Project: Unbranded 38 x 5mm LED DIY Lamp Kit

Trawling eBay listings for some more cheap kits to build resulted in some rather interesting discoveries. The kit in question was a 38 LED do-it-yourself LED lamp kit for AU$1.47.

At that price, I thought I’d jump on it despite some level of risk being a mains-powered project. Unlike my younger years, I am much more confident in handling mains power, but it does seem potentially dangerous as it might encourage less experienced people to attempt to play with the mains.

It is cheap, but is it worth trusting your life to it? I’d have to say, if you’re not particularly experienced, you probably should stay away from this kit … any mistake could have rather loud, smelly and explosive effects!

The Kit

The kit comes in familiar packaging – namely the clear zip-lock bag. But I notice something is missing – there are no LEDs! Alas, I didn’t read the fine print in the listing which said that no LEDs are included in the package. That explains why it’s so cheap, not that 5mm LEDs are particularly expensive nowadays. There are also no instructions, although none are likely to be necessary.

The first part is an Edison screw base with leads pre-soldered to the contacts internally. The base has slots for ventilation and relatively thin wires (which should vaporize in case of any major mistake as a makeshift fuse).

The main PCB that forms the LED panel is a resin-cured paper phenolic type PCB with single-sided copper. The top side has mostly white silkscreening to work as a “reflector” while marking the orientation of the LEDs. The underside has green solder mask and a few white silkscreen indications.

The other components come inside its own zip-lock bag, which is good since they could puncture the outer bag and get lost.

The components here form the driver circuit, and two very thin black wires allow for it to be connected to the LED panel.

The PCB is very much the same sort as the LED panel.

Construction and Testing

Instead of building a boring, low-quality white LED lamp, I chose to dig out my high intensity green LEDs to build myself a green LED lamp instead. After all, that’s my favourite colour.

Before I start, lets look at the total number of connections required:

LED PANEL
38 LEDs x 2 connections
1 Power Input x 2 connections

DRIVER PCB
1 Power Input x 2 connections
1 Power Output x 2 connections
4 Diodes x 2 connections
3 Resistors x 2 connections
2 Capacitors x 2 connections

TOTAL: 100 joints

The whole construction process took me about 1.5 hours including distractions and time taken to photograph as I went along. I started by constructing the driver PCB, which is a simple capacitive dropper with full bridge rectifier and smoothing capacitor.

This was relatively straightforward, although being a paper type PCB, it is more sensitive to overheating. As the pad condition being slightly dirty, soldering was not as easy as it could have been. I then attached this to the shell wires.

Constructing the LED panel was also straightforward but tedious, and the smell of the phenolic PCB is hardly the most enjoyable experience. The non-tinned pads did make soldering a bit more tricky than necessary, but at least the LEDs were laid out symmetrically.

Then, it was a matter of attaching the wires to the driver, and from the driver to the rear of the array. The drilled holes in the pads are unnecessary.

At this point, it’s clear that there’s no insulation between the driver board and the LED board which could result in catastrophe. As a result, it’s a good time to check for any stray solder balls or bridges, and wrap the driver up in heatshrink or electrical tape before popping it into the shell and snapping the PCB into the front.

Voila. The finished product looks pretty simple and neat.

It works just fine as well, with the LEDs glowing fairly brightly.

According to the power analyzer, it dissipates 2.6W of real power (but it depends on the LEDs in use). The current was 79.51mA, however, at a power factor of 0.142.

To analyze how everything works, I first checked out the driver PCB by overlaying a flipped underside image with the silkscreen top:

I drew out the schematic as follows:

In looking at it, there was a “fuse” trace which is slightly thinner as an added safety. There are appropriate discharge resistors, to prevent any unexpected tingles or residual voltage on the output. There’s also an output limiting resistor to avoid inrush damage to the LEDs. As it is a “crude” capacitive dropper, using the reactance equation gives us an effective current limit of about 49.6mA worst case at 230V/50Hz. This makes sense, as the output LED array runs pairs of LEDs in series (i.e. 18 series pairs of LEDs). Given the ~25mA maximum rating of many 5mm LEDs, this is pretty much at its limit. When unloaded, the output can “float” up to the mains peak voltage, so we should be careful of this. The 100 ohm output resistor will see up to 0.246W dissipation assuming an output short, so it seems it’s pretty much at its limit too.

Regardless, the design seems workable (even with the cheap non-class X2 rated capacitor of 400V DC working voltage and the ChongX capacitor on the secondary), but care must be taken not to contact the output to the LED panels under any circumstances when powered to avoid mains electric shock as the circuit offers no isolation from the mains.

Conclusion

This was another successful build of another cheap kit. This one involved mains power, so it was surprising that it would be so cheap and easily accessible. For beginners, it’s not a kit I would recommend based on the mains power requirement, the iffy phenolic-paper PCB with copper pads and a lack of instructions. But it is pretty nifty and cheap, if you wish to spend a lot of time soldering and clipping LED legs to make your own little (underpowered) LED lamp. I don’t think the capacitors would last particularly long, but it does look neat once built. Just remember to BYO LEDs and electrical insulation tape/heatshrink.

About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
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