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Just to add a little bit of context to the question, I am using an Arduino Uno and a breadboard.

I have a 5V power source running to the positive (longer) lead of an LED (rated at about 2v and 20mA). I then have a 150 Ohm resistor connecting to the negative (shorter) lead of the LED. The resistor is then connected to ground.

I am using a breadboard, so whenever I remove the resistor and connect it from the power source to the positive (longer) lead of the LED, I still see the same amount of resistance.

In other words, with no resistor, the LED is at X brightness. However, with a resistor attached to either lead of the LED, there is the same amount of resistance (thus, the LED is dimmer, at X÷resistance brightness.)

Based on my very limited knowledge of electronics, I would've assumed that you would have to connect the resistor to the power source, and then to the positive lead of the LED, so that the voltage and current would drop before they got to the LED. This seems to be false, based on my little "experiment" here.

Why is this the case?

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    \$\begingroup\$ Tip: Learn the meaning of the word CIRCUIT \$\endgroup\$
    – Majenko
    Commented Jul 3, 2014 at 18:36
  • \$\begingroup\$ ^^^ LOL, bold and caps. \$\endgroup\$
    – dext0rb
    Commented Jul 3, 2014 at 19:00
  • \$\begingroup\$ You can either lower the voltage before it the LED, or raise the voltage after the LED. Voltage is relative (between two points), so in either case the voltage ACROSS the LED (that is what is important for the LED) is reduced. (Actually it is the current that is reduced, but that is a next chapter.) \$\endgroup\$
    – Wouter van Ooijen
    Commented Jul 3, 2014 at 20:45

2 Answers 2

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It doesn't matter if you put the resistor before or after the LED. In either case the resistor will drop 3V and constrain the current to be 20mA. The LED will drop the other 2V and will light up the same.

See these other links for more information:

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In short, the brightness of the LED is roughly proportional to the current that flows through it. The current is determined by the voltage from your source minus the voltage drop across the LED (constant) divided by the total resistance of the circuit (the resistance of your resistor). The current is what matters for the brightness, and it is the same everywhere in the circuit, and is the same no matter which position the resistor is in. For more information check out the question linked in the first comment.

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