LED Light Therapy - Array of LEDs on PCB | How to?

Discussion in 'The Projects Forum' started by UncleGarage, Sep 29, 2012.

  1. UncleGarage

    Thread Starter New Member

    Sep 28, 2012
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    Greetings & Salutations,

    I am trying to make an LED light Board if you will. I want to pack about 100-200 LEDs onto a PCB board. The intended use case is described in this video. I cannot figure out a few basic things which is understandable since I am a n00b.

    1. How does one apply power to a PCB? Is there designated bus? I've seen people attached aligator clips to some but I cannot get this to work.
    2. I think I want to deploy the LEDs in parallel allowing the other LEDs to function if one goes out. Series=one big loop where one point of failure causes everything to fail doesn't make too much sense.
    3, I'd like to have portable PS like a battery but I am not sure how to calculate how much power my 100 LEDs simultaneously require. Can I power 100 LEDs of some type of battery?
    4. The longer-lead on the LED is the POS as I understand it. Do i have to put these in a certain way on the board?
    5. Do I need resistors?
    6. Do I solder the 2 leads to solely to the PCB or do solder them to the board and each other? If so, then POS lead to POS Llead?

    Right now I've only been able to solder 4 LEDs to my board. I did so in 2 sets with 2 LEDs each. 2 are connected horizontally and 2 vertically. I can take my 22AGE leads from the PS and make the LEDs light up by touching the leads to the solder leads from the LED. I can get them all to light up but not all at once.

    I thought I would be able to somehow take my power supply leads and attach them to the PCB would would then supply power to all LEDs on the boards.

    Parts:

    Grid-Style PC Board with 750 Holes Radio Shack 276-0158
    LED Uni-Color Red 660nm 2-Pin T-1 3/4
    Power Supply: 2AMP Regulated DC PS MW122A | Set @ 3V
    22 AWG solid wire

    Here are a few photos of what I'm doing. Warning...very rudimentary.

    [​IMG]

    [​IMG]

    [​IMG]
     
  2. MrChips

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    Oct 2, 2009
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  3. DickCappels

    Moderator

    Aug 21, 2008
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    If I may, below is the URL of a page that describes of one of several similar arrays I made. It should answer most of your questions.

    http://cappels.org/dproj/Array_of_180_UV_LEDs/Array_of_180_UV_LEDs.htm

    With all arrays I have made, from a simple 18 LED IR array to this 180 LED array, the major consideration has been heat; how to keep the LEDs from overheating and how to keep the circuit board from getting too hot to handle. I think the technique of leaving the LED leads long, as shown in the video, is good for keeping the board cool, but perhaps a little airflow through the LED leads will help keep them cool, and their light output high.
     
  4. UncleGarage

    Thread Starter New Member

    Sep 28, 2012
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    Thank you both for your replies. I've been reading the information you have recommended and have arrived at the following assumptions -

    My LEDs have a max fwd current of 40mA and max fwd voltage of 2.4v.
    The PS is a 12V.

    12v - 1v/2.4v = 11v/2.4v = 4.58
    round down to arrive at 4.0 maximum LEDs in a series.

    To calculate the correct current-limiting resistor -

    12v -(2.4 * 4) / 40mA
    12v - 9.6 / 40mA
    2.4/.04 = 60
    rLimit = = 60 Ohms

    I've learned how to calculate how many LEDs I can have in a series and how to calculate the resistor I need to run them in a series. This is both exciting and alarming. Alarming b\c I stayed up far too late last night reading about this subject and lost track of time ;)

    Can someone please tell me if I have done this correctly?

    Thanks
     
  5. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Your calculations are mostly correct but some of your design assumptions are not.

    If you look at the data sheet for your LEDs, you'll see that the Vf is indicated as a range from 1.5V to 2.4V with a current of 20mA at 25°C. The 40mA figure is an "absolute" maximum beyond which damage occurs and as you exceed 20mA and approach 40mA, the life of the LED is reduced.

    When you densely pack LEDs on a board you get some mutual heating so you need to derate the current to compensate for operation at a higher temperature. Similarly, you need to derate for any enclosure or other impediment to free air cooling.

    Your power supply is regulated at 12V so you need to use that as your design voltage. If the voltage sags under load, your LEDs will be less bright but undamaged. On the other hand; if you design using 11V and the supply runs at 12V, your LEDs will have a shorter life.

    The data sheet doesn't give a typical value for Vf at 20mA but it's probably around the median between the two range extremes given (1.95V). You can set up a 20mA constant current regulator on a breadboard and measure the Vf of a random sampling if you want to characterize the actual values.

    If I were doing this project, I would use 12V Vs and 1.95V Vf for calculation, design for 20mA and make sure that the voltage dropped by the limiting resistor was at least 10% of Vs.

    String length: Int ( 10.8V / 1.95V ) = 5

    Limiting resistor: ( 12V - ( 5 X 1.95V ) ) / 20mA = 112.5Ω :rolleyes:

    120Ω is the nearest higher E12 value so: ( 12V - ( 5 X 1.95V ) ) / 120Ω = 18.75mA. :)

    In the unlikely case that you're unlucky enough to have all 5 LEDs in a string have a Vf of 1.5V, the current would be 37.5mA, dangerously close to the absolute maximum. :(

    In the equally unlikely event that all 5 LEDs had a Vf of 2.4V, obviously there wouldn't be enough voltage to get anywhere near 20mA. :(

    You can compensate for this uncertainty by decreasing the string length and increasing the limiting resistor accordingly, but this decreases efficiency (more resistor heat) or you can avoid uncertainty by testing the LEDs.
     
    Last edited: Oct 2, 2012
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  6. DickCappels

    Moderator

    Aug 21, 2008
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    Very good advice. One other suggestion: Be prepared to adjust the values of the resistors if you are running with only a small voltage across the resistor because the variation in LED voltage can be significant (for example if you have 1 volt across the resistor in the nominal case and all four resistors are 01. volt high, your current will be reduced by 40%.
     
  7. UncleGarage

    Thread Starter New Member

    Sep 28, 2012
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    Thanks KJ6EAD for your detailed reply and thanks DickCappels for your comments.

    I did not realize there was an operating range but now I better understand. From looking closely at the data sheet I indeed see that the Vf min is 1.5 and max is 2.4 @ 20mA. I believe this to mean that the minimum vF to make the LED light up is 1.5 vF and the max is 2.4 at which point the LED will burn out.

    Question: How do I know what the optimal vF is in order to ensure max longevity of the LED? I think you answer my question when you mention the median and 1.95 vF.

    According to your revised calculations I need to use a 120Ω resistor and each string will have 5 LEDs. Does this mean that I need 20 strings of 5 LEDS each + a 120Ω resistir on each string?

    I am not clear at all on how a string would be assembled. Do i take any PCB board with enough space for 100 LEDs, pull the cathode\anode leads of the LED through and solder the leads together pos to pos and neg to neg on the flip side of the board? Where do I place the resistor and most importantly how do I apply power from the 12 volt PS to the board and\or the LEDs?

    Apologies for needing to be spoon fed here and thanks for your time and patience.

    Regards,
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    No.
    They cannot make the LEDs all the same. Some will have a forward voltage of 1.5V, some will have 2.4V and others will have a voltage anywhere in between when they are operating normally with a current of 20mA.

    So if your LEDs have a low voltage then the simple resistor current-limiter will cause their current (and brightness) to be high. If your LEDs have a high voltage then their current (and brightness) will be low.
     
  9. KJ6EAD

    Senior Member

    Apr 30, 2011
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    I'll elaborate on what the AudioGuru said and hopefully clear up a misconception you seem to have. The Vf (Voltage forward) of an LED is not a circuit parameter that you can control by your design. It's a manufactured property of the component that is difficult to precisely control in the manufacturing process. To use a crude analogy, it's like trying to make a bunch of snowballs all the same size by hand. You end up with a stack of snowballs ranging from 2" diameter to 3" but most are around 2.5". When you write the data sheet for the snowballs you indicate the range and if you're a good writer you include the 2.5" number as a "typical" value.

    I alluded to one method of testing the LEDs. Do you have a breadboard and a multimeter?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    I do not believe in "typical" values. They occur only if there are a lot of items in stock made at different times.

    But when the stock is sold then it is replaced with new ones that might be from a single production that are mostly low or are mostly high, not "typical".
     
  11. UncleGarage

    Thread Starter New Member

    Sep 28, 2012
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    OK. That was very helpful KJ6EAD. I see now that there is an operating range of Vf with the low at 1.5 and high of 2.4 @ 20mA.

    I have a Fluke 75 Series II MM but it is old and I don't know if it is even deigned for this type of application. Are a breadboard and a PCB one in the same? I have a PCB from RadioShack - photo up above in OP.

    My next goal is to obtain the required PCB or BB, resistors and figure out how to assembly my strings of LEDs + resistor. Where can I learn how current if applied to the PCB\BB? I've seen various video where aligator clips are used but there is little detail provided as to how this works.

    [​IMG]
     
    Last edited: Oct 8, 2012
  12. KJ6EAD

    Senior Member

    Apr 30, 2011
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    That meter is perfectly adequate. This is a breadboard:

    http://en.m.wikipedia.org/wiki/Breadboard
     
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  13. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Interesting reading: " Let There Be Light" by Darius Dinshah. A Practical manual for Spectro-Chrome { Light ], therapy. DINSASH HEALTH SOCIETY, P O Box 707, Malaga NJ 08328 USA. Phone No ??, 609-692-4686.
     
  14. UncleGarage

    Thread Starter New Member

    Sep 28, 2012
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    So, it looks like I use a breadboard to model, ticker and tweak my project. When it's ready I can create a permanent version on a PCB. Is there a reason why I would not just permanently use a BB?

    I am going to RadioShack to p\u a breadboard and assume the only thing I need to worry about is that it is big enough to accommodate my 100 LEDs. Here is the one I plan to buy - http://www.radioshack.com/product/index.jsp?productId=2734155#

    I researched and found good explanation of BB vs. PCB. I'm posting this for the sake of anyone following in my footsteps trying to sort this all out.

    Breadboard:

    A breadboard is a rectangular plastic box filled with holes, which have contacts in which you can insert electronic components and wires. A breadboard is what you use to string together a temporary version of your circuit. You don't have to solder wires or anything else; instead, you poke your components and wires into the little contact holes arranged in rows and connected by lines of metal; then you can connect your components together with wires to form your circuit.

    The nice thing about breadboards is that you can change your mind and replace or rearrange components as you like. You typically create an electronics project on a breadboard to make sure that everything works. If it's a project you wish to save, you can create a more permanent version.

    If you want to create a permanent version of your circuit, you need to create a soldered or printed circuit board; see below to find out how to go about that.

    Printed circuit boards:

    If you create a circuit on a breadboard and decide that it's worthy of immortality, you can make it permanent by soldering components in place on a printed circuit board. To do this, you have to get your hands on a universal printed circuit board. This is much like a breadboard except that you can solder all the connections you've made to keep them around.

    A universal printed circuit board has rows of individual holes throughout the board with copper pads around each hole and metal lines connecting the holes in each row, like in a breadboard. You mount parts on the face of the board and then pass leads through holes to the components. You can solder the leads to the copper pads on the bottom of the board.

    Universal printed circuit boards are available in a variety of patterns of contact holes and metal lines.

    You can get custom printed circuit boards made for your circuit; this is typically done by submitting a drawing of your circuit to a printed circuit board company. These boards eliminate the need to solder jumper wires between components.
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    The many contacts in a breadboard become intermittent after a while.
    The messy tangled connecting wires all over a breadboard pickup mains hum amd other interference in audio and video circuits.
    The high capacitance between the rows of contacts and connecting wires of a breadboard cause an audio or video circuit to oscillate and prevent proper operation of radio circuits.
     
  16. UncleGarage

    Thread Starter New Member

    Sep 28, 2012
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    AudioGuru,

    Judging from your comments it sounds like you agree with my post highlighting the deltas between BB and PCBs. From my perspective, using a BB makes a lot of sense to model\test a project and the ability to do so w\o soldering certainly makes it easier.

    I guess all of the interference you mention can be overlooked or at least temporarily accepted while modeling a project on a BB. I wonder how I can use the multimeter to see how efficiently\inefficiently my project runs.

    Question: Does the noise introduced by the BB have any effect on the vF of an LED? I understand that vF is determined at the time of manufacture but wondering if a BB shortens or otherwise adversely effects LED performance? I guess it does not matter b\c I will port the project to a PCB.

    Question: I still don't understand how I apply power from my PS to the BB. Do I do this with aligator clips?

    [​IMG]
     
    Last edited: Oct 9, 2012
  17. Audioguru

    New Member

    Dec 20, 2007
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    You can use a new breadboard for LEDs if you cut the longer pin of each LED the same length as the other pin then observe that the cathode has a flat spot on the rim of the housing.
    Noise and interference affects low level circuits not LEDs that use a few volts.

    Do not poke a wire that is too big into a contact on a breadboard or you will ruin the contact. Solder proper size wires to the ends of the wires from your power supply.
     
  18. KJ6EAD

    Senior Member

    Apr 30, 2011
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    You don't need a breadboard big enough for all your LEDs, just big enough to test 5 LEDs and a resistor.
     
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  19. UncleGarage

    Thread Starter New Member

    Sep 28, 2012
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    Indeed KJ6EAD, that makes a lot of sense :D
     
  20. Wendy

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    Mar 24, 2008
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