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LED Calculations

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LED Calculations

Postby Jon Chandler » Thu Apr 14, 2011 10:00 pm

Image

LEDs require a resistor to limit current through the LED.  Calculating the required resistor can seem like a mystery but it's very simple, even with LEDs of unknown types.

Read full article...
Jon

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Re: LED Calculations

Postby Jon Chandler » Thu Apr 14, 2011 11:15 pm

I've been thinking about this subject for several days - it seems like a topic that some people have trouble grasping. Today I was spurred into action after reading a 4 page forum thread on that forum which shall remain nameless where mis-information and confusion were the rule....

One point I didn't explicitly state in the article is that it doesn't matter how much current the voltage source is capable of providing - the current through an LED is limited by the resistor in the circuit. I really think this point should be self-evident but here it is just in case its not.
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Re: LED Calculations

Postby Jon G » Fri Apr 15, 2011 12:01 am

Jon Chandler wrote:One point I didn't explicitly state in the article is that it doesn't matter how much current the voltage source is capable of providing - the current through an LED is limited by the resistor in the circuit. I really think this point should be self-evident but here it is just in case its not.


I'm ashamed to admit this, but until I read your power supply article... I did not understand this concept. Understanding the role of the capacitor in the power supply made it perfectly clear what was going on.

Still need to read your most recent article.
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Re: LED Calculations

Postby Jon Chandler » Fri Apr 15, 2011 12:43 am

There is a big misconception among people that a power supply somehow "forces" current on a circuit. That somehow a circuit that's ok with a 5V 100 mA supply will somehow burst into flames if a 5V 1000 mA supply is used. The circuit will only draw the current it needs and no more.

There is one caution that needs to be taken however. Suppose that you're powering you dev board from a PC power supply capable of supplying 5 volts at 25 amps. The circuit will be perfectly happy using the few mA that it wants...but if there's a short of any kind, that 25 amp capacity may vaporize the wires to the circuit board or result in exploding parts. In the power supply article, I showed how a cell phone supply will just output less voltage if there's a short - they play a little more nicely.
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Re: LED Calculations

Postby Graham Mitchell » Fri Apr 15, 2011 3:03 am

Great article Jon. Well structured and awesome use of the equation editor.

A quick note on the LED polarity; check the datasheet! Nine times out of ten the flat edge on a LED package is the cathode, but not always. I've been bitten by this before ;)
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Re: LED Calculations

Postby jmessina » Fri Apr 15, 2011 3:32 am

The table even shows why certain LED's (InGaN-based ones like blue and white) won't work very well if all you have is a 3.3V circuit.
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Re: LED Calculations

Postby Jon Chandler » Fri Apr 15, 2011 6:10 am

Excellent point Jerry! I failed to mention that.

I recall seeing tables similar to this one of chemistry vs. forward voltage, but Google was not my friend yesterday. This table was boiled down from several specification pages on the Lumex web site, so I may have missed some options.
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Re: LED Calculations

Postby DavideAndrea » Fri Apr 15, 2011 8:49 am

Nice article... except there's room for improvement.

* You cannot calculate LED current using linear equations, because the LED's V-I curve is not linear
* You cannot determine LED polarity by looking at its guts

I have made some suggestions for your article in this page.

Thanks,
Davide
Last edited by Graham Mitchell on Fri Apr 15, 2011 11:04 pm, edited 1 time in total.
Reason: took the rough edge off
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Re: LED Calculations

Postby Graham Mitchell » Fri Apr 15, 2011 10:16 am

Thank you for the quick feedback Davide - the article has been updated.
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Re: LED Calculations

Postby DavideAndrea » Fri Apr 15, 2011 10:37 am

Graham wrote:Thank you for the quick feedback Davide - the article has been updated.


Thanks!

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