Why do we need a current limiting resistor with LEDs?

MrAl

Joined Jun 17, 2014
11,389
Excellent idea! I've taken many electronic courses at our community college here, and remember one experiment of biasing a diode and drawing it's curve. Know that I've learned so much here, I'll re-run the experiment - it will make MUCH more sense this time!
Oh that sounds great. Publish your results here too and we can talk about it more.
 

dl324

Joined Mar 30, 2015
16,839
I never thought of the voltage source/series resistor as a current source - I was always interpreting (incorrectly) it more as a voltage divider circuit (boy was THAT stupid)! Now that you point that out, I understand where my thinking was very flawed....
A current limiting resistor isn't the same as a current source. A current source will maintain a constant current regardless of supply voltage (providing there's sufficient head room). Current with a resistor will vary with supply voltage.
 

MrAl

Joined Jun 17, 2014
11,389
A current limiting resistor isn't the same as a current source. A current source will maintain a constant current regardless of supply voltage (providing there's sufficient head room). Current with a resistor will vary with supply voltage.
Hi,

Yes the resistor has to be of high enough value to keep the current *relatively* constant with change of other variables like supply voltage and LED voltage. So in other words, a pseudo constant current.
Normally some variation is certainly acceptable with an LED such as 80 percent to 120 percent of target current level, which means keep the max under maybe 15ma for a 20ma LED. Life goes up exponentially with decrease of current.
 

WBahn

Joined Mar 31, 2012
29,976
A current limiting resistor isn't the same as a current source. A current source will maintain a constant current regardless of supply voltage (providing there's sufficient head room). Current with a resistor will vary with supply voltage.
From the view of what is connected to it (which is what the discussion is about), a voltage source in series with a resistor is indistinguishable from a current source in parallel with that same resistor (with the voltage equal to the current times the resistance). The higher the resistance, the more it looks like an idea current source, the lower the resistance, the more it looks like an ideal voltage source.

Now, what is inside the box also influences how good a source (of either type it is) and you would not expect a battery and a resistor to be as good as an Intermagnetics General magnet supply.
 
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