Quick LED question

The reason for the question is because as you know, the high brightness LEDs

Sorry, but you must have misunderstood me about cost. I meant the reason I am asking is to avoid ruining my LEDs if I connect them up wrong. It had nothing to do with the resistors =)

Thanks for your reply anyway Jason

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No problem Jason - glad I could help.

... Steve C.

--- Since the LEDs will all have slightly different forward voltage drops, you may _not_ connect them in parallel and share a current limiting resistor.

What you _may_ do, though, is connect them in series and use a single resistor for each series group. However, since the Vf of the LEDs is 3.5V and you have a 12V supply, the largest number of LEDs you can connect in series is three, since voltage drops in series add and 4 * 3.5V = 14V.

If you've got a stiff supply you could connect the LEDs in two groups of three and one group of two, like this:

+12>------+---------+---------+ | | | [51R] [51R] [180R] | | | |A |A |A [LED] [LED] [LED] | | | |A |A |A [LED] [LED] [LED] | | | |A |A | [LED] [LED] | | | | GND>------+---------+---------+

That is, if the _typical_ forward current is 30mA. Check it again and make sure it isn't 20mA, which I believe is more common, and if it is, recalculate the resistor values.

-- John Fields Professional Circuit Designer

I would think each led set of three with a resistor in pretty standard for consumer stuff. Naturally the one resister per led is better, as it is with all electrical stuff to be isolated rather than share. White Led,s are less than a dollar now.

But john, I have a question about 12 volts. The assumption is using a quality source for power. These wall adapters rated at 12 volts are only good for the device it goes with. Otherwise the voltage can be as high as 20 vdc.

You are exactly right john, That is the way I did on a board holding over

1000 white led's and it works well. You can't imagine the brightness of that many white led's in a 8" by 10" space. If anyone want my pcb artwork I will send it to A.B.S.E. group. JTT

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John Fields warns you about that in his sentence that begins: "If you've got a stiff supply ..."

You will need to use a voltage regulator to stiffen the supply. You have 10.5 volts across the LEDs, the regulator will require some as well. Use one of the LDO regulators available from National Semiconductor:

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Sure, send away!

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Why?

ss LEDs

This is why I referred to the cost near the end, He was concerned about the expensive LEDs.

I was in the mind set that he wanted to whatever it takes to not damage the led,s regardless of efficiency. But your approach is the correct approach for getting maximum use of all resources. I did not know a stiff power source is, but I had a feeling that I should have known that it was a given based on your experience regardless.

Again I was looking at safeguarding the led's however unreasonable the method is.

I know it is a simplified view, I usually tell people that just in case they read the adapter incorrect.

That is something I should have not added to the solution.

That is my fault for reading 12v as other voltages.

I made my own led board once and I did like the 3 led one resistor setup you showed here. So I skipped some parts of your post, answered only with ideas to treat his led as the main concern. But things should be going better by now. ( I learned what stiff power source means by the way)

For some reason, I couldn't reply over on ABSE.

Nice layout! There's a possibility of a mis-scan

26 columns in going from left to right. Looks like some of the traces are touching. Since it works well for you, I'm assuming it scanned improperly.

How did you determine that you have sufficient cooling?

Ed

How much did that project cost or how much would it cost a consumer in parts? I only ask because I saw a unit of 1000 leds set into a board and sealed in epoxy or such. They were on sale for 400 dollars a unit a few year ago. Usage was for lighting streets and pathways with something vandals would have a difficult time with. I bought 500 yellow siemons branded leds and soldered them wire to wire and no resistors for running on 2 volts. Worked with one lead acid cell.

The led's I got were on eBay from a vender in china. For 1000 led's it cost a little over $100. I may try out some 10mm led's rated for 130,000 mcp I saw on eBay for 10.00 for 100 of them + 25.00 shipping just for curiosity. The pcb I used is an fr4 material with 1 oz copper. Mounted the pcb in a standard wood picture frame to keep it from warping.

Thanks ed. Maybe I missed that part on the traces touching, I scanned it from the printed output of the layout program into Photoshop. I'm going to have to take another look over my artwork and make sure now. As far as determining cooling, I just old fashioned touch to see if there were hot areas. With a nice air current flowing along the bottom of the board, the led's stay fairly cool. JTT

Thanks John. Terrific help as usual. See, I told you you would help me again in the future! Remember that motor controller about 3 months ago? =)

One more question though, about your 'series' connection: What is the likelihood of one LED failing for some strange reason and causing the rest of the LEDs in the string to get affected? Kind of reminds me of those fairy lights at Christmas time... you know, one bulb fails causes a whole lot of them to go out as well. Not a huge concern, but just a little curious.

Jason.

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My pleasure. :-)

How bright is this 1000 led panel you put together ( did you do it one by one or discovered a short cut?) in real life. What wattage florescent light would be the same, or if you used white leds, halogen or softwhite bulbs. Volts X Amps = Watts could be a common ground. Or just something simple like it is as bright as a x bulb.

Thanks

PS, what angle are the leds? wide or narrow?

I don't have a meter to check the brightness output, but it is more then enough to replace the lamp in an lcd projector. I did a small board of 96 led's running on a 12 volt NiCad battery pack and took it outside at night, just to see how it stood up to a flashlight. That small panel would light the trees across the street from my house. The light it makes also seems to make everything in high contrast. Give it a try and you will see what I mean by that.

As far as wattage, (350 strings drawing .025 ma.) = 8.75 amp = (12v * 8.75 amp) = 105 watt on the panel. But you can judge the led brightness by that, as Led's are more efficient then Incandescent lamps.

The angle on the led's were 20 degree if I remember correctly.

No, schematic not necessary. Thanks John. I think I'll be right from here. =)

Best regards, Jason