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?

  • 1
    \$\begingroup\$ you should put a circuit diagram of what is the actual setup \$\endgroup\$
    – Mitu Raj
    Nov 4, 2019 at 10:33

1 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.

enter image description here

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.

  • \$\begingroup\$ So, If I dont measure using a multimeter, there wont be any voltage drop across LED during its OFF state, right? \$\endgroup\$
    – user220456
    Nov 5, 2019 at 4:55
  • 1
    \$\begingroup\$ Correct. Your post doesn't mention the type of meter. Most digital ones are 10 MΩ. Most analogue meters are 20 kΩ/V so they change with the range selected. See if my article What is an LED? helps. \$\endgroup\$
    – Transistor
    Nov 5, 2019 at 10:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.