Determining Forward Voltage of LED's





Introduction: Determining Forward Voltage of LED's

In this Instructable, I will show you how to measure the forward voltages of LED's if it isn't provided. I recently ordered 200 pieces of assorted 20 mA LED's but there was no information on the forward voltages. This may also work for higher power LED's too but you have to be really quick.

Step 1: What You Will Need

LED's to test
Forward current of the LED's
Voltage source higher than the LED's forward voltage
Multimeter (having two would be very useful if you are going to test many LED's)
Alligator clips or anything to hold on to the test leads
Solid wires (or leads from burned out components)
500 or 1000 ohm variable resistor

Step 2: Measuring the Forward Voltage

To measure the forward voltage, set the multimeters to their proper settings (ie. current and voltage). Always set the resistance to the highest value before testing it to avoid frying the LED. It may be easier to clamp the multimeter leads by inserting solid core wires to the breadboard. Lower the resistance until the current is up to 20 mA and record the voltage and current.

The forward current of the blue LED was 3.356 V at 19.5 mA. If you are powering it with 3.6 V, the resistor value to use is the next higher value for R=(3.6V-3.356V)/0.0195A)=12.5 ohm.

Measuring high power LED's with >350 mA forward current can be a bit tricky because when they heat up quickly, their forward voltage drops continuously. This means the current will be higher at a given voltage. To measure high power LED's, follow the same procedure and set the current. Quickly hold the value on both multimeters. It if's too late, you would have to let the LED's cool down to room temperature before taking the measurement again. For this reason, current limiting resistors are not used to power them. They must have a constant current source.

Don't know the forward current?
If the LED is part of an indicator light of a device, you can desolder one of the LED's lead and measure its current. This may not be necessary because you can measure its forward voltage across the leads while it is on.

Increase precision of current adjustment
I was unable to set it to 20 mA because the variable resistor had a large range of 50k ohm. You may use 500 ohm or 1k ohm. To allow coarse and fine adjustments, you may connect a higher and lower range variable resistor in series.



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    You don't need to be quick testing high powered LEDs and should NOT try to be quick!

    The relevant forward voltage is the one it actually has in your implemented design, based on the actual temperature it will be in that design. As such, you want it on the same heatsink, same everything within reason, and to let it heat up to the maximum temperature it is expected to see.

    This is the only way the data can be valid in a real use, or of course if you did have the datasheet and it had a graph for that, well then none of this was necessary but if there is no graph you still need to do that kind of test to verify your design falls within the parameters you want.

    Perhaps it is time we started trying to have frank discussions with LED manufacturers about a more sane baseline temperature than 25C, since it is practically never that you would have a design putting full rated current through an LED and have it stay at 25C die temperature. IMO, 70C would be a better value to start the discussion with, especially for the purposes of lifespan rating and lumen derating.

    What does the "diode test feature" icon on the meters look like?

    Because, I have a meter and would like to use it instead.


    It probably has a diode symbol and it's usually one of the resistance settings.

    diode symbol.png

    What does the "diode test feature" icon on the meters look like?

    Because, I have a meter and would like to use it instead.


    Looks like the symbol being pointed to on the DMM in this picture. It looks like the normal diode symbol.

    I notice the two meters you have there have a diode test feature, which displays the forward voltage on the screen automatically, is there a reason why this "manual" way is more effective or are you just demonstrating so that anybody with any meter can find the forward voltage of a diode?

    What does the "diode test feature" icon on the meters look like?

    Because, I have a meter and would like to use it instead.


    The diode test function does not work for LED's although they glowed dimly. The reading was 1.7 for the red LED. For the diode test, the auto ranging multimeter puts out 0.9 mA @1.6 V and the manual ranging meter puts out 1.2 mA @2.7 V.

    I see. I have found reasonable results using them. They can't deliver the current for high brightness, but it gives you a good idea of forward voltage. From there you can select a resistor for 10mA @ X volts, then judge the brightness and find a more suitable resistance. LEDs are pretty forgiving, for a short time at least.