# Measuring the Forward Voltage Drop Of LED

We Can Measure the Forward Voltage Drop of Normal Diode. If I take A multimeter and put it on Diode Mode and connect the positive terminal of diode to the red probe and negative terminal of led to the black probe it is Showing 0.5.

But When I do the same for LED it is Showing OL. Is there any way I can measure the Forward voltage drop of LED with out Connecting it in a Circuit?

An LED has the same junction voltage behavior as a normal diode but it's value is more in the range of 1V5 and above. That voltage is too high for your meter, resulting in the OL message, over load. You can measure the voltage by connecting the LED to a 5V source with a 1K resistor in series. This will give you about 3mA of current. That's enough to create the diode junction effect. Now you can measure the voltage across the LED.

It depends on the construction of your meter and how much voltage it puts out for resistance measurements. It also depends what polarity it puts out in resistance mode.

My old analogue AVO meter puts out positive on the black lead and negative on the red lead during resistance measurements. This in the opposite of modern DVMs.

With my DVM I can see a very faint glow on the LEDs when I measure them.

I have several DVMs and just tested my el cheapo Harbor Freight meter. It measures normal, 1N4004, and Schottky diodes but shows only the number '1' for a red LED. The meter chip logic did not anticipate anything over approximately 1V0 or common diodes.

When this was previously discussed I was surprised to find that the open circuit voltage of my (digital) multimeter on “diode test” was nowhere near the 9V of the battery, but limited to (just checked it again) 3.07 volts on the “UNI-T” and 3.77 on the Fluke.

In general the meter will be using a 2V range and will show “OL” on other than red and IR LEDs, but most, even white and blue will just glow faintly on this limit.