Resistors and LEDs

Thread Starter

Jasper Davis

Joined Nov 16, 2013
3
Moderator Edit:

Welcome to AAC!

A thread belongs to the OP (original poster). Trying to take over someone elses thread is called hijacking, which is not allowed at All About Circuits. I have therefore given you a thread of your very own.

In addition, you have practiced the arcane art of necromancy, the revival of a long dead thread. Likely the OP (Original Poster) has solved his problem in the years that has passed, or thrown it away, or something.

This was split from why we use resistor with LED.


I have a similar question. I've been tinkering with an Arduino Uno I'd been given by my structural design teacher. It came with a few circuits to try on the bread board and I went through all of them without difficulty, even if I didn't follow orders exactly. in all circuits dealing with LEDs, the resistor was put between the LED and the ground. The circuit uses 5V LEDs and I don't understand how putting a resistor *after* the LED would assist the LED in any fashion. could someone explain this to me?
 
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MikeML

Joined Oct 2, 2009
5,444
There is no such thing as a 5V LED. There is such a thing an LED that operates on a specified current, like 20mA.

There is a math concept called the "commutative property". It applies to circuits, as well as math.

Look at this. Notice the current through the two LEDs. The current-limiting resistor determines the current through the LED regardless which circuit topology you use... It max nix...
 

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shteii01

Joined Feb 19, 2010
4,644
I have a similar question. I've been tinkering with an Arduino Uno I'd been given by my structural design teacher. It came with a few circuits to try on the bread board and I went through all of them without difficulty, even if I didn't follow orders exactly. in all circuits dealing with LEDs, the resistor was put between the LED and the ground. The circuit uses 5V LEDs and I don't understand how putting a resistor *after* the LED would assist the LED in any fashion. could someone explain this to me?
MikeML has showed it well.

But to add. Often when people try to explain current, they use water flowing through pipes as analogy. And while it is helpful at times. This is one of those times where this analogy does not work.

As long as resistor is in series with LED, that is all that matters. The exact placement (in front of LED, after the LED) does not matter.
 

Thread Starter

Jasper Davis

Joined Nov 16, 2013
3
I have a few smaller questions for you now.
First off, I didn't know how to express the LED's operating voltage so I stated the Voltage the Arduino outputs instead (if you haven't realized yet, I am new to electricity's properties and circuits in general). the package which holds the LEDs states they have a brightness of 150 - 200mcd, a wavelength of 620-625nm, and a Vdrop of 20-24V. what can I understand from the brightness and the Vdrop?
What did you mean by saying "it max nix"?
lastly, which is used more often in this community when referring to the flow of electricity through a circuit: saying the circuit flows from negative to positive or saying the circuit flows from positive to negative?
 

Thread Starter

Jasper Davis

Joined Nov 16, 2013
3
MikeML has showed it well.

But to add. Often when people try to explain current, they use water flowing through pipes as analogy. And while it is helpful at times. This is one of those times where this analogy does not work.

As long as resistor is in series with LED, that is all that matters. The exact placement (in front of LED, after the LED) does not matter.
Is this connected with the idea that electricity always flows through the path of least resistance?
 

shteii01

Joined Feb 19, 2010
4,644
Is this connected with the idea that electricity always flows through the path of least resistance?
In your specific example the analogy would sound like this:
You have water flowing through LED. But if there is too much water through LED, LED will die/break. So you need to reduce the amount of water going through the LED. How you do that? You use a more narrow pipe. The how narrow the pipe is correspond to the value of the resistor. So. If you use this water analogy, you would need the narrow pipe in front of the LED, the narrow pipe lets through less water, the LED does not get swamped and does not die, translating to electrical, you need to place resistor in front of the LED. This is how you were thinking of the problem.

Like I said earlier, the water/pipes analogy for current can be useful. But in this case it does not work. In real life you just need a resistor in series with LED, it can be in front of the led, it can be behind the led. The choice is your's, it can be dictated by your own habit, by circuit layout or some other criterion.
 

shteii01

Joined Feb 19, 2010
4,644
I have a few smaller questions for you now.
First off, I didn't know how to express the LED's operating voltage so I stated the Voltage the Arduino outputs instead (if you haven't realized yet, I am new to electricity's properties and circuits in general). the package which holds the LEDs states they have a brightness of 150 - 200mcd, a wavelength of 620-625nm, and a Vdrop of 20-24V. what can I understand from the brightness and the Vdrop?
What did you mean by saying "it max nix"?
lastly, which is used more often in this community when referring to the flow of electricity through a circuit: saying the circuit flows from negative to positive or saying the circuit flows from positive to negative?
I think you are missing a period in the Vdrop values. 20-24 volts sound like way too much. Typically they are 2.0-2.4 volts.

For example. Earlier this year I was working with Arduino Due. It is 3.3 volt system (your's is 5 volt). This was my first time working with it. So just to see how things work I hooked up LED to one of the digital pins. And I wrote short program to turn the digital pin on for a couple of second, then turn it off for a couple of seconds. So when the pin was ON, there was 3.3 volt drop across the LED. If I remember right, Due can source 10 mA of current from its pins. So while I was overvolting the LED, I was not running max current through it. It worked fine. It was generic red LED that I had laying around so I don't even know what kind of Vdrop it has or what kind of max current it can tolerate.
 

#12

Joined Nov 30, 2010
18,223
Macht nichts = German for, "makes nothing".
A dismissive reply to, "thank you" (danka)
makes nothing, means nothing, etc.
A few sloppy translations and you get max nix. It kind of sounds the same but is so distorted that it is nearly impossible to trace to the original unless you already know German.
ps, this site does not allow any language except English.
We only tend to cheat on that among friends.
 
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