Voltage during LED ON and OFF

Question

I have an LED Controlled by a signal to its cathode. If I make the cathode Low, LED Glows, and if cathode is High, LED is OFF.

The voltage applied at LED Anode is always 12.5V.

I observed that, when I turn ON the LED,

Voltage at LED Cathode = 0V

Voltage at LED Anode = 2.1V

LED Glows

And, If the voltage at the cathode is made high,

Voltage at Anode = 12.5V

Voltage at Cathode = 11.2V

LED Doesnot glow.

My question is, in my second case, when the LED Doesnot glow, why is there a voltage difference between the LED Cathode and Anode?

I understand that in the second case, the voltage difference between the cathode and anode is not greater than the forward voltage of the diode which is why it is not glowing. But when it is not glowing, why is there a voltage difference between the anode and cathode?

Answer 1

But when it is not glowing, why is there a voltage difference between the anode and cathode?

This reminds me of "If a tree falls in a forest and no one is around to hear it, does it make a sound?" In your case it would be re-written as "if noone is measuring the voltage will there be a voltage drop?"

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. The equivalent circuit.

Your meter has an impedance and will pass some current while taking the measurement. This current flow will cause a voltage drop.

Figure 2. I-V curves for typical LEDs. Source: LEDnique.

Your measurements indicate a 1.3 V drop. You can see from Figure 2 that all of the visible LEDs will pass tiny currents at that voltage.