The top right and the lower left OR the Lower right and the top left would each work with a Positive and negative lead from my mutlimeter. Any idea on what is going on here?

To further explain (looking at them horizontally (not vertically like in my photo)

Top Right = Positve / Bottom Left = Negative - LIGHTS
Bottom Right = Negative / Top Left = Postive - LIGHTS

So its as if the Top of the LEDs are positive and the Bottoms are negative

Also, all of my LEDs worked. So now i am going back to the beginning of your tutorial to test the other parts of this bulb.

Thanks, very eductional


mrstan (author)atillakocsis2017-04-07

you might try using the ohm meter to see if the top right and left are connected togehter and same for bottom two as well.. Maybe there are only two contacts, but for some reason they used two blobs on the pads. I noticed there are little black dots on all your LEDs. Mine all have a blackdot if they are bad.. Are yours working you say? I can not see very clearly on the picture when I zoom into it. Might take a picture of the top angle or bottom angle to see the pair of connectors on each end of the chips...

atillakocsis (author)mrstan2017-04-08

I looked at each LED, they all have a similar mark, i think its manufactured that way. But each one worked. I took the LED board out and below it found the electronics submerged in a rubber mass. So i wont be doing any testing of the rest of parts of this one. As for the bottom of the LED board there were no wires there. so the wiring must all be internal.

This is a first generation LED. the case is all metal with huge metal heat disapator fins. it must weigh 12 oz.

nonobadog (author)2017-04-06

I have a quite large 12 Volt DC LED bulb like this with the same type screw-in base that I accidentally put in a 120 Volt AC lamp and turned it on. It didn't last over a fraction of a second but it was pretty special with neat sparks and smoke. I might try to fix it after seeing this.

Speliobod (author)nonobadog2017-04-07

Once that smoke genie gets out she's a devil to get back in!!!!

mcelhiney (author)nonobadog2017-04-06

DC resistance vs. AC impedance...

nonobadog (author)mcelhiney2017-04-06

This is true, but irrelevant. The problem was the 120 Volts applied to a lamp designed for 12 Volts.

mcelhiney (author)nonobadog2017-04-06

My error, I misread it as "120 Volt". I have never seen a 12VDC lamp with a candelabra base... When I was in USAF tech school, the entire classroom building had a 120 VDC generator-powered emergency lighting system. We learned that you could not install a 120VAC lamp in those sockets because there was virtually no DC resistance.

JohnC430 (author)mcelhiney2017-04-06

No Larry, you learned that u cannot put a 12V bulb into a 120V socket.

Waste Of Space (author)JohnC4302017-04-06

No, he learned that you can put a 12v bulb into a 120v socket, but that it is a bad idea to do so if the longevity of the 12v bulb is important to him.

JohnC430 (author)Waste Of Space2017-04-07

LOL! u really made me laugh. yeah that too. i was commenting about what he learnt in military school and ure commenting about the lesson he learnt from his mistake. good and very true! thanks for the laugh for the day :))

JohnC430 (author)nonobadog2017-04-07

dont waste your money, time or energy on that. it is dead!
you should first check it out and see if there is anything worth salvaging apart from the mechanical structure. after all the way u described it, was it went off like a sparkler on the 4th of July. so many little parts to replace. and I am sure since it was made for 12 VDC, there is no rectifier. so any parts in the 120 VAC stream would be fried. dont keep it. throw it away. it will take hours of ur time to replace the parts. from the pictures it looks like 60 LED's or maybe more.

mrstan (author)nonobadog2017-04-06

wow! It exploded? My goodness!

nonobadog (author)mrstan2017-04-06

Yes, it was like a very rapid series of tiny explosions and pops as various little electronic pieces were zapped by the excess voltage way beyond their limits. It is strange they make 12 Volt lamps with the same base as 120 Volt lamps.

It was a big one too, one of these -

JohnC430 (author)nonobadog2017-04-06

yep! that is priced at $20. too bad and also you are correct, even though they say it is a 12V bulb they should not be selling it with an Edison base. where on earth do they use that kind of base for 12V?

lubi570 (author)JohnC4302017-04-06

Older Motor homes used these Edison base bulbs at 12 volts dc and the whole system in the motor home was 12 volts lighting. then when they wanted 120 Volts they used a circuit just for 120 V, and used an DC-AC inverter that was connected to the 120 V circuits. the 12 V lights were more safe in the ungrounded Motor homes for camping and was easier to supply 12 Volts when no mains were available.

Roy697 (author)JohnC4302017-04-06

You will find some older campers use the same base. It does not take long to pay attention when you have bulbs with the same kind of base, I have bought a few over the years and made the same mistake ONLY ONCE.

mrstan (author)Roy6972017-04-06

boats use the E37 as well...

mrstan (author)nonobadog2017-04-06

maybe it was the voltage going back and forth more than the over-voltage since it would be reverse-driving the LEDs with nothing to block the negative side of the AC waveform... That makes my heart sink when I think of your LEDs popping that way!!! How disheartening!!

nonobadog (author)mrstan2017-04-06

Yes, I'm sure the back-voltage was over the limit for the LEDs but the forward voltage was too so it would be hard to say which fried it, probably whichever hit it first. And the voltage was probably too much for those tiny SMD resistors and even the paths in between the components. I really felt stupid since I knew it was a 12 Volt lamp all the time.

juliadee (author)2017-04-06

Not to be overly pedantic, but I believe words and terminology are important. This is not chip-on-board, these are SMD LEDs. The LED chips themselves are inside self-contained surface-mount packages, with bonding wires from the chips to the SMD mounting terminals. In COB, the chip (piece of semiconductor wafer) is glued directly to a PCB, gold bonding wires are attached from the chip to the PCB using highly specialized machinery, and the whole thing is covered with a glob of epoxy. There is no practical way to repair COB, but then again COB is never used in LED lighting AFAIK.

duodreamer (author)juliadee2017-04-06

Absolutely correct, juliadee. However, there have been many instances of companies using COB LED for their products, going back 40+ years to early LED-segment pocket calculators that would have a plastic magnifying lens over the whole group of COB, all the way up to todays actual COB LED lighting units by Bridgelux (Vero) that are multiple diodes bonded to a board and coated with one large phosphor goop window.

Bodragon (author)duodreamer2017-04-07

These are cob lamps not COB LEDs. (as in corn cob).

juliadee (author)duodreamer2017-04-06

I would argue that even though the manufacturers call them "COB" because _they_ bond the LED chips directly to a ceramic substrate, products like Vero's Bridgelux are in fact self-contained components, or modules, as far as the user is concerned. Vero, in fact, calls them "modules" throughout their literature. Traditionally (in my experience), COB means that dice are glued directly to an end-product's PCB, the "B" in both acronyms meaning "board" :)

david.roseman (author)juliadee2017-04-06

"I would argue that even though the manufacturers call them "COB" because _they_ bond the LED chips directly to a ceramic substrate, products like Vero's Bridgelux are in fact self-contained components, or modules, as far as the user is concerned."

So what are you saying? What to you is a COB LED, compared to what a Bridgelux is? How is it not die that are bonded to a circuit board? How else can you get power to the die array? No one every said a PCB has to be a thick fiberglass board. Besides, their main page for the Vero states "Vero Chip on Board (COB) LED arrays are available in four LES configurations..."

JohnC430 (author)juliadee2017-04-07

im not familiar with the term. what does AFAIK mean? thanks

Bodragon (author)JohnC4302017-04-07

AFAIK, it means as far as I know, but as I say, that's just AFAIK.

Bodragon (author)juliadee2017-04-07

Dear juliadee,

In this case COB should really be written as just cob (or Cob if at beginning of a sentence). This type of lamp is known as a cob type since it resembles a cob of corn. It looks like mrstan looked up what COB means but got the wrong meaning and has remained as much in the dark (no pun intended), as you! I hope this shines a light (yes, pun intended this time), on the confusion.

jinpa108 (author)juliadee2017-04-07

So since your into precision I just thought I'd let you know that I agree these are smd's but I have many COB LED light bulbs. Actually I prefer them because they are generally brighter and have a mor concentrated light.

DaveM275 (author)juliadee2017-04-07

I thought COB meant Chinese Opprobrious Bulbs.

Gary Viveiros (author)DaveM2752017-04-07

Dave!, Dave! Stop, what with the $10- words! You're bankrupting my time budget having to look them up. Even if I were to object to the selection of that word, I'd risk being labeled, 'opprobrious'.

About This Instructable




Bio: Clinical Engineer. PhD, MBA, CET, BMET, MCSE Works with electronic, mechanical, medical, and automotive stuff. Systems Design, Repair, Modification, Repair.
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