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Out of pure curiosity, I purchased a LED string powered by 2x AA cells. There are 20 LEDs, all wired in parallel and in the sequence Blue, Green, Yellow, Red,- repeated 5 times. There are no other visible electronic items such as dropper resistors, and certainly no inverter of any kind.

When connected to a variable supply the string has this characteristic:

enter image description here

The slope of the curve suggests an equivalent series resistance of 1.5 Ohms, or 30 ohms if each LED has its own series resistor. The LEDs all shine with roughly the same brightness.

I'm just curious to know why this works, given that red and blue LEDs normally have quite different forward conduction characteristics?

The ESR of your average cheap AA cell probably provides sufficient current limiting but wouldn't a red LED win the current battle if directly paralled across a blue LED (which I thought required a good deal more than 3v, anyway)?

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  • \$\begingroup\$ You can get blue LEDs made with InGaN that light up as low as 2.7 V \$\endgroup\$
    – Finbarr
    Mar 9 at 15:57
  • \$\begingroup\$ also: LEDs are less constant-voltage than a lot of people will say, though it's still a good approximation. \$\endgroup\$
    – user253751
    Mar 9 at 15:59

1 Answer 1

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There are LEDs with internal resistors available, if you take a look into the LED-housing (without current flow), then you will see in the middle the LED-chip and on the side a little black dot, this is the resistor. They are made for a specific voltage, 3.3V or 5V or 12V. The resistor values do match to this voltages to generate a defined current flow.

Make a picture of the LED while there is no current flow and use macro-modus to get a picture of the chip clear, sharp and in focus.

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  • \$\begingroup\$ Thanks, I will try to examine the LED chips. I couldn't see how this could work with a blue LED supplied with 3.0v however, as I thought their Vf was closer to 4v and above. Research has proved me wrong. this chart of Vf vs colour is quite useful \$\endgroup\$
    – rossmcm
    Mar 9 at 20:05
  • \$\begingroup\$ Blue (and white) LEDs start to glow at 2.4V and the maximum output power is close to 2.9V or 3.1V, but we do not use the maximum under normal circumstances, so it is more close to 2.8V what is needed. - - - I have here some very old white LED from Osram, they need around 4 Volt. I have here modern (but very cheap) green 0603 SMD LEDs, the light is visible if I=380nA, 19µA (2.12V) good visible as status LED, 109µA (2.28V) very bright status LED, 218µA - extreme bright. \$\endgroup\$
    – MikroPower
    Mar 9 at 21:43

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