Novice LED questions

They aren't merely in parallel. They are arranged so if the voltage is applied in one direction, one LED will light up, and if the voltage is applied in the other direction, the other LED will light up. Useful if you want to determine polarity of a DC voltage. Apply an AC voltage, and both will appear to light up simultaneously, but in reality they will be each lighting for half of the time depending on which side of zero volts is applied to the LEDs at a given time.

Michael

Let's say you put four 20ma LED's in parallel, cc-aa, with a resistor to limit the current to

The LED with the lowest forward voltage conducts first, directing all 80ma through it. If/when it blows, the second diode conducts all the current, and so on, in a cascade failure.

That's why each LED (or series string) gets its own limiting resistor.

In an opposite configuration, only one LED can light at one time no matter what polarity of voltage is applied.

The schematic you describe sounds like a two-color LED; one lead goes positive, the red LED lights. Reverse the polarity, the green LED lights. They come in various colors.

I'm not sure what you mean here. If you mean you want to light several of these at once, just hook them in series with a resistor as you would normally. You can then reverse the polarity to light the string the opposite way.

This assumes I'm correct about the devices you're describing...