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Basic Electronics » Powering L.E.Ds

August 08, 2012
by mikey6879
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Hi, I'm just playing about with LEDs at the moment because i'm hoping to create a small screen of some sort which i can program soon, however i'm already having a few problems. My circuit has 4.5V (3AA 1.5V batterys) connected. I am able to light up 2 at a time but no more, after playing around and using diffrent voltages i was able to work out that i needed more volts, however when i come to build the screen which will probably take 25+ leds i don't want have several 9v batterys attached. I also don't want to trust myself with the wall socket and i'm sure my parents would agree. I have seen plenty of people power many more leds at one time on youtube, is there a way to change the voltage or another way around this issue? Any advice would be helpfull

Heres some info about the leds which should help:

Emitting color: Green Diameter: 5mm Lens color: Water Clear Forward voltage(V): 2.0-2.2 Current(mA): 20 View angle: 25 Luminous intensity(MCD): 8,000 - 10,000

Sorry to ask what is probably a really basic question but i have done my own research into the problem and i'm still stumped.

Thanks, mikey6879

August 08, 2012
by rajabalu21
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You may want to take a look at this site.

25 LEDs

LEDs could be connected in Parallel and in Series.

-Raja

August 08, 2012
by Ralphxyz
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You also need to understand POV (persistence of vision).

All of the leds do not have to be turned on at the same time to appear to be all turned on at the same time.

Ralph

August 08, 2012
by mongo
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There are two things you need to look at. First is the forward voltage for the LED's. Typically around 1.2 volts per. If you have four in series, like shown above, you are looking at 4.8 volts to light them up at their full rated brightness.

That is all at once.

Then there is the little trick called "multiplexing" (or Muxing for short).

In this scheme, only one is lit at any given time but they are rapidly strobed and as Ralph mentioned, that thing called "persistence of vision" comes into play. Flash each one brightly for a very small period of time and do them all in sequence over and over, they have the appearance of all being lit up.

Adding something that makes sense to us humans out of the lit LED's, is where the magic happens.

Many displays are muxed out there in the world. It's a rare thing these days to find things under full power with all of the LED's lit at once.

August 09, 2012
by BobaMosfet
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If you have a digital multimeter, see if you can do a 'diode test' on the LEDs. If it lights, look at the reading and see what voltage the meter says- that will give you the actual lowest point required to fire the LED. Calculate resistor size for about 10mA at just a little above that voltage, to keep the math easy.

Based on the datasheet for the average Energizer AA battery, 3 in series will give you the full useful life down to .8V (from each = 2.4V from all 3) while supporting a single parallel row of 16 LEDs at 2.4V, drawing 10mA each, for approximately 580 hours. 32 LEDs for about 180 hours. I have not included resistor dissipation, so time will be less.

BM

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