I'm wondering if it's possible to calculate the forward voltage of some LEDs I have, given that I have their other specifications:

Base type Aluminum Power consumption 1W Voltage 12V <- I believe this is the power supply this calls for, not a forward voltage. Lumens 11.7-84.2 lumens Beam angle 25 degree narrow spot beam Color temperature 3300-6500K LED working temperature -20 to 60 degree (around 54 degree) IP Class IP68 Lifespan 50,000+ hours

Would anyone be able to provide some advice for the calculation I'm looking to make? Thanks!

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    \$\begingroup\$ Do you have the datasheet? The forward voltage should be there. \$\endgroup\$ – Gustavo Litovsky Mar 7 '13 at 0:51

From the specifications, the LED in question is an integrated module, not a bare LED. Therefore, the Forward Voltage of the LED is not really relevant from a design perspective: Integrated current limiting circuitry, i.e. a resistor, would dissipate the difference in voltage between the actual LED junction's Vf, and the supply.

However, if this is an exercise in academia...

A diode characterizing device, or even an oscilloscope in X-Y mode plus a high-current signal generator, could be used to identify the approximate forward knee of the device's current curve. While a bare LED die would have a sharp knee, even a resistor-integrated part will show a somewhat representative curve, such as this:

LED current curve.

Of course, if you have a few sacrificial pieces for such characterization, a more definitive range of Vf values could be approximated.

Do note that LEDs have minor tolerance variations in Vf even between parts from the same batch, and adding a current limiting resistor increases such variation.

Also, the current graph in such cases will definitely not be sharp - at best, a mild knee will be observed, broadly indicative of the underlying LED junction knee.

The specific junction chemistry determines the characteristic band-gap and light emission threshold, so if more were known of the LED in question, this would help.


If the part is specified as "12 volt" then I would assume that it includes current limiting, and that you won't have access to the bare LEDs, so don't need to know the LED forward voltage.

Even for a bare LED, it is not possible to calculate the forward voltage from the information you gave. The forward voltage of an LED depends on its colour and chemistry - for a bare LED it will be shown on the datasheet.


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