Never Seen Before- LED Bulb Post Mortem And Repair
Wow, one of my LED bulbs broke down! According to my earlier calculations I should not have seen any of those dying until I reach the respectable age of 99 years old! Let’s go back to the math…
I bought them in mid-2012 when I moved from an apartment to a town house. I can remember perfectly when I replaced the permanently fixed fluorescent tube mounted above my desk with 4 desk-type lamps fitted with LED bulbs, two above the desk and two above the workbench. The LED bulbs were guaranteed 25000 hours, as printed on the box! The lighting in my office is quite good, so I only use those auxiliary lamps when I need a bit of extra light, for some delicate work or to take photos… If I say a maximum of 2 hours a day it is probably overstated but let’s take this as a base for calculations:
25000 hours divided by 2 hours/day = 12500 days
12500 days divided by 365 days/year = 34 years (ignoring the decimals)
So from 2012 the bulbs are supposed to last until 2012 + 34 = 2046! Yes, I would be 99 years old! Total disbelief, I tried the bulb in other sockets in other lamps, nothing. It was really gone.
Well, the good thing of this early demise is that I had the opportunity to open the LED bulb and see what is inside. The first problem was to find out where to start. It is all plastic and there is a groove around the bulb somewhere in the middle. Maybe using a cutter we shall be able to cut it open? Not really, the pictures below summarize the battle:
Finally we had something to work with, a nice little PCB and a LED board. Let’s start with the LED board. It is fitted with 22 LEDs and has a provision for 4 more:
Each LED tested OK individually with the analog ohmmeter, giving a bright light on the X1 range. The Peak Atlas gives a forward voltage of 2.6 V. At this stage it was not possible to test the whole board because the LEDs seem to be connected in series. We would need a voltage of at least 22 x 2.6 V = 57.2V. None of my instruments can do that…
Now let’s have closer a look at the board:
Nice and tidy, no sign of overheating. However we can spot a number of cold joints. Would this be the problem?
I started by measuring a few obvious components, capacitors, diodes, transistor etc… no one was found defective. Then I re-soldered the cold joints. Now it was time to reconnect the LED Board and do some measuring. And guess what? It was working!
The voltage across the LED board indicated 66.6 V and the current 140 mA. This confirmed that the LEDs are connected in series. Checking at the datasheet of the EMC 3030 LED confirmed that the current of 140 mA was within specs.
Everything indicates that the controller is a mini-SMPS acting as a constant current source, contrarily to our SMPS which usually act as constant voltage sources. To prove this I needed to use a load corresponding to the LED board. Then I should vary this load and see if the current remains constant. With 66.6 V and 140 mA the LED board corresponds to a resistance of 475 Ohms (and approx. 10 W). I don’t have this value but 10 x 47 Ohms resistors in series will do the trick. To vary the load I would either short one of the resistors or add one more resistor.
With 470 Ohms the values are 64.68 V and 139 mA.
With 470 – 47 Ohms (423 Ohms) the values are 60 V and 140 mA
With 470 + 47 Ohms (517 Ohms) the values are 69.7 V and 138 mA
We can see that the controller nicely adjusts the voltage to maintain a current of around 140 mA. Like this the manufacturer can add or remove a few LEDs to make different wattage bulbs without changing anything on the controller board.
The last thing I was keen to check was the input power. At the output we have 66.6 V with 140 mA which corresponds to 9.3 W. Measuring at the input I found 244 V and 55 mA which corresponds to the marking on the bulb (220-240 Vac 57 mA). So the power would be 244 x 0.055 = 13.42 W? Is this correct? The marking on the lamp is 10 W so are we getting ripped off? Is this lamp using 13.42 W, 30% more than advertising?
The answer is not so simple. In DC or AC with a purely resistive load, the power is given by Voltage x Current. In AC, if the load is not purely resistive, there will be a shift between Voltage and Current. This means that when the voltage is maximum, the current is not etc… so we cannot simply multiply voltage with current because they don’t have their maximum value at the same time. We have to consider the difference of phase. This phase difference is called the Power Factor (PF). It is in fact the Cosine of the phase difference in degrees! When there is no phase difference, PF =1 (Cosine of 0 degrees = 1). A PF of 0.9 is considered as normal. Less that is usually not good.
And the real power is expressed as:
P = V x I x PF
A quick survey on the Internet taught me that the power factor of LED bulbs if far from ideal and can vary between 0.5 and 0.9. As I do not have the equipment to measure the power factor I will assume that the input power is 10 W, as advertised. In such case:
P = V x I x PF
PF = P/VI = 10 W / 13.2 VA = 0.75 ?
I couldn’t resist finding a way of looking at the current’s shape. For this I needed a current transformer so I could connect the oscilloscope safely. I found one in my junk box, recovered from an old UPS. For the voltage I used a 240V/6V transformer. Like this my oscilloscope is safe. Never try to connect your oscilloscope to any part of the 110V/220V/240V mains! It would be instant destruction!
Using a standard light bulb (filament type) first I could see the shapes of voltage and current:
The phase shift is due to the use of transformers. Indeed for a light bulb both should be exactly in Phase as the light bulb is a pure resistive load.
Then replacing the light bulb with our LED bulb:
The sine wave is still our voltage. The hairy funny wave is the current (Note that it is shifted by about 2.5 ms because of the transformers). No wonder why, by multiplying the voltage by the current we do not get the expected result! Maybe the power factor could be the subject of another article.
For being one of the earlier LED bulbs that we could find on the market this one is not too bad. The construction is solid and there is no sign of overheating after a few years of use. With a better soldering quality, without cold joints, this bulb might have had a chance to live until 2046! Who knows? While de-soldering a few components to measure them I found that the solder used was difficult to melt and needed a much higher temperature. Probably Lead Free Solder…
We could repair those bulbs. Not for the money worth but for fun, to sharpen our skills and improve our knowledge. Providing we find a less destructive way to tear them down!
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