Xmas Lights (cheap)

Black Hole

Black Hole

May contain traces of nut
I've discovered that a 20-LED string, powered by a two AA battery holder with switch, bought from my local discount store for 99p (!), has the LEDs in parallel and no current limiting resistor - relying on the internal resistance of the batteries to bring the operating voltage down to about 2.5V. Fortunately NiMH rechargeables would only kick out 2.4V-ish, otherwise their lack of internal resistance would make them a hazard in this scenario.

A similarly cheap string, this time with a holder for 3xAAA, has a 10Ω series resistor. I think the Chinese makers of this crap decide by trial and error.
 
You'd think someone would do some basic design with these items. Not just trial and error. Anyway it's electronics not physics. :duel:
 
I'm with you on that one, but considering the market there's no real harm in going for what happens to work. What I was saying is that I don't think it would be possible to arrive at "10Ω" purely by calculation.
 
I'd need to see the characteristics of the LEDs, the power supply, the resistance of the wires - and then brush up on my basic electronic theory (I've forgotten most of it) - to find the resistance. Only then could I comment on whether it was trial and error. (Please don't post the details - I was just saying :D )
 
You would need to know the forward voltage (Vf) of the LEDs, to some extent this depends on the LED colour Blue (or White) can have twice the Vf of a Red LED, this can range from 1.6V to 4.4V so obviously some LEDs would survive without current limiting and others wouldn't
 
These are multicoloured strings, and surprisingly the red LEDs are dimmer than the other LEDs. Regardless of being different colours, obviously all the LEDs are running at near enough the same voltage.

The resistance of the wire is negligible, IMO. The power supply is dry cell batteries!

Anyway, as these are intended for a project where I intend to run two strings in series, I hooked them up (in series) to my bench PSU with current and voltage limiting. Sticking 20mA through them requires 5.5V, at which point they are at (I estimate) about 90% brightness. Driving them to 40mA makes very little difference (maybe 95% brightness). Even as little as 1mA (shared between 20 LEDs!) is enough to make them light up a bit.

My project has a 12V AC supply. I'll full-wave rectify it and run the LED strings in series through a 330Ω ½W resistor.
 
Black Hole said:
I've discovered that a 20-LED string, powered by a two AA battery holder with switch, bought from my local discount store for 99p (!), has the LEDs in parallel and no current limiting resistor - relying on the internal resistance of the batteries to bring the operating voltage down to about 2.5V. Fortunately NiMH rechargeables would only kick out 2.4V-ish, otherwise their lack of internal resistance would make them a hazard in this scenario.

A similarly cheap string, this time with a holder for 3xAAA, has a 10Ω series resistor. I think the Chinese makers of this crap decide by trial and error.
Click to expand...
Did they display the CE LOGO?
 
Black Hole said:
These are multicoloured strings, and surprisingly the red LEDs are dimmer than the other LEDs. Regardless of being different colours, obviously all the LEDs are running at near enough the same voltage.
Click to expand...
The red LEDs are dimmer for a reason. Conventional LEDs all have different forward voltages, from 1.8 and 3.3V depending on the colour and process, with red at the low end, blue and UV at the top*. So connecting red, green, yellow and blue LEDs in parallel would end up with just the red LEDs being lit as they have the lowest Vf.

So what's going on? Well they're not ordinary LEDs - they're blue LEDs with a phosphor on the die just like white LEDs, only in this case the phosphors are single colour. I'm now making an informed guess that the red phosphor is the least efficient as it's the furthest colour shift from the blue light that's exciting it.

* It's a standard method for switching between a green and a red LED simply by connecting the red LED across the green.
 
EEPhil said:
Wouldn't have thought so.
Just to add to the confusion there is going to be a UKCA mark as well.
Click to expand...
I had wondered. First I'd heard of this but then I'm no longer in industry. Hope they've registered it as a trade mark - the EU didn't do that for the CE hence it's misuse.
 
Yes, even though the spelling is different, somebody is going to pronounce ukca in the same manner as the yucca plant (minus the y, of course) and cause endless more confusion. Why, you would then ask, is the Universities Central Council on Admissions (UCCA, usually pronounced as above) certifying products. :roflmao: . Too many uccas, I think.
 
Anyway, you should report them as a fire hazard before someone's house burns down.
 
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