I can’t get past the electronics. This has been the case since childhood. And when something, even the most ordinary garland, breaks at home, before throwing it away and buying a new one, I first prefer to take it apart and see what’s inside. And then as things go, if it’s cheaper and easier to buy a new one, then I buy it, and if the breakdown is simple, then I repair it. The renovation also helps the environment a little.
This time the built-in ceiling lamp, under a certain brand ED-SON, stopped working. It was immediately clear that water had got into it, since it was clearly visible on the light diffuser.
I took the lamp out of the stretch ceiling and immediately saw that it consisted of two parts: a lamp and some kind of power source. They are connected to each other by a cool wire-to-wire connector with an overmold. For the first time I see that the current source is made separately, and not on a board with LEDs, but oh well. I’ll get to him too.
First I opened the lamp. Opens easily, not glued. The light diffuser, also known as the lampshade, simply has a plastic rim with which it snaps onto the block with the LED board. What do I see:
The lamp is located in a room where the roof leaks a little when it rains.
Upon closer examination, I realized that I would not restore it, but that it would be easier to buy a new one. In fact, it could have been repaired, it was a simple breakdown. But I’m in another country without any of my soldering equipment…
Here you can immediately see that moisture has penetrated into some LEDs and they simply need to be replaced.
The open lamp showed that the LEDs are connected in pairs (two in parallel) in series (all parallel blocks of two LEDs are connected in series) and this block is powered by direct current.
The board is made on an aluminum base, like most modern lamps of this type. Has 90 LEDs per bor. It is attached to the top cover not very elegantly with simple rivets.
And now about the power supply.
A very light and compact plastic box with something dangling inside. The wires are not fixed at the outputs (this is a minus).
I never cease to be amazed by plastic cases.
Especially fake plastic screws. It’s so funny. It is cast from plastic and naturally is not used for anything. The case itself is attached to flimsy latches located on the sides. The board inside is not secured, but simply dangles. That is, the body is larger for appearance and so that there is something to stick a label on.
But let’s get to the interesting stuff. Inside the block I see a small board on which there is a resistor that limits the turn-on current, a diode bridge and a filter capacitor. It’s not bad anymore, I thought I’d see just a single-period rectifier on one diode.
Next, the SDH7711SN chip (https://www.silan.com.cn/en/) is installed with a small connection in the form of a pair of resistors and a choke. A capacitor is also installed at the output.
I was a little surprised by the inductor, made in exactly this form factor; probably there was a lot of such ferrite left in the manufacturers’ warehouses from the production of fluorescent energy-saving lamps… I also noticed that one component was missing, judging by the silk-screen printing – a diode.
The board is quite simple, but two-layer. The gaps, approximately by eye, correspond to IPC. And the input electrolyte is even properly diluted (you can see how the path passes through the place where it is soldered).
Now a little about the scheme.
This block contains a circuit of a non-isolated step-down voltage-to-current converter. The chip is designed specifically for applications in LED lamps. Block diagram of the microcircuit:
We see that power minus control is implemented here, this is quite logical. It’s simpler and cheaper, and quite justified for this application. There is an adjustable OVP block (the level of overvoltage protection is adjusted by an external resistor). And the external resistor connected to the CS pin is a shunt, the drop across which will set the output current. It is calculated according to Ohm’s law for a reference voltage of 400 mV.
This module is configured to produce a current of 300mA with a voltage of up to 60V.
The datasheet shows a reference diagram for connecting the microcircuit.
It is implemented to supply current to the LEDs that are located in the lamp itself. The developers decided not to install only the D1 diode on the boards.