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Topic: LEDs without the use of current limiting resistors (Read 5 times) previous topic - next topic

myownway

Hi, this is my first post and I'm sad to introduce myself with a question. But I'd really appreciate an answer.

Well, I'm in a project and I was searching some info on the internet when I found this code: http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/LED/_7Seg_Example.pde

The thing is that I would love to know wich rules/relations/equations does this guy use to get the current average just with the amount of time that the led is on and off, in this part of the code:

Code: [Select]
//Display brightness
//Each digit is on for a certain amount of microseconds
//Then it is off until we have reached a total of 20ms for the function call
//Let's assume each digit is on for 1000us
//If each digit is on for 1ms, there are 4 digits, so the display is off for 16ms.
//That's a ratio of 1ms to 16ms or 6.25% on time (PWM).
//Let's define a variable called brightness that varies from:
//5000 blindingly bright (15.7mA current draw per digit)
//2000 shockingly bright (11.4mA current draw per digit)
//1000 pretty bright (5.9mA)
//500 normal (3mA)
//200 dim but readable (1.4mA)
//50 dim but readable (0.56mA)
//5 dim but readable (0.31mA)
//1 dim but readable in dark (0.28mA)


I wonder that this may be related to joule/second = w, but I'm not sure.

Thanks in advance.

Grumpy_Mike

#1
Jan 07, 2013, 02:49 pm Last Edit: Jan 07, 2013, 02:53 pm by Grumpy_Mike Reason: 1
The rules he is using is pig ignorance coupled with a lot of stupidity.
This is crap.

See this:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

Boffin1

I am driving LEDs with a constant current driver, and wanted to increase the current ( pulsed ) to increase the brightness.

I asked my LED manufacurer what the safe pulsed current is for the particular LED, and they said 30mA - its rated at 25mA normally.

So its definately not safe to start thumping huge current pulses through the LED, ( or from the Arduio pin )

GMikes tutorial sums it up nicely, but as he says, different LEDs have different pulse tolerances, so check with the manufacturer.

Unless you have a constant current drive, include the 2c resistor.
With my mobile phone I can call people and talk to them -  how smart can you get ?

Hippynerd

I dont know where he is getting his figures from, but He seems to be using a 6.25% duty cycle, so maybe is is using a calculation based on duty cycle and the parts forward voltage rating. Lets say your LED is fully lit with 20ma at 3.5, and you are giving it 5V, at 5v lets say you are using 100ma, but with a duty cycle of 6.25 % its using 6.25ma.

Those numbers are made up just to try to make sense of what he is probably doing, Its not too clear from his documentation.

I can tell you that I have run LEDs without resistors, and right now I have a couple sets of 192 LEDs running with out a single resistor, and they have been running for months now without issue. The really crazy thing is that no matter how angry mike gets, my LEDs still shine brightly.

Heres 192 LEDs, 0 resistors, 1 arduino, 1 4-pack AA batteries (6v).
https://www.youtube.com/watch?v=1FElbyUbTHM

Boffin1

You are saying its OK to pulse it at 100mA,  my manufacturer says 30mA  -   hmm .

Some TV remote controls use the internal resistance of the battery to limit the current through the IREDs, perhaps your running 192 LEDs from a couple of AA batteries is doing the same, how many LEDs are on at the same time ?

Whichever, its bad practice.

I run my 25mA rated LEDs at 18mA, and out of the last 25,000  LEDs since October,  I have had only a couple of failures, so I stick to that current.

I am pulsing the latest project ( 50% ) at 21 mA, still well inside of the recommended current.
With my mobile phone I can call people and talk to them -  how smart can you get ?

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