I am terribly frustrated with the vendor, there are no specification listed, nor does customer support have a part number for the leds. I have a link to the item... it won't help though.
I can do this, however, I never made a CC source before. All I would need is the Lm317, a 2W resistor valued to 12.5Ω, some luck, and wiring to the board in between the VDC source and the leds?As far as the range, he could simply rely on the Monte Carlo distribution of Vf's; odds are reasonably decent that jumbling up 78 LEDs and picking them randomly into strings of 6 that the total Vf's in the strings will be about the same.
If he really wanted to do it the right way, he could make up a simple constant current circuit using an LM317, 12.5 Ohms' worth of resistance from the OUT terminal to the ADJ terminal, and his 12.82v supply together to make a 100mA constant current circuit; then measure each and every LED's Vf, and sort them into bins by voltage.
Then match up the low Vf LEDs with the high Vf LEDs.
I see your point and agree that they will not all be the same in bulk. Its unfortunate that they cannot even supply any specs... poor CS on their part I think.Averaging the forward voltage of many LEDs doesn't work when they are all from the same production run (the same wafer).
That's pretty much it. You can use 4.3 Ohm and 8.2 Ohm resistors in series to get 12.5 Ohms. The regulator will get fairly warm if you have it powered on for awhile; but it'll only be dissipating around 1W power.I can do this, however, I never made a CC source before. All I would need is the Lm317, a 2W resistor valued to 12.5Ω, some luck, and wiring to the board in between the VDC source and the leds?
They can only supply a range. If they did the screening beforehand, you would probably pay several times as much for them.I see your point and agree that they will not all be the same in bulk. Its unfortunate that they cannot even supply any specs... poor CS on their part I think.
We don't know if you're planning on making a custom board or not. If you are making a custom board, leaving large copper areas as heat sinks will help quite a bit. Forced air cooling will likely be a must.As for the cooling aspect, I was hoping to have a grille on the back of the project box, perhaps with a small 12VDC fan from an old PC. (That is still up in the air) but until I receive these leds, I will not be making any enclosures yet.
Any ideas for the cooling? Maybe a string of heat sinks on each string of leds?
Ok, I can find those easily. The LM317 have different confirgurations though, does it matter which format I use? Like the To-92, or the To-220?That's pretty much it. You can use 4.3 Ohm and 8.2 Ohm resistors in series to get 12.5 Ohms. The regulator will get fairly warm if you have it powered on for awhile; but it'll only be dissipating around 1W power.
I understand that completely, this is why I went with this instead of smaller, more expensive packages of IR leds. I do wish they could have put a range of expected ratings for the lots. Even if its a ball park figure, at least it will give the purchaser some idea as to what they are getting.They can only supply a range. If they did the screening beforehand, you would probably pay several times as much for them.
I am using perfboard, I was going to place out the leds in a pattern that will fit my board, and project box, then wire them together ont he other side. It isn't a custom board job, will take that much more time for me to SMT everything. (I do not have a means for drilling the pads). None-the-less, by placing the heat sinks on the rear of the board, the side where the soldering will be, this should help some? If it isn't enough, I will have to salvage an old DC fan from one of the PC relics I have in the closet and frankenstein-it into the box.We don't know if you're planning on making a custom board or not. If you are making a custom board, leaving large copper areas as heat sinks will help quite a bit. Forced air cooling will likely be a must.
Ok, so I need to redo this whole thing then?The IR LEDs are in a package (5mm) designed for ordinary 30mA max LEDs. They will cook to death with 90mA to 100mA. But you can pulse them at 100mA.
A fan will not cool them much because the package is not designed to have a fan for cooling.
Yes, a camcorder with Night vision is being used with this IR light. The camera comes with a little tiny single IR led and is good for about 4 feet in darkness. This light should light up the whole room and then some. If I don't pulse them, I would have to assume 20-30mA current to feed them instead? I am worried about "brightness" or output of these leds as a result.We don't know why you are using many high power IR LEDs so we don't know if pulsing them will allow the detector (a video camera?) to work.
by Jake Hertz
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