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Topic: Full brightness multiplexing. (Read 6 times) previous topic - next topic

Boffin1

Hmmm,   I asked my manufacturer about pulsing the LEDs, and they replied :-

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Hi John,
I see . It's ok with leds . We did pulse testing on such leds.
But it will increase the decay of light a little bit .
You can try an samples .


So I asked what pulse width and frequency they tested at  ?  they said :-


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Frequency is 1.0hz .
1 time per second. We tested 3 monthes . And if leds ok . then we use the chips.


which I work out to being on all the time  :-)

so I asked if I can pulse them 150mA at 10% M/S ratio, and they replied :-

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Hi John,

No . 30mA is the max current .

150mA will burn the leds.

If need 150mA . You should use 0.5W


So it looks like I am abandoning multiplexing for now as I have a couple of thousand of these LEDs left, even though I can't get specs on them :-)
With my mobile phone I can call people and talk to them -  how smart can you get ?

Headroom

I is somewhat off topic, however the statement that an LED appears to be half as bright at 50% duty cycle then at 100% is incorrect for several reasons.
The relation between drive current and luminance is not linear.

It is pretty well explained here:

https://ledshield.wordpress.com/2012/11/13/led-brightness-to-your-eye-gamma-correction-no/

Boffin1

Thanks Headroom,   thats an interesting link.

So running the LED for half the time only loses a quarter of the perceived brightness,  and you could go down to 1:7 ratio before losing half the perceived brightness.

I just googled LED graph current luminance, to see what increase in current would restore full brightness, and again found pages of conflicting stories about PWM,  i.e. from electronics-tutorial.ws  .
Quote
So pulses at a frequency of 100Hz or more actually appear brighter to the eye than a continuous light of the same average intensity.


A theory from an old HP document according to another page http://electronics.stackexchange.com/questions/17528/does-pulsing-an-led-at-higher-current-yield-greater-apparent-brightness

I think I will have to knock up 2 samples of my 7x5 display and actually test it in the sun while varying the duty cycle of the one.
With my mobile phone I can call people and talk to them -  how smart can you get ?

magagna


So running the LED for half the time only loses a quarter of the perceived brightness,  and you could go down to 1:7 ratio before losing half the perceived brightness.


That makes sense, your eyes perceive light intensity as a logarithmic function. Here's the short table I use:

Step | Duty
===========
  0  | 0
  1  | 1
  2  | 3
  3  | 7
  4  | 15
  5  | 31
  6  | 63
  7  | 127
  8  | 255
http://en.wiktionary.org/wiki/magagna <-- My last name.  Pretty apt.

Boffin1

Another point I have been ignoring,  while trying to maintain maximum brightness,  is that each time I buy another batch of LEDs they are much brighter than the previous ones anyway !

It was only a couple of years back that normal ( 25mA ) cheap LEDs broke through the 1000 mcd mark, now 11,000 are common.

With my mobile phone I can call people and talk to them -  how smart can you get ?

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