What is hidden inside the LED bulb?

In recent years, LED bulbs have made their way into our homes for good, replacing the now-discontinued tungsten filament designs. Not surprisingly, LEDs compare surprisingly well with other light sources – they consume little energy, can shine in a variety of colors, turn on instantly and do not heat up as much.

Recently, one of these types of bulbs in my home was damaged, out of professional curiosity I decided to look inside. The damage turned out to be minor, one of the connections on the board broke, but in the process I thought it might be interesting to compare several types of LED bulbs. After all, there are quite a few designs on the market that differ in build quality and price, and it would be interesting to see if there are more differences. Influenced by this thought, I bought three classic white LED bulbs and a rather interesting RGB design, which we will look at later.

Introduction of the characters

LUMINOVA

OSRAM

PHILIPS

12W

6W

5W

4,39zł (0,98$)

6,27zł (1,40$)

19,89zł (4,44$)

1310lm

470lm

470lm

After accessing one of the more popular auction sites, I sorted the auctions by popularity and ordered three of the most popular designs. The cheapest one is the LUMINOVA LED, which should also provide the most light. A slightly more expensive bulb is OSRAM, but its power is half that of the 12W design. The most expensive and also the most “branded” is PHILIPS, whose power is similar to the OSRAM branded bulb.

Having already known today’s heroes, we can move on to the most interesting part, which is to look inside them.

The inside of a LUMINOVA bulb

LUMINOVA printed circuit board

Looking inside the design signed with the Luminova logo, we can see a rather thin laminate of 0.8mm thick it was made with MCPCB technology, that is, the circuits were placed on a metal substrate to facilitate heat dissipation. On the board itself we can find 15 white LEDs, two integrated circuits and two resistors. As you can see there are missing components in a few places, probably this laminate is quite universal and could have been used for different types of bulbs.

The other side of the LUMINOVA tile

On the other side, there was still a single electrolytic capacitor of 3.3µF at 400V and a resistor hidden in a shrink wrap, whose function is to limit the inrush current. These elements, along with a single power wire, are connected to the thread of the bulb. In turn, the connection of them to the PCB was realized with small plastic sockets, which also crumble quite easily.

How does the LUMINOVA bulb work?

Typical SR5131EC application (http://www.sxcai.com/uploads/soft/181225/1-1Q225133328.pdf)

The principle of operation of such an LED bulb is nothing fancy and is well illustrated by the diagram of a typical application included in the main driver’s note. The whole thing is powered by 230V mains voltage, passed further through an integrated bridge rectifier, designated MB10F, which can deliver up to 800mA of current. The rectified voltage is smoothed by an RC filter consisting of resistor R3 and an electrolytic capacitor, besides, the resistor’s task is also to discharge the capacitor when the power supply is disconnected. The voltage prepared in this way is fed to a string of LEDs controlled by the brain of the whole structure, the SR5131EC chip. This is a small, SOP-8 housed driver, dedicated to controlling serially connected LEDs. Despite eight leads, only three are used. D (Dren), to which the cathode of the last LED is connected, is actually the drain of the MOSFET transistor placed inside the circuit. CS connected to a resistor described by the same index, on the board it is R2, the current flowing through it is the reference value of the current flowing through the LEDs. The last lead is GND, the meaning of which I rather do not need to explain.

Internal schematic of the BP5131 twin chip (https://datasheetspdf.com/pdf-file/1258911/BPS/BP5131D/1)

How, on the other hand, does the SR5131 controller itself work? We can describe its operation as a kind of resistor, whose resistance can be changed in real time. Inside, however, we will not find any magic potentiometer. The central element of the circuit is a MOSFET transistor, the appropriate driving of which allows us to close the circuit in which the LEDs are placed and thus make them glow. The degree of opening of the transistor channel is controlled by a module that analyzes the current flowing in the circuit and the temperature, if it is too high, that is, in this case, exceeding 150°C, the circuit will be able to “close” the transistor and thus limit the current flowing through the LEDs.

Single LED in a LUMINOVA bulb

Let’s stop for a moment at the LEDs themselves, because they are not quite ordinary LEDs. There are actually two luminescent structures placed in a single housing, so the manufacturer was able to achieve even higher light levels. The diode is also powered by a much higher voltage than the typical 3.2V for white LEDs, it needs as much as about 14V to start working at all, the correct level of emitted light can be observed at about 18V. The reason for such a high supply voltage of a single structure is quite simple – somewhere more than 230V must be deposited (after rectification). The bulb is powered by the mains voltage and it must be lost somewhere.

Temperature test

After describing the operation and construction of the LUMINOVA bulb, it’s time for a temperature test, because it’s the temperature that is the killer for this type of design. In this and the other bulbs, I conducted a simple test in which the main driver with a thermocouple placed slightly higher was heated with warm air from a HotAir station. The temperature reading appears on the multimeter, but it is important to remember that this is only a reference value and actually shows the temperature of the air stream, not the semiconductor element itself. For safety reasons, I did not want to place the thermocouple directly on the circuit, after all, there is line voltage, which can be dangerous to humans as well as equipment.

However, let’s get right to the test itself and what you can see, or actually can’t see, on it. Throughout the test the bulb lights up virtually identically, which is a bit of a problem. As I mentioned earlier, in theory, the SR5131EC chip should have thermal protection inside, which, when it exceeds 150°C, should limit the current flowing through the LEDs. Here, however, nothing of the sort happened, despite the fact that at its peak I heated it with a stream of air of more than 220°C. Even if the current was slightly reduced, this did not translate into brightness of the LEDs.

The inside of an OSRAM light bulb

PCB in OSRAM light bulb

Once we know the structure and the idea of how led bulbs work based on the LUMINOVA design, we can move on to the next hero, the OSRAM bulb. When you open it, you can see the identical white MCPCB-type laminate, except that here it is slightly thicker at 1.2mm, certainly this improves heat dissipation. The electronic components themselves are somewhat fewer, eight LEDs, two resistors and a single IC. But in essence, the bulb’s design is almost identical to that of the LUMINOVA design.

The other side of the OSRAM laminate

On the other side, we also find a single electrolytic capacitor of 3.3µV at 400V, and an inrush current limiting resistor placed in a heat shrink band. The connection of the components to the PCB is also implemented in an identical manner.

How does the OSRAM light bulb work?

In terms of design and operation, this bulb is even more minimalist than its cheaper competitor. Here we have only one chip, designated JWB1891, for which, unfortunately, there is no catalog note. Only documentation for the JW1891 can be found, but it is a chip almost identical to the SR5131. Taking into account that the bulb driver is connected directly to the mains voltage, and the other components are placed identically as in the LUMINOVA bulb, I am inclined to think that the JWB1891 must contain a rectifier bridge inside, along with a linear LED driver based on a MOSFET transistor. Unfortunately, through lack of documentation, it’s hard to say whether it is equipped with thermal protection, but later in the material I will try to check this.

A single diode in an OSRAM bulb

The OSRAM bulb, like the previously described design, uses high-voltage LEDs with two luminescent structures inside. In this case, a voltage of about 28V is needed to trigger the LED. Why is it even higher than in the LOMINOVA bulb? Because there are fewer LEDs themselves, and the power supply voltage is also mains, and it needs to be smothered somewhere.

Temperature test

The mid-shelf bulb in the temperature test already performs slightly better than the cheapest design in the list. In this case, it can already be observed from about 1:10 that the LEDs were slightly dimmed, this means that the thermal protection worked properly. Unfortunately, due to the lack of documentation for the JWB1891, it’s impossible to tell what the minimum temperature is at which it will be activated, but given that at this point in the test I’m increasing the supply to around 200°C, the circuit probably reacts similarly to the SR5131EC at around 150°C.

Defective OSRAM bulb

Unfortunately, after the test, it turned out that the already somewhat damaged power supply resistor socket was completely destroyed.

PHILIPS bulb interior

PHILIPS bulb center

The most expensive bulb in the set, signed with the PHILIPS logo, is quite different from the previously described designs. Here we have only seven LEDs, two integrated circuits, a rectifying diode and three resistors. A metal laminate of similar thickness to that of the OSRAM bulb was additionally glued to a metal plate, which further improves heat dissipation.

The underside of the PCB in a PHILIPS bulb

It gets interesting on the other side. Here we found two electrolytic capacitors, a single film capacitor, a choke and a small transformer. The laminate itself is also connected to the thread via a single wire and a resistor hidden in a shrink wrap. These components, however, are connected to the PCB itself via tin, rather than a fragile plastic socket.

How does the PHILIPS bulb work?

Example diagram for DU8622 chip (https://datasheetspdf.com/datasheet/DU8622.html)

Despite the multiplicity of components, the operation of the PHILIPS bulb does not differ much from previously presented designs. The whole thing is powered by mains voltage, with the difference that it is reduced earlier by a small transformer. It is further rectified and smoothed by an integrated bridge rectifier marked 10U10 and an RC filter. The voltage prepared in this way feeds the main integrated circuit DU8622B, which is a linear LED driver of slightly more advanced design than the chips in previous bulbs. Its operation is also based on MOSFET transistors, but it needs slightly more components to work properly. This is illustrated quite well by the schematic of a typical application included in the chip’s catalog note and shown above.

Single LED in PHILIPS bulb

As before, PHILIPS LEDs are based on two luminescent structures, but their voltage is much lower, the diode responds already at about 4.8V. As we already know, this bulb is powered by a voltage reduced by the transformer, so it is not necessary to wipe out all the power of the mains voltage on the LEDs and the designers could have used more standard designs.

Temperature test

In the test of the PHILIPS bulb, the limitation of the current flowing through the LEDs can be seen quite well from about 1:00. The LEDs are clearly dimming, and according to the documentation, this is the point at which the circuit reached about 150°C. Unfortunately, the test does not end positively, as you can see in the video. At the end of the test, the bulb suddenly goes out.

Defective PHILIPS bulb

As it turned out later, one of the diodes burned out, and the other two, although they shine, you can already see charred spots on them. For some time I heated the whole thing with air at a temperature of about 300°C, so I think that the diodes simply did not withstand it. It should be remembered that these are just semiconductor components quite sensitive to such a drastic increase in temperature, about what happens in semiconductors when the temperature rises I told in the material on “Burning of the Soviet Transistor“.

Is it worth paying extra for an LED bulb?

Finally, it would be appropriate to somehow sum up and answer the question of whether it is worth paying extra for more expensive LED bulbs. In my opinion – yes. Although the task of each design is the same, they differ in construction and technical solutions. The thickness of the laminate, the specificity of the LEDs, the power supply voltage, heat dissipation…, the differences are in total quite a lot, and it seems to me that they are worth the price and if we want to enjoy you the light of the LED bulb for many years, however, it is sometimes worth paying extra.

How is an RGB LED bulb constructed?

China RGB LED kit

As a matter of interest, I’d like to show you yet another led bulb, as popular with consumers as the previously described designs. This is a light source that can glow in different colors, or simply an RGB bulb. The color of the light, as well as the light effects, because these are also available, are selected using the included remote control. I think you are familiar with this equipment, because identical remotes can be found in virtually every kit that includes the abbreviation “RGB” in its description. The bulb itself is 9W, and I could not find any information about the manufacturer on the packaging. Only the sticker with the data of the Polish importer reveals that the design comes from Shenzhen, more precisely from Shenzhen Passion Int’l Corporation, and from what I was able to check, these are the data of the exporter, not the manufacturer itself, which will remain a mystery. Interestingly, on the packaging we also find that the bulb is RoHS and CE certified. Taking a closer look at them, it looks like they are indeed genuine, and CE does not stand for the famous “China Export”.

In the video you can see one of the lighting effects generated by the bulb. After testing it, I have to say that it works quite well, and the light produced has a very pleasant color to the eye.

What hides the inside of an RGB bulb?

RGB bulb interior

I wouldn’t be myself if I didn’t take a look inside this equipment, and it’s really interesting here. There are quite a few components inside, we have here an MB10F rectifier bridge along with a voltage smoothing filter, a VS1838B infrared receiver, a main driver devoid of any markings, and three transistors.

Unfortunately, by the lack of markings on the main control circuit, it is difficult to say something about how the whole thing works. All that remains is conjecture. It is powered by mains voltage and one of its leads is connected directly to the output of the infrared receiver. Thus, by pressing a button on the remote control, we send data to which the circuit must respond and drive the diodes accordingly. These, in turn, are connected in series in three circuits corresponding to the color of their illumination. The individual ranks of diodes are controlled by small transistors located on the board.

The inside of the remote control

We can also look inside the remote control, but unfortunately we won’t learn much from it, because the control circuit is a classic black “glue”. The hardware is powered by a CR2025 battery, and data is sent out into the world via a small 3mm IR diode.

Also see:

Sources:

  • https://steemit.com/gadgets/@proteus-h/what-s-inside-a-cheap-led-light-bulb-an-explanation
  • http://www.sxcai.com/uploads/soft/181225/1-1Q225133328.pdf
  • https://www.diodes.com/assets/Datasheets/MB10F.pdf
  • https://datasheetspdf.com/pdf-file/1258911/BPS/BP5131D/1
  • https://hackaday.com/2021/05/27/investigating-a-new-chip-in-a-minimalist-led-lamp/
  • https://www.joulwatt.com/Upload/file/201905/20190528134540_1972.pdf
  • https://datasheetspdf.com/datasheet/DU8622.html

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