I tested two parallel LED circuits. In the first circuit, only the red LED glows. In the second circuit, both of the blue LEDs glow.
I know the forward voltage rose in the second circuit, but why did this happen?
Why does the forward voltage just fit the red LED (low voltage) not the blue one (higher voltage) in the first circuit?
The loads are are in parallel so there will be same voltage over both LEDs.
The red LED will turn on at much less than 2V or so and it starts to conduct current and there will be much less than 2V over both LEDs.
The blue LED requires much more than 2V to start conducting current so it will not light up as it only has much less than 2V.
The red led acts as a shunt regulator. By putting various voltage shunts in parallel, the one with the lowest voltage "wins" and doesn't allow the voltage to increase. That's what a shunt regulator is, by definition: it limits the voltage by drawing current as the voltage goes up too high.
If you want to learn more about this concept, search using the term shunt regulator.
Now, LEDs may be crude shunt regulators, but consider that the circuit you've shown allows to use a SPST pushbutton as if it was an SPDT button. Shown below, when the button is depressed, the blue led turns off, and the red led turns on. The leds alternate, even though just a simple button is used.
simulate this circuit – Schematic created using CircuitLab
After opening the schematic in CircuitLab, click on SW1 to select it, and use Space to toggle the switch.
