12V - 7.29A ac/dc converter project Need Help

Discussion in 'The Projects Forum' started by Butterworth, Nov 6, 2011.

  1. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    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. :rolleyes:
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,201
    1,809
    OK, if those specs are good, then it's 1.8v @ 100mA.

    The numbers look OK when plotted in a spreadsheet.

    So, that'll be 6 LEDs per string, 13 strings, total of 78 LEDs, and you will need 13 resistors measuring 20 Ohms that are rated for 1/3 to 1/2 Watt.

    I actually found these 20.5 Ohm 1% tolerance 1/2 Watt metal film resistors on Digikey:
    http://search.digikey.com/us/en/products/CMF5520R500FHEB/CMF20.5HFCT-ND/1554845

    Or, you could go with these:
    http://search.digikey.com/us/en/products/SFR2500002009FR500/PPC20.0YCT-ND/596952

    Or get them from another supplier. Those are both metal film, which I suggest using as they are more stable than carbon film resistors.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,201
    1,809
  4. Audioguru

    Expert

    Dec 20, 2007
    11,251
    1,350
    If the LEDs are enclosed then they will be extremely HOT!
    81 x 1.2V x 0.09A= 8.8W. How will you cool them?

    I made a light with 36 LEDs of 2V and 53mA (only 3.8W). The LEDs are SuperFlux type with 4 pins rated at 70mA. Boy oh boy they got extremely hot even though the case (for a compact cassette tape) had many holes around it.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,201
    1,809
    A.G.,
    We're looking at 1.8v *100mA * 78 LEDs at the moment; that's just a tad over 14W for the LEDs, and 16.666 Watts for the whole enchilada.
     
  6. Audioguru

    Expert

    Dec 20, 2007
    11,251
    1,350
    What about the RANGE of forward voltage? Some might be 1.6V and others might be 2.0V.
    The cheap surplus LEDs do not have a part number, no manufacturer's name nor datasheet so what is their maximum allowed current?

    I think they will melt.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,201
    1,809
    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.
     
  8. Audioguru

    Expert

    Dec 20, 2007
    11,251
    1,350
    Averaging the forward voltage of many LEDs doesn't work when they are all from the same production run (the same wafer).
     
  9. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    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?

    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.

    As for the cooling aspect, I was hoping to have a grille on the back of the project box, perhapse 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?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,201
    1,809
    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.

    They can only supply a range. If they did the screening beforehand, you would probably pay several times as much for them.

    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.
     
  11. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    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?
    I noticed that they are rated for different amperes, the To-92 was rated at .1A, the To-220 was at 1.5A.

    I understand that completely, this is why I went with this instead of smaller, more expensive packages of IR leds. :D 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.

    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.

    Question about current driving these leds, why can't I just limit the current to 90mA on them instead of the full 100mA? Will that not help with the amount of power being emitted through them, in turn reducing the amount of heat generated? (Just a thought).
     
  12. Audioguru

    Expert

    Dec 20, 2007
    11,251
    1,350
    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.
     
  13. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    Ok, so I need to redo this whole thing then?

    My design was based on 90mA current driving 81 leds, I cannot do 81 now so I am limited to 78 leds (13 strings of 6 leds in series), and how do you pulse it? I already have the 12.82 VDC adapter, and the project box. I am waiting on the leds, and still need to purchase the resistors. I want this project to be as simple & effective as possible... If I can get away without a backpack to carry this thing, I would certainly appreciate it. ;)

    Oh these leds will be open to the air on the face side of this project box, the resistors and back sides of the leds will be enclosed only. Will this help with heat dissipation?

    What do you guys suggest I do to pulse these leds at 100mA?
     
  14. Audioguru

    Expert

    Dec 20, 2007
    11,251
    1,350
    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.
     
  15. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    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.
     
  16. Audioguru

    Expert

    Dec 20, 2007
    11,251
    1,350
    Beware of LEDs that have their weight as their only spec. Just like RadioShack.
     
  17. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    Yeah you get what you pay for right? ;-)
     
  18. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    My leds have arrived!. They look sweet, or as sweet as fresh leds can look :cool:

    I also want to do the constant current testing to ensure that they are segmented per Vf's, but rummaging around my misc. electronics, I found a whole whack of IC's! One of which is an LM723CN. It is a dip14 style IC. It has a max output of 150mA according to the datasheet.

    Can I use this as my regulator?
     
  19. SgtWookie

    Expert

    Jul 17, 2007
    22,201
    1,809
    Well, that would make the circuit more complex.

    The 723 is a really good linear regulator, but it can be somewhat complex to use.
     
  20. Butterworth

    Thread Starter Active Member

    May 6, 2009
    135
    3
    Ok i would like to use it. if i am going to be making more of these lights... i need something to test the leds. can you help me get a schematic together for this ic?
     
Loading...