decorative-lighting LEDs - internal ballast R???

The shorter wavelength LEDs have a higher voltage drop (due to a higher bandgap or something...). So IR leds have only ~1.2V, red leds

I doubt they have any resistor. Imagine the expense! They must simply have a V/I curve such that they can in fact share current, well enough for the job.

Or perhaps they have a few hundred Chinese sorting them into voltage bins :)

OK, I had it in my head that blue LEDs have about 2.5V forward drop, but in fact a couple of data sheets I looked at just now indicate closer to 3.6V. Must remember not to rely on myth.

That thought had indeed crossed my cynical mind. Given the humungus quantities involved, they probably beat up their suppliers to do it for them.

Thanks

Happy New Year, you old fart!

Those tiny LED chips have a lot of internal resistance. You can too connect them in parallel, though it's not generally the best, for Xmas decorations cheap is best.

Check out a typical blue LED:

Eyeballing the graph (after a couple of single-malts, so do your own checking if you want accuracy), I get 3.05V at 10mA and 3.3V at 15mA. Your nominal is 12mA, so they're like a 3.18V perfect diode with approximately 50 ohms in series at 12.5mA +/- 2.5mA. That's just the way things work out, m'kay?

If fact LEDs from a given batch will match closely, and they don't likely do much more than slap them together and check they light up. The 50R is plenty, especially given the log response of human eyes. BTW, you get a LOT of LEDs from one wafer.

Best regards, Spehro Pefhany

...

There are several different types of cheap hand torches on sale in my local hardware shop, I have bought three different types so far - and none of them even bothers with a series current-limiting resistor.

One of them has two groups of 12 white LEDs paralleled straight across three AAA cells in series, with only the voltage drop of a saturated control transistor in series with each group. The total current with new alkaline batteries was over an amp, until I fitted a couple of limiting resistors.

The most amazing thing is the price: You get 24 LEDs, a push-button switch, a controller, 2 small switching transistors, a PC board, a reflector, a lens, two plastic housings with battery contacts, a hanging hook and a fairly powerful button magnet for attaching it to metal surfaces. All this plus transport from the Far East for less than 4 UKP

I have just adapted the body of a tiny conventional pocket torch, which uses two AA cells,a mechanical switch and originally had a pre-focus tungsten bulb. It now has a parallel pair of LEDs (salvaged from one of these modern torches) which draws 40 mA in total. It gives as much light as the original tungsten arrangement with a very much longer battery life. It doesn't seem to need a series resistor when only two cells are used.

The mechanical switch has no volt-drop, is not damp-sensitive and is more difficult to turn on by accident. To my mind this is a much more sensible arrangement than push buttons and electronic controllers

...

There are several different types of cheap hand torches on sale in my local hardware shop, I have bought three different types so far - and none of them even bothers with a series current-limiting resistor.

One of them has two groups of 12 white LEDs paralleled straight across three AAA cells in series, with only the voltage drop of a saturated control transistor in series with each group. The total current with new alkaline batteries was over half an amp, until I fitted a couple of limiting resistors.

The most amazing thing is the price: You get 24 LEDs, a push-button switch, a controller, 2 small switching transistors, a PC board, a reflector, a lens, two plastic housings with battery contacts, a hanging hook and a fairly powerful button magnet for attaching it to metal surfaces. All this plus transport from the Far East for less than 4 UKP

I have just adapted the body of a tiny conventional pocket torch, which uses two AA cells,a mechanical switch and originally had a pre-focus tungsten bulb. It now has a parallel pair of LEDs (salvaged from one of these modern torches) which draws 40 mA in total. It gives as much light as the original tungsten arrangement with a very much longer battery life. It doesn't seem to need a series resistor when only two cells are used.

The mechanical switch has no volt-drop, is not damp-sensitive and is more difficult to turn on by accident. To my mind this is a much more sensible arrangement than push buttons and electronic controllers

Correction--- 3.18V is the voltage across the ideal diode plus