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When current limiting resistors are needed with LED?
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Electro Fan:
Generally resistors are used with LEDs but it is not an absolute requirement.

LEDs can handle some amount of current - they will get brighter as you increase current up to some point that too much current (driven by too much voltage) will overheat and burnout the LED.  The threshold for burnout will vary based on the specifications of the LED but if you keep the current below 20mA the LED will probably be ok (but it’s good to have some spares available just in case).

if you hook a DC Power Supply directly to a LED - without a resistor - and you set the PS for a maximum of 20mA you can then start applying some voltage.  At 1 Volt nothing much will happen.  Gradually turn up the voltage 1/10 Volt at a time.  Somewhere around 1.6 -1.8 Volts you will see a (red) LED start to faintly emit light, and get brighter as you increase the voltage in 1/10 steps.  On the PS (or with multimeter in series with the circuit) watch the current increase from a few mA until the voltage gets high enough that the LED draws 20mA.  At this point the PS is limiting the current - you can probably let the circuit run like this indefinitely and the LED will be fine.

If you raise the current limit on the PS, say to 100mA, and keep turning up the voltage at some point instead of getting brighter the LED will burn out.  With a resistor in the circuit the LED could have survived more voltage.  How much more can be calculated if you know the specs of LED including maximum rated current and it’s forward voltage - then you can size the resistance of the resistor using Ohm’s Law.  After you see the relationships expressed by V=IR all of this will make more sense, but these circuit making experiments will also help the math become more tangible.  In the process the math will help you see the forces at work that cause the shapes of the curves posted above.  After you burnout a few LEDs be prepared to burnout a few small resistors - at which point P=VI will become more clear. 
Youkai:

--- Quote from: IanB on April 27, 2018, 04:27:14 am ---One way to answer this is to say that a bare LED, all by itself, is never rated for a voltage. Not ever. An LED, by its very nature, is always rated for a current. For example, 20 mA maximum, or if it is a big power LED maybe 300 mA or so (with a heat sink).

--- End quote ---

Ok so that makes a lot of sense I guess. Looking at this red LED: https://www.superbrightleds.com/moreinfo/through-hole/5mm-red-led-120-degree-viewing-angle-flat-tipped-1200-mcd/279/1206/ the site says it's rated for 30mA. But also for 2.0V. So is it "rated for 2.0V" or is that somehow a misnomer/incorrect way to think about it?

EDIT: Oh or is the 2.0V it's forward voltage and it's just poorly labeled?
ebastler:

--- Quote from: Youkai on April 27, 2018, 05:50:48 am ---Ok so that makes a lot of sense I guess. Looking at this red LED: https://www.superbrightleds.com/moreinfo/through-hole/5mm-red-led-120-degree-viewing-angle-flat-tipped-1200-mcd/279/1206/ the site says it's rated for 30mA. But also for 2.0V. So is it "rated for 2.0V" or is that somehow a misnomer/incorrect way to think about it?

EDIT: Oh or is the 2.0V it's forward voltage and it's just poorly labeled?

--- End quote ---

You got it, it is 2V forward voltage. The "rated for 2V" seems to be your own misinterpretation -- I can't find the term "rated" anywhere on the page?
ogden:

--- Quote from: ebastler on April 27, 2018, 06:17:26 am ---
--- Quote from: Youkai on April 27, 2018, 05:50:48 am ---Ok so that makes a lot of sense I guess. Looking at this red LED: https://www.superbrightleds.com/moreinfo/through-hole/5mm-red-led-120-degree-viewing-angle-flat-tipped-1200-mcd/279/1206/ the site says it's rated for 30mA. But also for 2.0V. So is it "rated for 2.0V" or is that somehow a misnomer/incorrect way to think about it?

EDIT: Oh or is the 2.0V it's forward voltage and it's just poorly labeled?

--- End quote ---

You got it, it is 2V forward voltage. The "rated for 2V" seems to be your own misinterpretation -- I can't find the term "rated" anywhere on the page?

--- End quote ---

Indeed you want to know forward voltage at rated current.

Identical LED's, both have +/- identical initial currents around 20mA. Voltage of both supplies increases by 10%, yet resulting currents differ. I would say LED powered by voltage becomes destroyed, one with resistor lives:



[edit] Answer to original question "When current limiting resistors are needed with LED?" is: always when you power LED using voltage source.
Zero999:
The comments about LEDs being rated for a certain current and their voltage being non-linear are correct.

To complicate matters further, the forward voltage can vary significantly between LEDs, of the same type, from the same manufacturer and decreases, as the temperature increases. For example, you might have a blue LED, with a typical forward voltage of 3.3V, when the current is 10mA, but when connected to a 3.3V, constant voltage, supply it's brighter than expected (draws more than 10mA) and problems are experienced with overheating. When the LED starts to pass current, it heats up, dropping its forward voltage, but the regulated 3.3V supply keeps its output voltage constant, so the current increases, to the point when the LED overheats. What's worse is this might not happen, when bench tested at room temperature, but is more likely in the field, when the operating temperature is higher.

The solution to the above problem is to add a small series resistor. The blue LED will be more than bright enough, at 10mA. A forward current of 1mA would probably suffice. If high brightness is desired, then a more elaborate solution is required, such as a constant current supply.
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