Most standard Red, Yellow, Green LEDs operate at: approximately 1.8V to 2.2V and most Blue, White LEDs: Approximately 3.0V to 3.3V.
The typical Forward Current (marked/symbolized as: IF)/operating current for standard LEDs is 20 milliamperes (mA). Exceeding this current can reduce the LED’s lifespan or cause immediate failure.
Wrong/Reverse Polarity can cause No illumination/The LED will not emit light when reverse-biased. Also, LEDs have a maximum reverse voltage rating, typically around 5V. Exceeding this reverse voltage can cause breakdown and permanent damage to the LED.
Normally, The longer lead is the anode (positive), and the shorter lead is the cathode (negative). Some LEDs have a flat edge on the housing, indicating the cathode (negative) side. Internal Structure: Observing the internal components, the larger electrode usually connects to the cathode (negative).
But my led, when tested with diode mode of MM, showed 1.77v (forward voltage flow) when red probe touched shorter leg (which was shorter in the inside the led also), and black probe touched anode (positive). If it showed higher forward voltage or even OverLoad-OL, it could mean that the led is faulty, or there is some extra resistance down the line (in case its is AC led, in this case you can check for resistance on each of its sleeved legs separately), or you’ve connected multimeter in wrong mode/reverse probes/polarity. Also nothing wrong will happen to a good led by this wrong testing.
Also, with MM’s resistance mode, neither the good one dc led or the ac led (good/bad don’t know yet) showed any continuity or current passing thru them.
The big Ammeter’s continuity mode also lit the small led.