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I have a bunch of red LEDs, I don't have their data sheet and I am trying to figure out a safe resistor to use to lower the current across them safely.

My current setup is as follows:

schematic

simulate this circuit – Schematic created using CircuitLab

I figured a 100 Ohm resistor and 68 ohm resistor in series because, assuming a forward voltage of the led of 3 volts, and a desired current of 20mA (which I assume is ok?) I get:

R=V/I
R=3.3/0.02
R=165 Ohms.

So, I pop that into the circuit lab as shown below, click run, then click DC simulation, but then it just throws an error saying : "unable to get solution"

How do I use this tool to allow me to inspect various values at various points in the circuit? Have I got this wrong somehow?!

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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Add a GND to the schematic so the simulator has a zero volt reference.

enter image description here

Figure 2. Add nodes at the points you want to monitor. Rename them to suit your purposes. Run the DC solver.

enter image description here

Figure 3. Click each named node to add it to the DC solver tab.

enter image description here

Figure 4. Click on a wire to monitor the current.

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  • \$\begingroup\$ Note with Vled=2.1V@25mA with 3 LEDs in series =6.3V (for this LED model values) , no series R is needed. and with 2.1-1.8= 0.3V /25mA the ESR is 12 Ohms within the xx% tolerance of my model of 15 Ohms which depends on supplier binning and quality. Thanks @transistor for validating my experience and showing the details of CircuitLab \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 4 '16 at 23:45
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If 6.3V is stable, you can use AlInGaAs RED LED x3 in series with NO resistors safely.

Then use as many 3S strings in parallel as your regulated PS can support.

If Vout is 6.5 or more then add 1Ohm per 100mV per 100mA of LED current. imagine each 5mm Red LED as a 1.8V diode with 15 Ohms inside, give or take xx%

  • edit (typo , 1.8 not 2.8)
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  • \$\begingroup\$ Tony, I think that's an answer to another question somewhere in cyberspace. Read the title and his final paragraph again. \$\endgroup\$ – Transistor Oct 4 '16 at 21:08
  • \$\begingroup\$ you are probably right but then sometimes the best answer is a better solution not asked for. I gave a better model too for Ohmslaw manual trivial calculations \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 4 '16 at 21:12
  • \$\begingroup\$ My red LEDs are around 1.8 volts, and common LEDs have a maximum current rating of 20 mA or so - I usually aim for about 10 mA for safety. \$\endgroup\$ – Peter Bennett Oct 4 '16 at 21:24
  • \$\begingroup\$ 1.8 is good and indicates high efficacy and lower ESR than typ. actual Vth is less than 1.8 (which is an asymptotic value only. ) 20mA is still safe unless you have an unregulated supply, then 10mA is wise \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 4 '16 at 21:34

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