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LED lights wiring advice

  1. Jun 29, 2015 #1
    Ok, so please folks could you advise me which way would be better.

    I've given a job to build a LED light.the light is a cube shaped lamp which i built myself and put all the diodes in , and now I have to think which way of wiring them would be the best.basically I have 98 red colored 3w led's. around the corners for effect there are some 12 blue ones and 6 orange.all of them are 3w rated power ,
    now I checked the datasheets and they say that the red ones have 700mA of current and 2.0 to 2.5 of voltage drop across.
    the blue and orange ones have the same 700mA but a bit higher about 3.3 to 3.5 volts across them.

    the dilemma here for me is how to wire them given that i have only few choices, first one is to use a pc power supply and somehow arrange the leds parallel in chunks and use the 3.3 and 5 and 12v lines from the psu or just wire them all in series and use a higher voltage DC source , the latter case would be easier.
    I could take the mains 230v and rectify then filter with some caps and I would get 325v DC adding some leds to the count would make them enough to be able to put 325v across them.in this case I would use a 1:1 isolation transformer before the bridge rectifier.

    I'm bothered with this because the pc psu way would be better to my mind but due to the difference in the LED forward voltages and the voltages I can get out of the pc psu I would have to use some current resistors in series and they would waste quite some power and also take up space and heat up.

    what's your advice given the numbers i gave you?
  2. jcsd
  3. Jun 29, 2015 #2


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    Hi. In order to power the LEDs the best practice is to utilize constant current LED driver power supply with adjustable voltage level at output. As all mentioned LEDs have 700 mA so when you connect them with this type of power supply it would automatically adjust voltage level while keeping current constant at 700 mA. Now suppose if power supply voltage rises then with V=IR relation current would also try to rise. However as this power supply is of constant current regulation type and hence would lower the voltage to keep current constant and vice versa.
  4. Jun 29, 2015 #3


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    To keep the voltages safe (and within the 12V output of your PC power supply, you will use groups of 4 Red LEDs in series, and groups of 3 blue & orange LEDs in series.

    Agreed, this is the best way to drive LEDs.

    But, if you do some math, you will find that the PC power supply is going to have to be pretty big to power all of those LEDs. If you use a bunch of parallel strings, each with its own CC driver, you will get about (96/4) + (12/3) + (6/3) = 30 strings, each with 700mA. 30 * 700mA = 21A at 12V, or about a 250W power supply. That is a big PC power supply, but maybe do-able. There are also some 12V-only versions of PC power supplies that I saw mentioned in this wikipedia article:


  5. Jun 29, 2015 #4


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    You should do a search to find LED Driver ICs that can accommodate the 700mA drive (or sink) current. Some of them can drive multiple strings, but watch your power dissipation -- you may do better to use 30 individual LED Driver ICs with adequate heat sinking (and probably a fan...).
  6. Jun 30, 2015 #5
    thanks for the replies, the thing here is that I'm doing this for another person , and he wants the thing to be rather cheap as he already has spent some money on the led's and other parts.this is not for home but for garden lights.

    as you said berkeman , I was also thinking od dividing them in series groups and then connecting those groups in parallel once each group has enough diodes in series to match the voltage of the psu.for example to fully utilize all the pc psu outputs , I could do some red LED's just in parallel and connect them to the 3.3v output adding a series resistor.as the 3.3 has quite alot of amps , on average some 20A of current , then the 5v line has good current also so i could do some red ones, two in series and then those two series in parallel to match the 5v.

    what do you think about the mains rectified case? if one uses some good isolation and probably a UPS, the only drawbacks would be that if one led burns out all of them stop shining and you have to replace the broken one and also since some of the LED's have different voltage those would be dimmer than the majority of red ones since the other ones have a higher voltage rating , I assume.
  7. Jul 1, 2015 #6


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    Please don't do this. It is dangerous. With or without 1:1 isolation.

    As to the voltage rating it dies not matter if you need equal currents. As long as the input voltage is sufficiently higher than the total string voltage to accommodate the drop across the limiting resistor then you will have the same current in each of them. One issue you might have is that you may want to have different currents through the different colors to balance the brightness. Then you must have only one color per string. Also use one resistor per string.

  8. Jul 1, 2015 #7
    Is isolation from the mains necessary? For comparison, are incandescent garden lamps typically isolated?
  9. Jul 1, 2015 #8


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    If by incandescent garden lamps you mean the small buried wire pagoda lamps then yes they are. Or at least they should be. You would want them isolated as a single wire short to the soil will not cause a hard failure. Cheap imported crap might not be isolated.

    If you mean standard Edison type incandescent lamps used in the household then no they are not isolated and they represent a shock and fire hazard when broken.

    The problem with non isolated power systems is that you can get a fault in the active components that will energize the user accessible parts to mains voltage and that is generally a bad idea. Just think if your computer mouse got up to mains voltage with only ~30V insulation between you and it. There is usually two layers of insulation in the transformers in addition to the winding insulation.

    A hand built craft item is unlikely to have built in safeguards that would make it safe using an un-isolated mains supply or any mains supply for that matter (no offense to the OP). By design some parts of the lamp will be directly connect to the high voltage input voltage. This represents a hazard. If one component fails open then potentially the whole thing could be at the input voltage. If something fails short then the stress increases on the other LEDs and this could cause a cascade failure and a fire.

  10. Jul 1, 2015 #9
    rbelli, it's fairly implicit that the OPs customer wants a cheaply produced, marketable, retail grade device, so that isolation is debatable. I would explore and present alternatives that may present themselves.

    I'm inclined to think this a customer with more money than sense. But the customers pays the bill.
  11. Jul 2, 2015 #10
    Well no worries , I kinda got it sorted out., the customer is not so much a customer as my friend.
    Well yes the mains rectified idea is kinda dagerous but there is nothing bad about it if one handles it safely.my friend once built a house lamp from white power LED's. Since the lamp was hanging at the ceiling it was ok.He just took the necessary amount if white led's to represent the series voltage of a rectified mains 230v which turns out to be 325v DC and the Led's shine just normal. and if one of them burns out instead of getting higher voltage on others you get none because the series connection gets interrupted.

    In this case I took the LED's and piled them in chunks of 4 and 5 leds , each chunk is connected in series so that by making 4 or 5 led's in series i get the necessary voltage +12v.then I put the chunks in parallel.
    I will use a self made transformer , by self made i think I wound the right amount of turns on the secondary to make the output voltage after bridge rectifier +12 volts.
    I see ebay sells some cheap 12v 300 to 600w diode power supplies so in the long run I guess I will advice to buy one of those.
  12. Jul 2, 2015 #11
    Have you considers a 12 or 24V Power supply for Garden Lighting? It is designed just for this(outdoor - wet environment), mass produced ( so good cost point) and offers the isolation you are looking for, reduced the Voltage to a safer level - etc....
  13. Jul 2, 2015 #12


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    Those are usually AC, though, aren't they? Just a step-down transformer, I believe.
  14. Jul 2, 2015 #13
    Well I could use just a transformer and then rectify the output but since the smps power supply from ebay is so cheap and promises the needed power output I kinda lean towards that choice.
  15. Jul 2, 2015 #14
    Sure these are usually AC-- but if you do a little investigation I'll bet you can overdrive them at 50% duty cycle - in a anti parallel set up an they will appear brighter - or just throw a $10 20A rectifier at the set up. Still keep in mind the LED datasheet values may be at 25 Deg C - if you are putting this outside - what is the hottest ambient temp they will see - get this wrong and you will kill the whole lot.
    Regarding a SMPS power supply - is it isolated?, that is usually a large cost factor, and if you plan on putting it outside, there will be leakage.

    Ether way -- the more you know about the problem (question) the better the solutions ( answer)
    Last edited: Jul 2, 2015
  16. Jul 2, 2015 #15
    Oh - as a follow up to the overdrive and duty cycle - have you ever looked at the tail / brake lights of the car in front of you and wondered if they are LED? -- Sweep your vision back and forth quickly - and if they appear to strobe they are LED.
  17. Jul 2, 2015 #16


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    I have noticed that and it is distracting to me.
  18. Jul 3, 2015 #17
    oh by the way , I calculated the led current and voltage needs today after I finished soldering them together.And I have 13 strips of 5 led' s in series , then I have 10 strips of 4 leds in series and then 3 strips of 3 led's in series.the voltage on the latter one is higher (different color) that's why it has only three in series.
    Now each led has a current of 700mA , so each strip connected to the 12v supply draws a current of 700mA or 0.7A.
    after calculating all the strips together , I got that the whole set would draw about 18 amps at 12 volts.
    in this case I actually think about using a pc power supply.It has isolation and also quite ok regulation for my task.

    so I have this question (not sure whether to open new thread or ask here) I have a pc power supply and it is a 400w unit.it states that it has two 12 v rails, rail 12v1 and rail 12v2, one rail has stated output of 10A and the other has 14A, now each of them separately is insufficient to power my setup but if I could just pin them together in parallel they would be ok.
    So the question is can I pin them together in parallel ? As long as I know the majority of two or more rail power supplies actually use a single transformer and single rail just split in two rails with a current limiter and safety on each of those split rails , also I took a multimeter and probed the yellow wires (12v + rails) and they all show short circuit to one another , so I assume their basically just one output winding labelled as two rails while in reality they are just one.
    What do you think?
  19. Jul 6, 2015 #18


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    You have 116 of these leds crammed into a small cube shape? What dimension is your cube? What material is it made of? Are you planning to power it continuously, or just flash all the leds on for a few seconds?
  20. Jul 6, 2015 #19
    It will be continous as a lamp.I did calculate the current need and I turned up with about 19 amps at 12 volts.This should fit in the atx psu range.atleast if i can trust the specifications atleast approximately.

    as I said I do believe both 12 volt lines atleast on the cheapt and middle segment psu's are the same line just separate safety circuits, please comment if you know otherwise.

  21. Jul 6, 2015 #20


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    I believe what Nascent was referring to was the heat output of the LEDs. Even though they are very efficient as far as light generating devices go, they are somewhat under 20% efficient for current technology. More than 80% of the power you put in will come out as heat.

    You are building at least a 182 Watt toaster. Can it dissipate the heat and keep the LEDs in their comfort zone? Can the materials withstand the temperatures?

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