Analysis of cheap 1080-LED colored light string

[davekni]

Purchased a cheap string of 1080 LEDs:
https://www.amazon.com/dp/B0B8JSQFMB?ref=ppx_pop_dt_b_product_details&th=1
(BTW, amazing how prices bounce around.  Was $24.99 a couple weeks ago.  Now $55.99.  When searching earlier, many strings were ~$50.  Purchased this one because it was cheap.)

String runs on +-30V provided by a small supply/controller, drawing +-200mA.  String is wired 9S120P, with 60 of each 120 connected reverse polarity.  Allows effects such as fading from one half to the other.  Down-side is that "steady-on" mode is slightly under 50% duty cycle, not as bright as when one half or the other are lit.  255Hz chop frequency, 100us dead time one direction, 140us the other direction.

LEDs are all blue.  Phosphor converts blue to red, yellow, and green.  Less saturated color than with normal LEDs.  Advantage is that identical forward voltage allows direct paralleling.  Appears to have no per-LED series ballast resistors.  Ballast is provided by thin copper interconnect, total around 27 ohms for the string.  Wiring keeps all LED voltages matched:


I plan to make a higher-power supply, steady-on mode only.  About +-38V increases current from 200mA to 400mA.  Efficiency of red and blue drops as current increases.  Unfortunate that DC powers only half the LEDs.  200mA DC would be better than 50% duty cycle of 400mA.  However, green slightly increases efficiency as current increases.  Not sure if phosphor is non-linear, or if the change in blue wavelength with current matches phosphor better.  Yellow efficiency is roughly linear with current.

[klugesmith]

Nice drawing. Thanks for sharing, and for comment about price fluctuation. Guess we can hope for a clearance sale in January, or maybe after Chinese New Year.
My own seasonal lighting focus is to restore 3yo octahedron of T5 fluorescent lamps.  If the weather were nicer I would get out a traditional string of 120 volt C7 colored lights, which was converted from 25P to 25S by snipping some wires. MOT is one way to drive it.  Safer than NST, because if one bulb burns out the string will probably go dark instead of catching fire.

[davekni]

Made a small supply to run this LED light string in steady-on mode at higher power, about +-0.4A instead of +-0.2A.  Chops at 21kHz.  Runs from a 19.5V Dell laptop supply.  It is a simple cross-coupled two-FET oscillator.  Frequency is determined by R/C time constant of cross-coupling capacitors and gate resistors.  Certainly not a precise frequency, as precision isn't needed here.  Frequency varies with gate threshold voltage, load current (which affects threshold voltage), and supply voltage.  Not regulated.  Output is connected from one drain to the other, +-38V for 19Vdc input.





And here is the schematic:

[klugesmith]

Nice. Where did you get the bobbin and core?  Is it ferrite or laminated steel?

[davekni]

Nice. Where did you get the bobbin and core?  Is it ferrite or laminated steel?

I think these were from EBay, probably PC40 (ferrite).  I have a fair stash of cores, some from DigiKey etc., but most from EBay or from old work projects.

[Mads Barnkob]

String runs on +-30V provided by a small supply/controller, drawing +-200mA.  String is wired 9S120P, with 60 of each 120 connected reverse polarity.  Allows effects such as fading from one half to the other.  Down-side is that "steady-on" mode is slightly under 50% duty cycle, not as bright as when one half or the other are lit.  255Hz chop frequency, 100us dead time one direction, 140us the other direction.

Thanks you! I was at the zoo one of the days during Christmas holidays and was staring into a very long LED light chain and was wondering how they did the alternating fade on a single wire. I was considering that each LED was a single addressable DMX channel, but then I came to my senses, bought some coffee and walked around looking at the animals instead