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Antonio Martinez-Torteya's picture



Antonio Martinez-Torteyajoined February 3, 2016

posted Monday, April 18, 2016 | 2:12 pm PDT

I'd love to include LEDs that actually lit up for teaching purposes.

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Great idea!

That's on our list.

may I support that request

 a LED part would be very welcome. More than visual appearance I would appreciate the electric model and the usual symbol. Unfortunately I am not to deep in this topic, but I guess in some cases LEDs are used in circuits for no other purpose as to provide a well defined voltage drop which is differing by colour. Am I right?

New LED Model

We have added a new LED model to our open-source component library, under the "Analog Electronics" category. The model title is "LED with datasheet parameters". As the title implies, it allows the user to specify some parameters that would often be available on a manufacturer's datasheet. For example, you can specify the forward voltage drop V_fwd, and the corresponding forward current I_fwd. The model uses the standard diode equation to have the expected  non-linear I vs. V relationship, but will pass through that particular point on the curve, (I_fwd, V_fwd).

Per the request from Antonio, we have included a parameter to specify the "Lumens_nominal", the light output that is generated at (I_fwd, V_fwd) conditions. Then internally there are two quantities that track the expected light output during operation, "lumens_instantaneous" and "lumens_perceived". The first, lumens_instantaneous, simply provides a scaled value of lumens_nominal, proportional to the instantaneous power i_led * v_led, relative to the nominal power level I_fwd * V_fwd. That may not be exactly correct for every (i, v) operating point, as there can be a color shift or other changes to the light output, per Armin's comment. But near the nominal operating point, the value should be close. The more important quantity is the "lumens_perceived", which is intended to be the "average" light output. This is actually just a low-pass filtered version of "lumens_instantaneous", where the low-pass frequency is set to 15 Hz, approximately the visual bandwidth of the human eye. This value is useful when PWM dimming is used. The LED is actually operating at the nominal (I_fwd, V_fwd) condition, but simply modulated ON and OFF in time to give an average reduced light output. For this condition, the value of "lumens_perceived" should settle to the accurate value of the actual perceived light output.