Max current for multiplexed LEDs

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Just an FYI in case you missed it in my other post, I mentioned L*di/ dt as a good reason not to mux such a large display. Not to mention RFI.

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"The Journey is the reward"

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That is an interesting experiment you are proposing. I think you would have to make sure the LED is in a light tight enclosure since ambient may influence the measurement.

I've looked around for 45-degree cross-hatch louvers -- they seem better for outdoor viewing (if you can accept the viewing angle limitations.) To avoid ghosting effects, some translucency (10-15%) is helpful in the louvers, themselves.

#M has had something called "Light Control Film" for some time. You might google that. It appears that it is available now for computer monitors, ipods, and pretty much anything that may get used outdoors.

There's this company to look at:

I know nothing about them, but it appears they are in the business area I'm talking about here, too. Namely,

Their president claims their films reduce energy consumption in LED displays for outdoors, by 80%.

Apparently, they are working with Wahl Optoparts, which got rebranded as Jenoptik Polymer and is now also working with Lumenova:

But if you read about these things, you'll see why there is a need to focus on the optics side to gain some real advantage in the display. Just brute force alone will get you only so far.

Jon

Yahoo groups may be a pain, here. I just went there, via my login account, and it is definitely there and downloadable.

Forget the direct link. Probably has my session stuff in it. Just go to the web link I provided before that. You'll probably have to join or something, though. Want me to just send the file to you?

Ah, hell. I'll load it to my own site. Here's the link:

Jon

One thing I see in a lot of LED datasheets is pulse ratings amounting to maximum RMS current not exceeding maximum continuous DC current. I forgot how short the maximum pulse repetition period is for most of these, maybe

1 millisecond. Whatever that maximum pulse repetition period is, the RMS current over that amount of time may have to not exceed the LED's maximum DC current.

It appears to me that *somewhat* higher RMS current is OK (possibly

1.5-2 x the DC max. current rating? ) as long as the repetition period is nice and short. With duty cycle in the 10% ballpark, the limiting factor is usually bonding wires and maybe contacts to the die.

If duty cycle is short, this will put a crimp on average current.

I have pulsed LEDs with peak current as high as a majority of an amp - and RMS maybe double the maximum DC rating of 30 mA. I had pulse repetition rate of 5 KHz. The purpose was to achieve spectral change in LEDs of a few types whose spectra change in interesting ways when instantaneous current gets extreme. I do not guarantee that LEDs will reliably survive this.

- Don Klipstein ( snipped-for-privacy@misty.com)

On a sunny day (Fri, 01 Jan 2010 16:46:01 -0800) it happened Jon Kirwan wrote in :

OK, got it. I am looking at PRJ_12.CPP The comment says: "The basic idea for recomputing a new TACCR1 value is to multiply the on-time by 5/4ths if we are increasing the brightness or else multiply the on-time by 3/4ths, if we are decreasing the brightness."

I will read some more of it.

I include a gamma correction in my color PIC control software a long time ago, see bottom of this page

I have experimented with and without his gamma correction, both gives interesting results. I will try a log function on top of that to see what it does.

I think there are 2 feedback mechanisms at work, the closing of the iris (fast), and dark adaption of the eye (slow). So I think it depends on if you want to make for example a light show (fast variation), or want to set the room light (slow variation).

More experimentation is needed :-)

Yeah. That's the one. The idea is to just use geometric progression. It's not tricky.

It also points out, "This isn't an even-handed method, as the dimming period will be shorter. But it is cheaper in terms of code and doesn't harm things much."

Anyway, the only 'trick' in the code is that I switch to a linear rise instead of an exponential one near the bright end of the cycle to "visually blunt" the transition there. The reason for doing it on the bright end is to "soften" the transition when turning the direction around. To me, it's more "pleasing" to see it that way. YMMV.

Jon

Hi folks. Sorry about being silent for some time. My ISP was down for more than 24 hours and I fell ill during that time. Nothing serious, but I still don't feel up to going through your replies in detail and answering them. Will come back ASAP. Thanks for your interest.