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Note: I am very new to the more advanced side of electricity. I do not have a firm grasp on the exact mechanics.

Lets say I have an example circuit:

5V Power Supply -> LED -> Resistor -> Ground

Now the LED is receiving 20ma and 5V. If I were to switch the LED and the resistor however, the LED should receive less voltage but the same current, so wouldn't it be dimmer? Or would it receive the same voltage?

I've looked at these two threads and their answer doesn't help. Why doesn't it matter if a resistor is before or behind an LED wrt voltage drop? Why does a resistor need to be on the anode of an LED?

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tjpc3 The Redstoner
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3 Answers3

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Switching the positions of the LED and resistor will have no effect on the brightness of the LED because they would still be in series. The current through them would be exactly the same. Thus the brightness of the LED would be the same. The voltage drops across the resistor and the LED would be the same since the current through them is the same. This is a characteristic of all series circuits.

Barry
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The led DOESN'T receive 5 V across its terminals But it only DROPS some amount of voltage across it

Any Diode has a property called Forward voltage this is simply the amount of voltage that is needed to turn this diode on (Let the current flows through it)

Lets assume your LED has a forward voltage = 1.7 V

And this LED is connected in Series with a 250 Ω resistance

Two possible configurations could be achieved

  1. Source -> Resistance -> LED -> GND
  2. Source -> LED ->Resistance ->GND

For the 1st configuration

enter image description here

If we measured the voltage drop across the resistance it will be 3.3 V So your source provided 5 V , 3.3 V dropped across the resistance and 1.7 V dropped across the LED (Forward voltage) The sum of the voltage drop across the circuit elements must equal to 5 V (3.3 + 1.7) Not more or less than 5

For the 2nd configuration

enter image description here

Same this will happen (1.7 V dropped across the LED , then 3.3 V are dropped across the resistance

So it DOESN'T matter where is the resistance as long as its connected in series with the LED

So why we put the resistance ?

Well in many electronics books this resistance is called the "Current limiting resistance" its used to avoid short circuits and its value is used to control the amount of current flowing in your circuit without this resistance huge amount of current will flow and burn your LED

What does this resistance has to do with the brightness of the LED?

Well as mentioned , the MORE the value of this resistance the LESS the amount of current flows in your circuit , the LESS the LED brightness

How can i control the brightness of the LED ?

You will need to know how much current is following in your circuit , since your circuit contains only a resistance (250 ohms) and a diode , Neglecting the resistance of diode

The amount of current is (Source voltage / Resistance) <- ohm`s law

Elbehery
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    Great answer. But, general note on style: SE has _italic_ as its recommended way of emphasizing text segments. That's a bit more subtle than **bold**, which tends to get distracting (in particular if you combine it with [screaming case](http://english.stackexchange.com/a/8679/9466)... – leftaroundabout Jan 10 '16 at 21:22
  • "Forward voltage this is simply the amount of voltage that is needed to turn this diode on". Not really, the forward voltage is simply a voltage where the anode is positive relative to the cathode. Datasheets give nominal values of Vf for a given operating condition (a certain forward current + temp.), and generally a graph showing the relationship. Diodes never really turn off (with the exception of exactly 0V), as there are always leakage currents. – Tom Carpenter Jan 11 '16 at 00:09
  • @ leftaroundabout Thanks for the tip @TomCarpenter Thanks for your comment, i just meant by "turning on" that the LED itself is turned on (Working) since the person asking this mentioned that "he is new to the more advanced side of electricity" so i wanted to mention how the LED will work properly – Elbehery Jan 11 '16 at 15:40
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As explained, it would receive the same voltage either way, but an experienced engineer would say there is only one correct way to normally do this. The LED should be connected to ground, with the resistor between it and 5 V. The resistor has two functions. The first is to set the amount of current into the LED in normal use, as you have imagined.

The second is to protect the circuit from accidental damage. If you are working on this circuit, you might be using something like an oscilloscope that has one clip lead that is normally ground. If you connect the LED to the 5V, and accidentally touch the other end with ground, you will have no current limiting and may burn out the LED.

If you connect the resistor to 5 V, and the LED to ground, then touching the connection between them with ground draws more current through the resistor, but the LED gets no voltage since it is now grounded on both ends, draws no current, and therefore no damage is done.

Dave Morgan
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