A 2011-08-07 watsonseblog post from the Way Back Machine: (the second resistor is a 1 ohm resistor in series with the LED for measuring the LED current.)
2011-08-07
2011 Aug 07 Dual LED Odd Colors
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When a white LED is connected to a power supply or 6V battery, in series with the required current limiting resistor, it will light up and the forward voltage across the LED will be about 3 volts. Then if I connect a red LED in parallel with the white LED, the red LED will light up and the white LED will be dark. The reason for this is that the red LED has a forward voltage of about 2 volts, and that is much less than the white LED, so all of the current will go through the red LED.
Yet in the picture, I show a red LED and white LED connected in parallel, and both are lit! How could this be? Am I defying the laws of physics? It certainly seems so.
First off, let me say that this is just a conventional Joule Thief with the coil on the left, the transistor which is hidden behind the yellow clamp jaws, and the resistors. The red LED leads are inserted between the white LED leads and are just touching them. The red and white LEDs really are both lit, I didn’t make any mods to the picture. Anyone can do the same thing by putting a red LED in parallel with a white LED in a Joule Thief. The current through the white LED alone was measured at 18 milliamps (9 millivolts across a 1/2 ohm resistor). With the red LED in parallel, the current went up to 40 milliamps.
The white LED dimmed a lot, which is understandable since the red LED has lower forward voltage. The reason the white LED did not go out is that during the transistor’s off time, the peak current from the coil into the red LED is very high and the voltage across the internal resistance of the red LED rises to more than 3 volts, which is enough to allow some of the current from the coil to go through the white LED. Thus both LEDs light up, however the white LED is getting a fraction of the total current and the red LED is hogging most of the current.
But the seemingly impossible still happens: the red and white LEDs both light up. The secret is in the high current pulses from the JT’s coil.