2 replaced http://electronics.stackexchange.com/ with https://electronics.stackexchange.com/ edited Apr 13 '17 at 12:33 It's a valid assumption because it's close enough to true that the difference is negligible in many cases. See for example this graph from LTL-307EE: Notice that the voltage axis starts at $$\1.2V\$$, and over the typical operating range of the diode, the voltage only varies about $$\0.6V\$$. This is mostly due to the internal resistance of the diodemostly due to the internal resistance of the diode, which in this case is about $$\13\Omega\$$. If you are going to put this diode in series with a $$\4.7k\Omega \pm 1\%\$$ resistor, then the $$\13\Omega\$$ resistance of the LED is quite insignificant compared to resistor you've added, which might deviate $$\\pm47\Omega\$$ from the nominal value of $$\4700\Omega\$$. Put another way, that $$\13\Omega\$$ of resistance from the LED represents a $$\0.28\%\$$ error in your calculations. It's a valid assumption because it's close enough to true that the difference is negligible in many cases. See for example this graph from LTL-307EE: Notice that the voltage axis starts at $$\1.2V\$$, and over the typical operating range of the diode, the voltage only varies about $$\0.6V\$$. This is mostly due to the internal resistance of the diode, which in this case is about $$\13\Omega\$$. If you are going to put this diode in series with a $$\4.7k\Omega \pm 1\%\$$ resistor, then the $$\13\Omega\$$ resistance of the LED is quite insignificant compared to resistor you've added, which might deviate $$\\pm47\Omega\$$ from the nominal value of $$\4700\Omega\$$. Put another way, that $$\13\Omega\$$ of resistance from the LED represents a $$\0.28\%\$$ error in your calculations. It's a valid assumption because it's close enough to true that the difference is negligible in many cases. See for example this graph from LTL-307EE: Notice that the voltage axis starts at $$\1.2V\$$, and over the typical operating range of the diode, the voltage only varies about $$\0.6V\$$. This is mostly due to the internal resistance of the diode, which in this case is about $$\13\Omega\$$. If you are going to put this diode in series with a $$\4.7k\Omega \pm 1\%\$$ resistor, then the $$\13\Omega\$$ resistance of the LED is quite insignificant compared to resistor you've added, which might deviate $$\\pm47\Omega\$$ from the nominal value of $$\4700\Omega\$$. Put another way, that $$\13\Omega\$$ of resistance from the LED represents a $$\0.28\%\$$ error in your calculations. 1 answered Jul 30 '13 at 13:18 Phil Frost 46.6k1414 gold badges116116 silver badges229229 bronze badges It's a valid assumption because it's close enough to true that the difference is negligible in many cases. See for example this graph from LTL-307EE: Notice that the voltage axis starts at $$\1.2V\$$, and over the typical operating range of the diode, the voltage only varies about $$\0.6V\$$. This is mostly due to the internal resistance of the diode, which in this case is about $$\13\Omega\$$. If you are going to put this diode in series with a $$\4.7k\Omega \pm 1\%\$$ resistor, then the $$\13\Omega\$$ resistance of the LED is quite insignificant compared to resistor you've added, which might deviate $$\\pm47\Omega\$$ from the nominal value of $$\4700\Omega\$$. Put another way, that $$\13\Omega\$$ of resistance from the LED represents a $$\0.28\%\$$ error in your calculations.