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The old fashioned way (without simulator) to do it relatively accurately is to draw a "load line" on the LED curve.

Here is a curve taken from this datasheet, on which I've drawn a straight red line. It's straight because resistors are linear. LEDs are not.

Where the load line crosses the Y axis represents the current through the resistor if the LED voltage was 0V (it would be 9V/1kΩ = 9mA). Where it crosses the 3V vertical line is the current if the LED voltage was 3V (6V/1kΩ = 6mA).

Where the two lines cross is where the actual current and voltage will be. In this case, with this LED, a 1kΩ resistor and a 9.0V supply, it will be about 7.1mA and the LED voltage will be around 1.93V.

As you can see, though, for currents that are 5mA+ the LED voltage will be somewhere between 1.9V and 2V so if we use 1.95 we'll get an answer that's good enough for most purposes in that range. After all, as the battery dies the voltage will change considerably. If you use the fixed voltage to predict the LED current (and therefore the brightness) as the battery voltage drops you'll get an overly pessimistic answer- in fact the LED forward voltage will drop as the battery dies, the current draining the battery will drop, and it will emit a weak light far longer that you'd predict using the fixed voltage approximation. This particular LED is a red LED. Generally shorter wavelength LEDs such as green, yellow and blue have higher forward voltages (2 or 3V typically) at similar currents and longer wavelength types such as the IR LEDs used in remote controls and optocouplers have lower forward voltages (a bit over a volt). Colors like pink and white generally use blue dies and some phosphor so they behave electrically like blue LEDs.

SPICE based simulators have a nonlinear equation (similar to the Shockley equation with some enhancements such as a resistive component) that represents the above curve (potentially quite accurately if the model parameters are good) and they solve this equation numerically, iterating to a precision that is better than the models generally. Different models of LED (even of the same color and chemistry) will have different parameters depending on many factors. Many closed-source circuit simulators (of which I assume Tinkercad circuits is one) are based on SPICE, originally developed as an open-source project at Berkeley in the 1970s- with even earlier military origins.

Here is what a typical set of diode model parameters looks like, for an SMT Kingbright 0603 red LED:

.model APT1608SURCK d(IS=2.01E-17 N=2.139 RS=2 m=0.431 Vj=2.32 Cjo=35pF IBV=10u BV=5 EG=2.26 XTI=3 Iave=30mA Vpk=5 mfg=Kingbright type=LED)

As you can see there are quite a few numbers associated with the LED and nowhere is the Vf explicitly stated. If you really want to go down a rabbit hole, the actual model is explained here.

The old fashioned way (without simulator) to do it relatively accurately is to draw a "load line" on the LED curve.

Here is a curve taken from this datasheet, on which I've drawn a straight red line. It's straight because resistors are linear. LEDs are not.

Where the load line crosses the Y axis represents the current through the resistor if the LED voltage was 0V (it would be 9V/1kΩ = 9mA). Where it crosses the 3V vertical line is the current if the LED voltage was 3V (6V/1kΩ = 6mA).

Where the two lines cross is where the actual current and voltage will be. In this case, with this LED, a 1kΩ resistor and a 9.0V supply, it will be about 7.1mA and the LED voltage will be around 1.93V.

As you can see, though, for currents that are 5mA+ the LED voltage will be somewhere between 1.9V and 2V so if we use 1.95 we'll get an answer that's good enough for most purposes in that range. After all, as the battery dies the voltage will change considerably. If you use the fixed voltage to predict the LED current (and therefore the brightness) as the battery voltage drops you'll get an overly pessimistic answer- in fact the LED forward voltage will drop as the battery dies, the current draining the battery will drop, and it will emit a weak light far longer that you'd predict using the fixed voltage approximation. This particular LED is a red LED. Generally shorter wavelength LEDs such as green, yellow and blue have higher forward voltages (2 or 3V typically) at similar currents and longer wavelength types such as the IR LEDs used in remote controls and optocouplers have lower forward voltages (a bit over a volt). Colors like pink and white generally use blue dies and some phosphor so they behave electrically like blue LEDs.

SPICE based simulators have a nonlinear equation (similar to the Shockley equation with some enhancements such as a resistive component) that represents the above curve (potentially quite accurately if the model parameters are good) and they solve this equation numerically, iterating to a precision that is better than the models generally. Different models of LED (even of the same color and chemistry) will have different parameters depending on many factors. Many closed-source circuit simulators (of which I assume Tinkercad circuits is one) are based on SPICE, originally developed as an open-source project at Berkeley in the 1970s- with even earlier military origins.

Here is what a typical set of diode model parameters looks like, for an SMT Kingbright 0603 red LED:

.model APT1608SURCK d(IS=2.01E-17 N=2.139 RS=2 m=0.431 Vj=2.32 Cjo=35pF IBV=10u BV=5 EG=2.26 XTI=3 Iave=30mA Vpk=5 mfg=Kingbright type=LED)

As you can see there are quite a few numbers associated with the LED and nowhere is the Vf explicitly stated. If you really want to go down a rabbit hole, the actual model is explained here.

Cleaner copy with additional information here, pp49-71, courtesy of @periblepsis.

The old fashioned way (without simulator) to do it relatively accurately is to draw a "load line" on the LED curve.

Here is a curve taken from this datasheet, on which I've drawn a straight red line. It's straight because resistors are linear. LEDs are not.

Where the load line crosses the Y axis represents the current through the resistor if the LED voltage was 0V (it would be 9V/1kΩ = 9mA). Where it crosses the 3V vertical line is the current if the LED voltage was 3V (6V/1kΩ = 6mA).

Where the two lines cross is where the actual current and voltage will be. In this case, with this LED, a 1kΩ resistor and a 9.0V supply, it will be about 7.1mA and the LED voltage will be around 1.93V.

As you can see, though, for currents that are 5mA+ the LED voltage will be somewhere between 1.9V and 2V so if we use 1.95 we'll get an answer that's good enough for most purposes in that range. After all, as the battery dies the voltage will change considerably. If you use the fixed voltage to predict the LED current (and therefore the brightness) as the battery voltage drops you'll get an overly pessimistic answer- in fact the LED forward voltage will drop as the battery dies, the current draining the battery will drop, and it will emit a weak light far longer that you'd predict using the fixed voltage approximation. This particular LED is a red LED. Generally shorter wavelength LEDs such as green, yellow and blue have higher forward voltages (2 or 3V typically) at similar currents and longer wavelength types such as the IR LEDs used in remote controls and optocouplers have lower forward voltages (a bit over a volt). Colors like pink and white generally use blue dies and some phosphor so they behave electrically like blue LEDs.

SPICE based simulators have a nonlinear equation (similar to the Shockley equation with some enhancements such as a resistive component) that represents the above curve (potentially quite accurately if the model parameters are good) and they solve this equation numerically, iterating to a precision that is better than the models generally. Different models of LED (even of the same color and chemistry) will have different parameters depending on many factors. Many closed-source circuit simulators (of which I assume Tinkercad circuits is one) are based on SPICE, originally developed as an open-source project at Berkeley in the 1970s- with even earlier military origins.

Here is what a typical set of diode model parameters looks like, for an SMT Kingbright 0603 LED:

.model APT1608SURCK d(IS=2.01E-17 N=2.139 RS=2 m=0.431 Vj=2.32 Cjo=35pF IBV=10u BV=5 EG=2.26 XTI=3 Iave=30mA Vpk=5 mfg=Kingbright type=LED)

As you can see there are quite a few numbers associated with the LED and nowhere is the Vf explicitly stated. If you really want to go down a rabbit hole, the actual model is explained here.

The old fashioned way (without simulator) to do it relatively accurately is to draw a "load line" on the LED curve.

Here is a curve taken from this datasheet, on which I've drawn a straight red line. It's straight because resistors are linear. LEDs are not.

Where the load line crosses the Y axis represents the current through the resistor if the LED voltage was 0V (it would be 9V/1kΩ = 9mA). Where it crosses the 3V vertical line is the current if the LED voltage was 3V (6V/1kΩ = 6mA).

Where the two lines cross is where the actual current and voltage will be. In this case, with this LED, a 1kΩ resistor and a 9.0V supply, it will be about 7.1mA and the LED voltage will be around 1.93V.

As you can see, though, for currents that are 5mA+ the LED voltage will be somewhere between 1.9V and 2V so if we use 1.95 we'll get an answer that's good enough for most purposes in that range. After all, as the battery dies the voltage will change considerably. If you use the fixed voltage to predict the LED current (and therefore the brightness) as the battery voltage drops you'll get an overly pessimistic answer- in fact the LED forward voltage will drop as the battery dies, the current draining the battery will drop, and it will emit a weak light far longer that you'd predict using the fixed voltage approximation. This particular LED is a red LED. Generally shorter wavelength LEDs such as green, yellow and blue have higher forward voltages (2 or 3V typically) at similar currents and longer wavelength types such as the IR LEDs used in remote controls and optocouplers have lower forward voltages (a bit over a volt). Colors like pink and white generally use blue dies and some phosphor so they behave electrically like blue LEDs.

SPICE based simulators have a nonlinear equation (similar to the Shockley equation with some enhancements such as a resistive component) that represents the above curve (potentially quite accurately if the model parameters are good) and they solve this equation numerically, iterating to a precision that is better than the models generally. Different models of LED (even of the same color and chemistry) will have different parameters depending on many factors. Many closed-source circuit simulators (of which I assume Tinkercad circuits is one) are based on SPICE, originally developed as an open-source project at Berkeley in the 1970s- with even earlier military origins.

Here is what a typical set of diode model parameters looks like, for an SMT Kingbright 0603 red LED:

.model APT1608SURCK d(IS=2.01E-17 N=2.139 RS=2 m=0.431 Vj=2.32 Cjo=35pF IBV=10u BV=5 EG=2.26 XTI=3 Iave=30mA Vpk=5 mfg=Kingbright type=LED)

As you can see there are quite a few numbers associated with the LED and nowhere is the Vf explicitly stated. If you really want to go down a rabbit hole, the actual model is explained here.

The old fashioned way (without simulator) to do it relatively accurately is to draw a "load line" on the LED curve.

Here is a curve taken from this datasheet, on which I've drawn a straight red line. It's straight because resistors are linear. LEDs are not.

Where the load line crosses the Y axis represents the current through the resistor if the LED voltage was 0V (it would be 9V/1kΩ = 9mA). Where it crosses the 3V vertical line is the current if the LED voltage was 3V (6V/1kΩ = 6mA).

Where the two lines cross is where the actual current and voltage will be. In this case, with this LED, a 1kΩ resistor and a 9.0V supply, it will be about 7.1mA and the LED voltage will be around 1.93V.

As you can see, though, for currents in the at least 5mA the LED voltage will be somewhere between 1.9V and 2V so if we use 1.95 we'll get an answer that's good enough for most purposes in that range. After all, as the battery dies the voltage will change considerably. If you use the fixed voltage to predict the LED current (and therefore the brightness) as the battery voltage drops you'll get an overly pessimistic answer- in fact the LED forward voltage will drop as the battery dies, the current draining the battery will drop, and it will emit a weak light far longer that you'd predict using the fixed voltage approximation. This particular LED is a red LED. Generally shorter wavelength LEDs such as green, yellow and blue have higher forward voltages (2 or 3V typically) at similar currents and longer wavelength types such as the IR LEDs used in remote controls and optocouplers have lower forward voltages (a bit over a volt). Colors like pink and white generally use blue dies and some phosphor so they behave electrically like blue LEDs.

SPICE based simulators have a nonlinear equation (similar to the Shockley equation with some enhancements such as a resistive component) that represents the above curve (potentially quite accurately if the model parameters are good) and they solve this equation numerically, iterating to a precision that is better than the models generally. Different models of LED (even of the same color and chemistry) will have different parameters depending on many factors. Many closed-source circuit simulators (of which I assume Tinkercad circuits is one) are based on SPICE, originally developed as an open-source project at Berkeley in the 1970s- with even earlier military origins.

The old fashioned way (without simulator) to do it relatively accurately is to draw a "load line" on the LED curve.

Here is a curve taken from this datasheet, on which I've drawn a straight red line. It's straight because resistors are linear. LEDs are not.

Where the load line crosses the Y axis represents the current through the resistor if the LED voltage was 0V (it would be 9V/1kΩ = 9mA). Where it crosses the 3V vertical line is the current if the LED voltage was 3V (6V/1kΩ = 6mA).

Where the two lines cross is where the actual current and voltage will be. In this case, with this LED, a 1kΩ resistor and a 9.0V supply, it will be about 7.1mA and the LED voltage will be around 1.93V.

As you can see, though, for currents that are 5mA+ the LED voltage will be somewhere between 1.9V and 2V so if we use 1.95 we'll get an answer that's good enough for most purposes in that range. After all, as the battery dies the voltage will change considerably. If you use the fixed voltage to predict the LED current (and therefore the brightness) as the battery voltage drops you'll get an overly pessimistic answer- in fact the LED forward voltage will drop as the battery dies, the current draining the battery will drop, and it will emit a weak light far longer that you'd predict using the fixed voltage approximation. This particular LED is a red LED. Generally shorter wavelength LEDs such as green, yellow and blue have higher forward voltages (2 or 3V typically) at similar currents and longer wavelength types such as the IR LEDs used in remote controls and optocouplers have lower forward voltages (a bit over a volt). Colors like pink and white generally use blue dies and some phosphor so they behave electrically like blue LEDs.

SPICE based simulators have a nonlinear equation (similar to the Shockley equation with some enhancements such as a resistive component) that represents the above curve (potentially quite accurately if the model parameters are good) and they solve this equation numerically, iterating to a precision that is better than the models generally. Different models of LED (even of the same color and chemistry) will have different parameters depending on many factors. Many closed-source circuit simulators (of which I assume Tinkercad circuits is one) are based on SPICE, originally developed as an open-source project at Berkeley in the 1970s- with even earlier military origins.

Here is what a typical set of diode model parameters looks like, for an SMT Kingbright 0603 LED:

.model APT1608SURCK d(IS=2.01E-17 N=2.139 RS=2 m=0.431 Vj=2.32 Cjo=35pF IBV=10u BV=5 EG=2.26 XTI=3 Iave=30mA Vpk=5 mfg=Kingbright type=LED)

As you can see there are quite a few numbers associated with the LED and nowhere is the Vf explicitly stated. If you really want to go down a rabbit hole, the actual model is explained here.