Yet another LED grow light

Discussion in 'The Projects Forum' started by blackice8r, Dec 19, 2011.

  1. blackice8r

    Thread Starter New Member

    Dec 19, 2011
    Please don't get mad, I did look at other posts they touch the subject indeed by they are not answering my question and I have spent many hours so far on this already.
    I'm posting here because I think my electrical understanding is incorrect and that costed me already enough time and frustration.

    I have two questions, and they might seem trivial to you.

    Question 1

    Why is my old LED array dying, what did I do wrong?

    I used a 12V at 1A power supply, to power 62LEDs (I think, I can double check at home).

    It's a mixture of RED and BLUE LEDs.

    12x BLUE LEDs: FORWARD VOLTAGE 3.0 - 3.2V ;MAX CURRENT 20 mA, wired in series of 3, 4 series.

    60x RED LEDs: FORWARD VOLTAGE 2.0V ; MAX CURRENT 20 mA, wired in series of 5, so 8 series.

    All series have one 1 Ohm resistor at the end, each.
    To design the array I used :

    I think I should have used a 120Ohm resistor for the BLUE that what killed my array?
    They started failing one serie after I only have a few of BLUE about 3-4 series left...
    I spent a full day soldering on this array, my lemon plants are doing great, I have a rose plant that seems to survive (give the roses to your wife when they are spent, cut the flower, leave 4-5 leafs, chip the bark, dip in routing hormone and you might get a rose),the dill and parsley are doing great too. It's all in a small printer box, a bit crowded but it works.
    So imagine my frustration now when they will all die.

    Question 2

    I just ordered 10x High Power Led Helixeon Emitter 1W Blue 24lm,350mA, forward voltage min 2.35V,max 2.65V
    and 5x High Power Led Helixeon Emitter 1W Red 55lm,350mA, forward voltage min 3.25V,max 3.55V

    I have 2x 12V at 1A I can use and 2x 18.5V at 3.5A (old laptop power supplies)

    How do I wire them?

    I discovered that seems to give more accurate and detailed results.
    Could the 12V supply handle al 15 LEDs? I think not...
    If I type in Supply Voltage 12V; Voltage Drop Across LED 2.35V (for the RED ones), Desired LED Current 350mA; and 10 LEDs , it designs an array with 2 series, using 1 Ohm resitors for the series.
    BUT the Actual Single LED Current is 250mA, not 350mA , it said below...
    Is that normal/safe?

    I did something similar for the 5 Blue LEDs; I typed in Supply Voltage 12V; Voltage Drop Across LED 3.25V (for the BLUE ones), Desired LED Current 350mA; and 5 LEDs , it designs an array with 1 serie, using a 6.8 Ohm resitors for the serie.
    Actual Single LED Current 330.9mA, again...not 350mA

    Doing it for 15 LEDs will show Circuit's total current consumption 1654.4mA, so my 12V power supply can't handle 15 LEDs since it give a maximum 1A.
    Also, as far as I know it should work at 80% capacity, so 800mA should be the maximum....

    so no go on 12v I guess

    would a 18v at 3.5A be able to do it? The calculator it's saying yes...
    Can I wire all LEDs, red and blue to the power supply?
    Using the calculator I tend to think yes but not sure if it will be safe for my array:
    - one serie of 7 RED LEDs, using a 6.8 Ohm resistor; ( Actual Single LED Current 301.5mA - that worries me a bit, it's it ok? )
    - one serie of 3 RED LEDs, using a 33 Ohm resistor (Actual Single LED Current 347mA - that worries me a bit, it's to high!!! will it burn my LEDs ??)
    - one serie of 5 LEDs using a 6.8 Ohm resistor

    So again...a mixed array, and I have mixed feelings...
    I tried to give as much info as I could, maybe to much...
    I have a feeling I'm missing something, possibly something important...
    I just spent 22£ on the 15 LEDs, I don't want to kill them and waste my time again...

  2. Georacer


    Nov 25, 2009
    A quick stop while I moderate today's threads:

    About your first question. The wizard told you that you needed an 120Ω resistor for each strand. This is not optional, it is mandatory. Without the correct limiting resistor, the LEDs will effectively short the power supply and get burned out by the excess current. 1Ω isn't remotely enough, you must use the suggested size for your resistor.

    I hope other members will answer the rest of your questions.
  3. KJ6EAD

    Senior Member

    Apr 30, 2011
    If your power supply is well regulated, a single resistor of the correct value on each series string will adequately limit the current to the LEDs. Switch mode power supplies usually are well regulated but many electromagnetic power supplies are not. You need to know your supply voltage at the intended load by measurement, not just by a printed specification on the housing. Have you considered using a constant current regulator instead of a resistor?
  4. blackice8r

    Thread Starter New Member

    Dec 19, 2011
    many thanks for your replies!

    I don't know exactly what a constant current regulator is :(

    the definition would be this I think:A Constant current regulator is a device that regulates current delivery, limiting it to a constant value. These are used with long tail pairs, series lighting strings, some LED lighting, and some other applications. Some IC (integrated circuit) regulators are designed to be used as either constant voltage or constant current devices.

    but I'm not sure if I fully understand it...

    I would appreciate an explanation on how to use it for my scenario.
    I trusted my power supplies are regulated.
    My 12 V power supplies are LED Drivers delivering 1A. And the 18.5v ones (delivering 3.5A) should be too , because they are laptop power supplies.

    I just bought a multimeter and I would be able to see exactly I hope, once i figure out how to use it...

    Georacer was correct, I know know why I messed up my first array. Instead of wiring 4 Blue LEDs in series, so I can use the 1 Ohm resistor, I wired 3 - according to the calculator I should have used a 120 Ohm resistor..

    I don't know about the intended load.
    I've read that the power supply should work at 60-80% capacity. Not exactly sure how to determine the load.

    10x blue 10* ( 3.25V*0.35 A ) = 10* 1.1375 W ? (I think)
    5x red % * ( 2.35V *0.35 A ) = 5* 0.8225 W
    (10*1.1375) + (5*0.8225) = 11.375 + 4.1125= 15.4875 W

    would that be correct for my case? would that be the load of my array?

    On the 18.5V, 3.5A , power supply it states, 65 W , so I guess I'm good? or it's too much? should I get a 24V power supply?

    I've calculated again, using the 18.5V supply on
    for the RED LEDs (350mA, 2.35 forward voltage), 5 in series, I need a 22 Ohm resistor of 2 W
    the calculator is saying: Actual Single LED Current : 306.8 mA

    Doe it sound good to you? Should I be worried because the LED gets 306.8mA instead of the 350mA?

    for Blue LEDs (350mA, 3.25 forward voltage), 5 in series, I need a 6.8 Ohm Ohm resistor of 1 W
    the calculator is saying: Actual Single LED Current : 330.9 mA

    Again, it's not 350mA.

    I'm also using the lower value , Ex: for the blue - forward voltage min 3.25V,max 3.55V, I'm using the 3.25V , is that good practice?

    If you could answer these questions this should clarify my case.
    Last edited: Dec 19, 2011
  5. bwack


    Nov 15, 2011
    Regarding power, could you post a picture of the 12V power supplies showing the ratings? Maybe there is some misunderstanding. The power you can deliver from a 12V 1A supply is 12*1=12W, but since its in use for large parts of the day I would not load it for more than 10W each.. Then you could use two of these for your LEDs.
    The laptop power supply is rated 18.5V 3.5A and is most likely regulated. Power performance is then 18.5V*3.5A = 64.7W. Plenty.

    You have 15W leds + 2*2W power loss over resistors (could be less could be more)..Fits two of your 12V supplies or one 18.5V supply.
    Again this will be used for many hours a day, I'd back off on the current thru the leds, but that depends of how well you will pull heat out of them.

    Haven't though so much about this yet, but you can mix blue and red LEDs in each string.
    You could also use two current source circuit (two npn transistors and two resistors) instead of the two resistors, this will keep your current rock stable even if the supply voltage or forward voltages change as long as you leave some headroom (voltage) for it to regulate. See the pdf in sheldons post here:

    Don't forget to drag out the heat of the led. Mount/Glue them on a aluminum or copper backplate. The best would be a heatsink with fins, but thats probably overkill for <1W LEDs. Glued using thermal glue. I know <1W per LED isn't that much dissipation, but you will appreciate them running cool for longer life.

    About the last question of using the minimum forward voltage for calculation is good practice? Then you are probably have an error in the calculation with voltage forward value is lower than actual, the actual voltages over the led is higher, therefor the actual voltage over the resistor is lower than calculated, wich means less current than expected.
    The typical value is better, but you should use a datasheet and see what the forward voltage is expected to be at the current you decide to run thru it.
    Last edited: Dec 19, 2011
  6. blackice8r

    Thread Starter New Member

    Dec 19, 2011
    "You could also use two current source circuit (two npn transistors and two resistors) instead of the two resistors, this will keep your current rock stable even if the supply voltage or forward voltages change as long as you leave some headroom (voltage) for it to regulate."

    I'm a bit lost here, it sounds good but I can't understand it. What are "npn transistors" ?

    I'm affraid besides some things I've read on forums I have no actual electronical knoledge...

    I will have to ask you to feed me the info as you will feed a new born with a spoon.

    I oredered a aluminium sheet to hold the LEDs and help with the heat, and also 5 more White LEDs.

    So, I have a 18.5V at 3.5 A power supply, regulated.
    10x High Power Led Helixeon Emitter 1W Blue 24lm,350mA, forward voltage min 2.35V,max 2.65V
    5x High Power Led Helixeon Emitter 1W Red 55lm,350mA, forward voltage min 3.25V,max 3.55V
    5x High Power Led Helixeon Emitter 1W White 95lm,350mA,forward voltage min 3.,max 3.8V

    How would you wire them all in one array?
    What resitors would you use, how may in series, what resitors would you use? (or other electronic components, regulators etc)

    And some infor that applied aparently to all the LEDs and for me it doesn't mean much :

    Parameter : 1W
    Peak Forward Current : 500mA
    (1/10 Duty Cycle at 1KHz)
    Continuous Forward Current :350mA
    LED Junction Temperature :120℃
    Operation Temperature :-40℃ ~+105℃
    Storage Temperature :-40℃ ~+120℃
    Soldering Temperature :JEDEC 020c 260℃

    Allowable Reflow Cycles :3 Times
    ESD Sensitivity(1) :> 8,000V Human Body Model (HBM)Class 2 JESD22-A114-B
    Reverse Voltage (V) : not designed for reverse operation

    The seller's lik:
    - the blue ones:
    - the white ones:
    - the red nes - the link doesn't work anymore, out of stock I think.

    So, how would you do it?

  7. Wendy


    Mar 24, 2008
    I have helped a lot of folks with these kind of projects. I would suggest reviewing the latest thread Led Grow Light

    There are quite a few other threads, I have made quite a few schematics.

    The easy way is to use a constant current source (a type of regulator), but it gets very hot and will generate a lot of heat. This is as simple as it gets.

    Another way (if you want to lay out a lot of cash) is a buck converter. It is similar to a constant current source, but does not get hot. It also uses a lot less electricity, but unfortunately they are not cheap.

    The LEDs themselves will also need heatsinking. No way around this, if you don't do it they will burn up.

    Because you linked to a commercial site and are a new user your last post was automatically moderated. Moderated means rendered invisable. I have approved the post.
  8. blackice8r

    Thread Starter New Member

    Dec 19, 2011
    many thanks for the reply, I'm going to start reading on what you suggested.

    I just received the Red and Blue LEDs and the aluminium sheet that I will use as a heat sink, the white LEDs should arrive tomorrow or the day after.
    The 1W leds I received are TINY, a lot smaller that what I expected...I just hope they are more powerful than the cheap 5mm ones I used in my old array....

    I'm still confused, I thought the 18.5V power supply is already regulated, you are saying that I need another device on top, to regulate?
    If the answer is yes, then how much would it be? how do I know what type to use? what about the buck converter?

    I've spent in excess of 30£ so far (not counting my previous array), and I'm still afraid that I might burn all that in a flash if I get this wrong... All for a small box to grow a few greens and see for myself if LEDs indeed work in growing plants.

    I will read the suggested topics, hopefully it will bring some light as now I'm confused. With the calculators for the LED arrays was simple, insert the data, get a representation of the array, solder the components, job done.
    Now it's getting more complex and time consuming :(

    I made a mistake when I gave you the specs for the LEDs, I inverted the values for the forward voltage for Blue and Red LEDs

    It should read like this:

    18.5V at 3.5 A power supply, regulated.
    10x High Power Led Helixeon Emitter 1W Blue 24lm,350mA, forward voltage min 3.25V,max 3.55V
    5x High Power Led Helixeon Emitter 1W Red 55lm,350mA, forward voltage min 2.35V,max 2.65V
    5x High Power Led Helixeon Emitter 1W White 95lm,350mA,forward voltage min 3V,max 3.8V

    Edit: I take back what I said about the LEDs, being TINY and worried they might be rubbish, I just powered now one Blue LED and I'm half blind....
    Last edited: Dec 21, 2011
  9. bwack


    Nov 15, 2011
    Atleast the use of the regulated power supply makes it easier to control current with a simple resistor. See the posts from Bill, much good info.
    I think you may have mixed up the forward voltage info on the red and blue LEDs. I'm guessing typical Vf for blue, red and white is: 3.4V, 2.4V, and 3.6V.

    suggestion for strings (leds in series).
    string 1: 4 blue + 1 red = 16V
    string 2: 4 blue + 1 red = 16V
    string 3: 4 white + 1 red = 16.8
    string 4: 2 blue + 1 white + 2 red = 15.2V

    Lets say we drive them at 320mA. Calculate the voltage over the resistor and divide by the current through it and you should get the resistor value you need. String 1&2: (18.5V-16V) / 320mA is 7.8 Ohm. To calculate the power dissipated by the resistor, use again Ohm's law P=U*I (power equals voltage (over resistor) times current (through resistor)), ~1W..
  10. Georacer


    Nov 25, 2009
    One should account for resistor tolerances too. The closest 5% resistor value to 7.8 is 7.5 and it can go as low as 7.5-0.05*7.5=7.125Ω. That results in a current of 1.5/7.125=350.1mA, barely at the limit. It would be wiser to get a 2% resistor of 7.87Ω and have a minimum value of 7.87-0.02*7.87=7.7126Ω and a max of 324mA.
  11. blackice8r

    Thread Starter New Member

    Dec 19, 2011

    I'm starting to understand that!!

    Thank you very much!!

    I always thought that it's safer to keep Blue with Blue, Read with Read, White with White because they have the same forward voltage, but I guess the overall value of the string is the one that matters.

    How is the 3.5A given by the power supply fit in all this? Only on the overall calculus of the power?

    What happens if one LED fails? The string stops working I guess, would that put more pressure on the remaining strings and kill them?

    many thanks to Georacer for the calculus check!

    of course Bill_Marsden , many thanks to you too, I will start reading what you suggested, I'm at work now and I'm too tired to start now, but I will, soon.

    Looking at the LEDs I just received, I see they have a small piece of metal at the back, I'm guessing that is for the heat transfer. I will need to create some base plates for the LEDs (plain cardboard will do? or does it have to be something that can't ignite ?) , use thermal paste to stick that end on to the aluminium sheet used as heat sink. My plan was to drill holes in the aluminium sheet and stick them in, but then how would the heat dissipate ? silly me :D
  12. Georacer


    Nov 25, 2009
    The current rating of your supply will dictate how many strings you can attach to it. For a safe 3A supplied, you can have at most 3/0.35=8.57, let's say 9 strings of 0.35A each. No more.

    I think diodes (and LEDs as such) fail shorted, which will mean that its voltage drop will be transferred to the resistor of the string. In the first string, for example, if a blue LED fails, the resistor will have a 2.5+3.4=5.9V on it, resulting in a string current of 750mA. It way to much for your other LEDs. That said, you may want to find the maximum string resistor value for an adequate brightness. That way, you may have time to notice the burnt LED before the others bite it too.

    However, only the LEDs of the same string are in peril. Next in line is your power supply that will see a very small 7.8Ω resistor after all 5 LEDs burn. For that reason, a 3.5A fuse in line with your power supply output would be a wise choice.

    This is where a constant current supply would save your project, but if I were you and would like to keep things simple, I 'd go for the simple solution of the fuse too.
  13. blackice8r

    Thread Starter New Member

    Dec 19, 2011
    0 makes sense all you have said, it turns out I'm not that thick after all

    the fuse suggestion sounds good, I have a feeling I didn't get the "constant current supply" part.

    I was under the impression that being a laptop's power supply the one I'm going to use will provide constant current anyway...was I wrong?
    If I was how to I can achieve that?

    I got something that will show you how pitiful I'm at electronics....might make you laugh a bit
    see my now dying LED took me about a day to solder all that...
    Plastic board , hole made with a soldering gun, made it curve a bit, covered on one end with aluminium foil to reflect light.
    And then the box...reused printer box






  14. Georacer


    Nov 25, 2009
    I see no problem with the construction quality. Wouldn't you need some air holes for your plants?

    About the constant current source. Most appliances need a constant voltage to operate. Their power supplies deliver just that, constant voltage. Your laptop power supply will give a constant 18.5V and vary its current accordingly to its load, in order to preserve that voltage.
    If you use it to supply your LED array and a string fails, then the effective load will be close to 0. 18.5V over 0Ω results to a lot of current, bad for your power supply.

    On the other hand, there are devices that give constant current, and vary their voltage output to achieve that. If you need 3A for all of your strings, you will chose a current source of that magnitude. If a string fails, then no excess current will be drawn. But, all of the current will go through the failed string, probably burning something. However, that something will be one LED or resistor and just that, not your whole supply. Smaller fuses in each string would be meaningful in that case.

    The idea of individual fuses in each string is more beneficial in the constant voltage source, where if a string fails, no change in the voltage or current of the other strings will occur.

    However, I 'm not convinced you will need so much protection, size your resistors within safety limits with some tolerance and you should have plenty of working hours with them.
  15. Wendy


    Mar 24, 2008
    You can use the resistors to measure the real current. It is basic Ohm's Law.

    I = V / R

    Measure the voltage across the resistor and plug the number into the above equation.

    I would not accept more than a 10% variation from your target current.

    You're plants are going to need air, just as you or I do. If you must enclose them either put air holes or force feed some air with a fan.
  16. bwack


    Nov 15, 2011
    Soldering looks good to me, just keep in mind not to short those naked supply wires with the alu-foil :) You could use electrical tape underneath the naked wire, and strip the insulation of those two supply wires much closer to the circuit?

    If you have lots of those resistors laying around, you could parallell them to get close to 7.8Ohms and to divide the power dissipation between the resistors. The resistors look like 1/4W resistors, then you could use four of those in parallel, or if they are 1/2W resistors, you could use this sheet of calculated parallel resistors for selection. Its easy to make. I used conditional formatting in openoffice to get yellow background on the values within 1% of 7.8 Ohm.

    Its vice to select the pairs of resistors that have the most equal values. In this case that would be 15 and 16 Ohms. This to make sure you distribute the heat most evenly between the two resistors.
    Last edited: Dec 22, 2011
  17. blackice8r

    Thread Starter New Member

    Dec 19, 2011
    ehehe..that array was the initial set-up, I isolated the wires and all the joints that could have created a short-circuit. I used adhesive hot glue to keep the wires from moving around, I even used the glue on the resistors...bad idea! maybe that accelerated the death of my array..

    My plants have have air holes , I have two fans: intake (bigger) and exhaust (smaller) but if I power them the plants think there is a tornado inside the box so they are not functional yet... The plants are small and I open the box every day so they are ok for now
    I have to lower the voltage for the two power supplies that power the fans from 12v to 7.5V I think, to make them spin slower...

    The resistors I used on the old array are indeed 1 Ohm (looking on my e-bay's account history) : 1 Ohm 0.25w Carbon Resistors 1/4w Resistor 1R
    I got a lot of them lying around, they came in a batch of 100.
    Placing then in parallel is a very good idea, would save me some money, cheers!

    I've been reading Bill_Marsden's posts on LED's , some info I managed to digest, some other doesn't stick to my brain yet...

    I've done some reading on the Buck Converters and laptop power supplies , apparently the laptops use a buck converter to regulate the voltage inside, as Georacer was saying...the power supplies are not regulated...

    I found a Buck Converter on e-bay, £3.29 coming from Hong Kong :

    LM2596 DC Converter Power Supply Buck Step Down Regulator In:4-40V Out:1.5-35V
    Features :
    --Input: DC 4V to 40V (input voltage must be higher than the output voltage to 1.5v or more. Can not boost)
    --Output: DC 1.5V to 35V adjustable voltage, high efficiency, maximum output current of 3A.
    --Features: 36u thick circuit boards, high-power high-Q inductors
    --Size: 33mmx33mmx14mm
    100% Brand New
    1x DC/DC Converter
    2x Cable

    Looks good to you? fit for the purpose?

    I'm hopefully going to have some quiet time to read everything again and start making some decisions and a shopping list that I will run thought you.

    thank you for all the support you have given me, much appreciated!
    Last edited: Dec 22, 2011
  18. bwack


    Nov 15, 2011
    I think as Georacer said before that you must have enough resistance in the strings. You probably had them run too hot and then they burn out. I've seen resistors connected to 1W LEDs for a RGB moodlamps (similar to your 350mA leds) potted with glue gun. Hasn't been a problem for him. But it depends on how much heat he needs to dissipate if it is to be a problem.
    (scroll down to the bottom of the page and you will see the potting).
    Btw I made this lamp myself as a present for my father. He still uses it after 3 years. (I didn't pot my components).

    Careful, if you parallel two 1 Ohm resistors you get 0.5 Ohm, you parallel 4 and you get 1/4 Ohm etc.
    You want 7,8 Ohm. You could solder eight of those 1 Ohm resistors, in series. In series you just sum them together and you get 8 Ohm, and for power rating, you get 1/4 W * 8 = 2W (1W more than you need and is good). Not bad that either except you would need many (not bad that either hehe). Its not always I have the correct resistors laying around. So the other day I wanted to try out the circuit that sheldon had used. I needed 33.3 Ohm, so I parallell 3 100 Ohm for that and got 33.3 Ohm. I also needed over 500 Ohm, so I put three 180 in series and got 540 Ohm.

    I did some messuring on this circuit. What you see on the bottom (transistors and R2) is a constant current configuration. I shorted out one of the led so that the voltage would increase, and still the other led got the same current :) and I also tried increasing from 5V to 12V, still almost the same current thru the led, and since this only was below 20mA, the transistor in series with the led didn't get hot.. for 320mA I think you may need a larger transistor, if you would use this kind of circuit.

    Oh btw, the talk about 7.8 Ohm was only valid for string 1 and 2.
    Last edited: Dec 22, 2011
  19. Wendy


    Mar 24, 2008
    You can do this with 2 components, a LM317T (which is available from Radio Shack, and much cheaper from anyone else) and a single resistor. If you want to make a better version this schematic will work well...

    [​IMG] .. [​IMG]

    The point of current regulation is you are freed up from variations in the power supply, LEDs, and for the most part, resistors. It will adjust the current in a active way, just plug and go.

    The current mirror has exceedingly low dropout voltage, a good thing. It will use less than .5V of the total to regulate, or less. This means the Vf of all the LEDs added up can be just ½V from reaching the power supply voltage and this regulator will still work.

    You can add as many legs on the Current Mirror as you want. For simplicity I would still recommend the LM317, or a simple resistor as you are doing.

    If you use a resistor do the current check I recommended in post #15.
    Last edited: Dec 22, 2011
  20. blackice8r

    Thread Starter New Member

    Dec 19, 2011
    Hi guys,

    Sorry for the very very late reply.

    I wanted to thank you all for the advice and to show you the results.

    I didn't feel like jumping on something big so I did a small LED array.

    For the LEDs I used:

    - 6 x High Power Led Helixeon Emitter 1W Blue 24lm,350mA, forward voltage min 3.25V,max 3.55V

    - 3 x High Power Led Helixeon Emitter 1W Red 55lm,350mA, forward voltage min 2.35V,max 2.65V

    Power supply: LED driver 12V , 1 A

    They are setup in 3 strings of 3 LEDs, 2 Blue + 1 Red
    Each string has a 10 ohm resistor ( I had lots of 1ohm 1/4W so I used 10 in series)

    2x 3.25V + 2.35V = 8.85 V it's saying , the current trough the LED is 315mA, so it should be fine

    Sounds ok to you?

    I made this for my yucca plant but I'm using it for now to bring up my cacti (in the propagator now)
    The next array will be much bigger as my cacti will need more light as they grow.

    (thermal paste in between the LED and aluminium LED base, and then thermal paste in between the aluminium base and the bigger aluminium sheet, the bigger sheet is warm after hours of usage )