I'm new to electronics and I'm learning the basics for a small home project involving LEDs, fiber optics, and microcontrollers.

I'd like to simulate the circuit I plan to build to get a better understanding of the different V drops and I values in each node as I change the structure of the circuit. I've experimented with various free tools and so far 5Spice (Win, desktop) and Circuitlab.com proved to be the best ones for me.

However, I'm stuck when trying to simulate the simple 5mm 24mA LEDs I bought (manufacturer link here). Both tools support adding LEDs, but I need to set values such as N, TT, M_J, I_S, R_S, C_J0, and V_J.

Even after reading the basics of Spice and reading this other StackExchange question, I'm not sure what values I can use, and the procedures described there are beyond my current (basic) understanding, at least for the time being. Can anybody help me providing these values?

My power source will be a constant 4.5V and the LEDs Voltage is in the range 1.9v-3.2v (depending on the color) What values should I use for a basic simulation? I can't find the datasheet in the manufacturer site.

Thanks,

  • Depending on what properties you need to simulate, a voltage source in series with a resistor can be a suitable model for an LED, and doesn't require so much data. I suspect if you need to simulate more complex behaviors of an LED, then you aren't going to get very far without a datasheet. – Phil Frost Apr 17 '13 at 17:35
  • @Phil: you mean putting a resistor instead of a LED? what resistor value should I use for these LEDs? will that make it possible to still measure I an V flows/drops? Thanks – Sebastian Apr 17 '13 at 20:14
  • You need the datasheet to create the model but for what you're doing that is overkill. Also, if you buy assorted anything you probably won't get a datasheet. Use a vendor that has datasheets or as @PhilFrost has suggested. FYI if you can find a curve in a datasheet, there are tools (like in Orcad Pspice) that will curve fit and output a spice model for you. – Analog Arsonist Apr 18 '13 at 22:37
up vote 2 down vote accepted

An LED typically has a voltage/current relationship like this:

graph of current vs voltage

A resistor's voltage/current relationship is a straight line, passing through the origin, with the slope defined by the resistance (from Ohm's law: \$ I = \frac{E}{R} \$).

The LED's curve is not unlike a straight line, once you get above 1.8V or so. So, at least for the LED's operating region between "current starting to flow" and "overheating", an LED is not unlike a resistor, with the voltage pushed to the right.

What's the value of this imagined resistor? Well, in this graph it looks like the curve goes through \$(2V, 5mA)\$ and \$(2.2V, 20mA)\$. If you rearrange Ohm's law, you can see that an Ohm is a volt-per-amp:

\$ R = \dfrac{E}{I} \$

We will deal with shifting this curve to the right later, so for now we can consider only the change in voltage and current between the two points we picked:

\$ R = \dfrac{\Delta E}{\Delta I} \$

\$ R = \dfrac{2.2V - 2V}{20mA - 5mA} \$

\$ R = \dfrac{0.2V}{15mA} \approx 13 \Omega \$

So that gives us the straight line. How do we shift it to the right? Simple: all simulation packages have a voltage source: a component that has a voltage you define across it under all conditions. To get the value, we can simply look at our graph and extend the line to see where it would intersect the Y axis (\$0A\$). Looks like about 1.8V. So here's our simple LED model, a voltage source and LED in series:

LED model schematic

This model breaks down if the current becomes very small. A real LED turns off, and blocks current like a diode, but in this model you start getting a reverse current, like a resistor would do. You can improve this by adding an ideal diode in series, if you like.

There are finer details that this also doesn't simulate: it doesn't simulate that smooth "knee" where the LED just starts to turn on. It doesn't simulate reverse leakage current, or temperature effects, or photocurrent. However, most of this is not significant in practical LED circuits.

On that practical note: there is a simpler model most engineers use day-to-day: just a voltage source. For this LED, if you just assume the voltage across it whenever it's on will be \$2V\$ whenever its on, you are probably close enough. If you are going to include a \$1k\Omega\$ current-limiting resistor in series, then the \$13\Omega\$ from the LED hardly matters. If the current gets high enough to deviate significantly from that \$2V\$ estimate, the LED is probably destroyed.

  • thank you for your time and for helping me, the quality of your answer is remarkable! – Sebastian Apr 26 '13 at 21:05

If your circuit simulator support importing a SPICE model, you can use the models provided below for different colors of LEDs.

*Typ RED GaAs LED: Vf=1.7V Vr=4V If=40mA trr=3uS
.MODEL LED1 D (IS=93.2P RS=42M N=3.73 BV=4 IBV=10U
+ CJO=2.97P VJ=.75 M=.333 TT=4.32U)

*Typ RED,GREEN,YELLOW,AMBER GaAs LED: Vf=2.1V Vr=4V If=40mA trr=3uS
.MODEL LED2 D (IS=93.1P RS=42M N=4.61 BV=4 IBV=10U
+ CJO=2.97P VJ=.75 M=.333 TT=4.32U)

*Typ BLUE SiC LED: Vf=3.4V Vr=5V If=40mA trr=3uS
.MODEL LED3 D (IS=93.1P RS=42M N=7.47 BV=5 IBV=30U
+ CJO=2.97P VJ=.75 M=.333 TT=4.32U) 

NOTE: There are three models available here since different LED colors have different characteristics. The lines starting with * are comments.

The following simulators support SPICE model importing to my knowledge.

And since in your case the software has an interface to fill in the parameters just manually fill in the blanks using the models I mentioned. e.g. I_S = 93.1

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