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Topic: Voltage drop on the LED (Read 378 times) previous topic - next topic

leonhard_euler

Hello,

Im new to this forum and quite new to electronics. Hope Im writing this post in correct section.

I know some basics like Ohms law etc. Im going through the 15 projects from arduino UNO book. Before I plug things into Arduino Im testing my circuits with TinkerCad.

Lets say I have a serial circuit powered by 5V pin (Arduino UNO). In the first project the 220 ohms resistor is chosen (would like to see calculation of why is it 220). Ohms law says that current in the circuit will equal to around 20mA. So far so good. According to the book thats around maximum that LED can handle. I understand that values are taken from some kind of LEDs datasheet (or whatever it is called :)). When I construct above circuit with TinkedCad I get a 2.08V voltage drop on the LED. What I dont understand is how TinkedCad knows exactly what is a voltage drop and why while increasing resistance a voltage drop on the LED decreases?

Additionally current in above circuit with LED is 13.5mA according to TinckerCad... Does a LED decreases the current in the circuit somehow?

Appreciate any attempt of explaining it to me :)
Cheers

vaj4088

I don't know anything about TinkerCad or why its calculations are right or wrong.

The voltage drop of an LED depends upon the current through it, but does not vary too much near the current needed for continuous light.  This voltage drop and current will be on the data sheet.

The current through the resistor is given by Ohm' s law, where the voltage is your supply voltage (in this case 5.0 volts) minus the forward voltage drop of the LED, and obviously the resistance is the value of the resistor.  This of course assumes that the resulting calculated current is "near" the current in the data sheet for the LED.  In this case, if the data sheet uses 20 mA and your calculation results in 15 mA, that is near enough for many purposes.


Grumpy_Mike

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What I dont understand is how TinkedCad knows exactly what is a voltage drop
All simulators has mathematical models to predict the behavior of components under different conditions. Any model can not hope to simulate all sorts of LEDs so a "generic" LED model is used. This is not the same as a real life LED and so the changes you see in the forward voltage drop across an LED when it responds to current changes are only what the model will predict. In real life these will almost certainly be different.

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and why while increasing resistance a voltage drop on the LED decreases?
Because that is how real LEDs behave. The forward voltage drop changes with current, but not in a linear way like it does in a resistor. Large changes in current will produce small changes in the LED's forward voltage drop.

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Does a LED decreases the current in the circuit somehow?
Yes.
By the fact that it has a forward voltage drop that is dependent on current means that as that drop changes so does the current for any given value of fixed resistor.

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