# FYI - LED Vf in LTspice models

Discussion in 'General Electronics Chat' started by WBahn, Dec 28, 2014.

1. ### WBahn Thread Starter Moderator

Mar 31, 2012
18,085
4,917
So I finally installed LTspice (actually did have a copy installed but had never used it) to help out someone in the Projects forum (How to have capacitor power only one part of parallel circuit?) and they are using a blue LED. So I needed a model for a blue LED but, unfortunately, the included devices only said LED. So my next step was to use one that had a forward voltage of about 3.2V, but that information wasn't listed, either. In their selection table they list only the breakdown voltage and the average current.

So what to do?

In their models they have Is and N. From these we can get Vf at a particular Vd very easily according to the relationship:

$
I_f \; = \; I_s \cdot e^{ $$\frac{V_f}{N V_t}$$}
$

Solving for Vf we get

$
V_f \; = \; N V_t \cdot \ln $$\frac{I_f}{I_s}$$
$

I used this to set up a little spreadsheet where I have sorted the list of stock LEDs based on the forward voltage at the specified average current and also added a column for the forward voltage at a specific current contained in another cell. The table is sorted in order of descending Vf@Iave. The following table uses If=20mA.

Part # Mfg Is (A) N Iave (A) Vf@Iave (V) Vd@If (V)
PT-121-B Luminous 4.35E-07 8.37 20.000 3.84 2.34
NSCW100 Nichia 1.69E-08 9.626 0.030 3.60 3.50
NSPW500BS Nichia 2.70E-10 6.79 0.030 3.27 3.20
LUW-W5AP OSRAM 6.57E-08 7.267 2.000 3.26 2.39
AOT-2015 AOT 5.96E-10 6.222 0.180 3.16 2.80
W5AP-LZMZ-5K Lumileds 3.50E-17 3.12 2.000 3.13 2.76
LXK2-PW14 Lumileds 3.50E-17 3.12 1.600 3.11 2.76
NSSWS108T Nichia 1.13E-18 3.02 0.040 2.99 2.94
LXHL-BW02 Lumileds 4.50E-20 2.6 0.400 2.95 2.75
NSSW008CT-P Nichia 2.30E-16 3.43 0.040 2.92 2.86
QTLP690C Fairchild 1.00E-22 1.5 0.160 1.90 1.82

Note that this doesn't take into account some of the other model parameters such as Rs, but it gives a good starting point.

Hopefully this will be helpful to someone.

Sinus23 and bwilliams60 like this.
2. ### eetech00 Active Member

Jun 8, 2013
676
116
Hi

If you have a part number I could model it for you..

eT

3. ### WBahn Thread Starter Moderator

Mar 31, 2012
18,085
4,917
The OP in that thread didn't supply a part number and it wasn't a big enough deal for me to worry about it. As long as I got the voltage at the operating current close to what most blue LEDs develop then that should be good enough.

The purpose of this post is that lots of people here use LTspice and many of them like to use the stock parts when possible. So this should make choosing a suitable stock part easier.

4. ### eetech00 Active Member

Jun 8, 2013
676
116
Oh..Ok...I understand now. So the last column is Vfwd@20mA...that's useful.