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What do I need to run 200 LEDs on 110V

  1. Jun 18, 2008 #1
    I want to build my own under cabinet LED strips for my kitchen. I am looking at wiring 200 Nichia T1 3/4 LEDs to one AC circuit. But I am having problems finding any directions to build the circuit or driver to power a system like this. If I need to buy a premade step down transformer then I could to that too, but I don't know if that is the best way or not. Can somebody help?


    Kevin
     
  2. jcsd
  3. Jun 18, 2008 #2

    russ_watters

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    What are the power requirements of these LEDs?
     
  4. Jun 18, 2008 #3

    mgb_phys

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    Is it practical to run LEDs from AC?
    Presumably any flicker at 60Hz won't be visible- but what does it do to the life of the diodes?
     
  5. Jun 18, 2008 #4

    russ_watters

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    I'm thinking no, but then a store-bought dc power source is pretty cheap...
     
  6. Jun 19, 2008 #5
    Ok cool, the one thing you want to do is to limit the current flowing the LED since you can break them if its too high, done this a couple of times myself :tongue:

    As has been said its easiest to go with a DC supply since AC wont be visible at 60Hz (if you're in the states) since the LED will flicker at about this frequency and our eyes cant refresh that quickly.

    I found a cute web app that might help you out

    http://led.linear1.org/led.wiz

    Looks like someone has done all the work for us :)
     
  7. Jun 19, 2008 #6
    I forgot about the possible flicker from AC, I knew I came to the right spot to ask about this. :)
    So, I need to drop the voltage and convert to DC. That's easily solved. But my next question is what would be the ideal voltage to run this many LED's? If I bought a 110VAC - 12VDC supply, would that be easy to work with and reliable?

    Eidos- THAT LINK IS PERFECT!! THANKS
     
  8. Jun 19, 2008 #7

    NoTime

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    The calculator above does not account for forward resistance,
    Not an issue for one or two LEDs but for long strings of LEDs this changes.
    However, worse case is they will be dimmer than expected.

    You might notice flicker since they would only conduct for a half cycle.
    I'm fairly susceptible to flicker and haven't noticed any for one off 1/2 cycle AC use.
    But then I think wide area bright illumination is a factor in the flicker effect.

    Adding a full wave bridge should fix that as well as eliminating any problem exceeding the reverse bias spec which is the only potential problem I can think of for AC use.
    Leakage variances in string mode can easily cause the reverse breakdown spec to be exceeded.

    If you use a ground fault and understand proper insulation techniques then, with 200 LEDs, they can be series connected for direct AC line use.
    Otherwise a stepdown transformer is recommended.

    Improper wiring/insulation can still be a deadly combination even with a ground fault.
    Particularly with the long strings needed for this application.
     
  9. Jun 19, 2008 #8

    mgb_phys

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    The typical way to do this would be to use a 12V dc supply.
    Then hook up the appropriate number ( probably 8-10) LEDs in series to give a forward voltage drop of 12V, you might have to experiment slightly adding/subtracting a few to get the best brightness.
    Then connect sets of these bars in parallel, checking the total current output of the 12Vdc supply.
     
  10. Jun 19, 2008 #9
    You will still want some sort of series resistor even if you series up about 6 LEDs. At 2 volts drop each 6 LEDs will get you 12 volts. But the amount each one drops is dependent on temperature and a few other things. If you hook up 6 LEDs that each are dropping 1.8 volts to a 12 volt supply you will most likely fry at least one of them if there is no series resistor installed.
     
  11. Jun 19, 2008 #10

    Redbelly98

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    If these are white LED's, each one has about 3 or 4 V drop. Not the 2V typical of red and yellow LED's.

    Also, one should not assume a "12V" supply outputs 12V. It should be measured, then double-checked when running a load.
     
  12. Jun 19, 2008 #11
    NoTime-If I were building these in a sealed enclosure then I'd look at running them at line voltage. Since this set up is going under cabinets, I am figured stepping down the voltage would safest, I just didn't know if I should convert to DC or what the best voltage would be for a circuit this large.

    Would it be wrong to just run them all in parallel and use 1 resistor per 3 LEDs? That is what comes up when i enter the numbers on the link Eidos posted earlier. To get that I entered the following:
    12V Source voltage
    3.6 diode forward voltage
    30 diode forward current (mA)
    200 number of LEDs in your array

    Thanks for all your help so far, this will be a fun project for a newbie like me.
     
  13. Jun 19, 2008 #12

    NoTime

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    If you use a real 12v supply then you may not need the resistors.

    Since there are several types of white LED and I haven't tried then out, I'm not sure what the forward resistance is on the ones you have.

    The common red LED has a forward resistance of about 22 ohms.
    If the white ones you have are similar then that would give you a built in 66 ohm resistor for three in series.
    So you would not need to use the 47 ohm resistor which is less than 66 ohms.
    Which is why I said the calculator need some work.

    However, If 30mA is the max current rating for your LEDs then you should select for a lower operating current.
    Running things at maximum specified ratings tends to fry them as the max rating also has a maximum operating temperature associated with it.

    Note: You might be able to scavenge a nice 12v wall wart switching supply from a computer peripheral.
     
  14. Jun 20, 2008 #13

    Redbelly98

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    That is the idea. The 12V power supply must be capable of supplying just over 2 Amps, if you are going to run 30 mA through all 67 strings.


    200 LED's and 67 resistors will take a while to wire together. It's a really good idea to be sure the circuit will work before you go to all this trouble.

    So:

    Before you wire up all 200 LED's, wire up a single 3-LED + resistor string. In parallel with that, connect a resistor that will draw roughly 2 Amps, in order to simulate the total load of the complete 200-LED string. Six 1-ohm, 5-Watt resistors (in series to get 6 ohms) would suffice for this.

    Double check that the LED's actually use the current you want for the resistor value you choose -- just measure V across the series resistor and apply ohm's law to find current. If necessary, adjust R (the resistor in series with the LED's) until you have the current you want.


    Once you have experimentally determined R correctly, you can wire up the entire 200 LED circuit.

    Good luck and have fun!

    Mark
     
  15. Jun 20, 2008 #14
    ACH --- Fuse --- R1 --- C1 --- ~ + -----|------------A
    ........................................ Bridge......C2........Diode Strings (~ 50 to 100 volts per string)
    ACN -------------------------- ~ - -----|------------C

    R1 - negative temperature coefficient thermistor
     
    Last edited: Jun 20, 2008
  16. Jun 20, 2008 #15
    You want to run the average current below specifications or the diode degrades in intensity and the lifetime is reduced. To be more refined, for any given duty cycle and frequency there is a maximum rated current. The higher the frequency and the lower the duty cycle the higher the current that you can run.

    There's actually a net gain in running AC or repeated-pulse current for most LEDs (but not the white output variety). The eye sees the peak output, so your LEDs will actually appear brighter for the same average current.
     
    Last edited: Jun 20, 2008
  17. Jun 21, 2008 #16
    You guys are just what I needed. Good honest, knowledgable answers from people NOT trying to sell something.

    Mark- That is so simple, it's no wonder I forgot about it. I've be researching and trying to find something to set up 200 LEDs, when I should have been trying to find out how to hook up a few and copy that setup to get my 200.

    Phrak, NoTime-Your both right, I missed the column header that said "Absolute Maximum Rating" when I put in 30mA. When I looked lower on the spec sheet I see it say 20mA is typical for these LEDs.

    NoTime, that's another good point about the internal resistance of the LEDs. I don't see anything on the spec sheet, but I'll test them when they come in.


    Below is the link to the spec sheet, I haven't ordered them yet, but now that you guys have helped me figure this one out, I will defiently be ordering them and testing (read:playing) around with them this week.

    http://www.nichia.com/specification/led_lamp/NSPW500CS-E.pdf

    Thanks again for all the help and I'll post the pictures after it's all done and running.:smile:
    Kevin
     
  18. Jun 21, 2008 #17

    NoTime

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    I'm curious about the point of C1.
    That doesn't make any sense to me.

    C2 isn't necessary since LEDs usually just care about RMS current.
    It will just make the math harder, unless there is something special about the white variety.
    It will also tend to eliminate the pulse mode effective brightness thing you mention.

    Not sure about the point of R1 either.
    Why the thermistor?
    If resistors are required they should be in line with the LED series strings, since there will always be the threshold voltage variance issue.

    IIRC only certain types of white LED have the current lifetime issue others do not.
     
  19. Jun 21, 2008 #18
    Start with a simpler circuit; C1 in series with two antiparallel zeners, all across 120VAC. C1 limits the average current without resistive loss. Now, insert a bridge on the downstream side of the capacitor to get DC current to the zener. It has to be downstream for the DC current in the capacitor to be zero.

    I don't know about C2 either. I threw it in to be complete. It could increase LED lifetime a little.

    There's a nice story about the thermistor. Originally, I gave this general architecture to a college, and forgot about it. He designed it in, sold it, and they blew up in the field! The thermistor is required to limit surge at power-up, should the circuit be turned on near an AC peak.

    I'm sure you know more about LEDs than I do. I was speaking toward thermal degradation over time. Is there actually a total-current lifetime for white LEDs?
     
  20. Jun 21, 2008 #19

    OmCheeto

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    Have you looked into just buying a strand of white-LED xmas lights?
    I got a strand of 70 lights from Ace Hardware for $5 the day after xmas a couple years back.
    You just plug them into your 110. No fuss, no muss.

    Actually, I bought every set the store had. Then I raced off to the store about 4 miles away, only to see some geek nerd with an arm load of them headed for the check stand. Fortunately he couldn't carry them all so I got two more sets.

    My entire house is lit with them now. And I have enough left over to make navigation lights for my boat, and brake and turn signals for my boat trailer.

    Also, from my experience, LED wiring is best done by trial and error. I use a proto-board and start with the "correct" number of LED's, based on the calculations. Then I add and subtract bulbs until the light intensity vs. current flow reaches what I consider to be optimal levels. For instance, with 7 or 8 bulbs, the light level remains the same, but the current flow is less with 8 bulbs. Adding a 9th bulb to the series reduces the current level to the point where the light intensity is insufficient. But you also have to monitor the current flow in the circuit. Lowering the number of bulbs will make them brighter, but you don't want to exceed their maximum current capacity, or you'll have burned out bulbs.
     
  21. Jun 21, 2008 #20

    NoTime

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    C1 isn't necessary then.
    The forward voltage drop with a string of about 32 white LEDs (3.6v) is sufficient.
    Adding it just makes the math more complex and can cause issues with forward drop variance.
    Without C2 you can put one downstream of the bridge.

    R1 -- ok, but it is only potentially necessary if C2 is present.
    I say potentially because other circuit parameters can perform the current limiting function it provides. You need to do a full circuit analysis.
    It seems unlikely to help lifetime issues since peak current per the data sheet kevin071572 listed is 100ma.
    Peak current with a 20ma RMS current is only about 28ma and heating is dependent on the RMS value.

    I don't have a lot of experience with the white ones, but yes my understanding is that the problem with the specified LEDs would be thermal degradation of the phosphors.
    I think the OLED variety has some direct issues with current as opposed to thermal effects, but I'm not entirely certain of this.
     
    Last edited: Jun 21, 2008
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