Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Need power supply for 200 5 volt leds

  1. Dec 20, 2007 #1
    I'm thinking of making a 200 led grow light using red and blue leds(5v). I have little knowlage on electronics. I was wonder if a computer power supply could be used and what other componets would be needed. I'm trying to get this built by late February to start growing my veggies (new project upside down tomatoe plants). Any help for this project would be a big help.
  2. jcsd
  3. Dec 20, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    How much current do the LEDs require?
    Assuming you connect them in parallel and they take around 30mA then you only need 6Amps. a PC power supply should be able to source 6Amps on the 5V line easily.

    You need to short a couple of pins on the connector of an ATX supply to turn it on.
    ps 5V is also available on the Molex 5.25" disk connector.
  4. Dec 21, 2007 #3
    First of all DON'T CONNECT LEDs IN PARALLEL. They don't current chare that well.

    Secondly, what is the total power you want? You could make one on your own or just buy an evaluation board from digikey. Looks like you needs a switching power supply to do your job here.
  5. Dec 21, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper

    Typical practice is to use combinations of serial+parallel grouping
    eg. 5 x 2.1V leds in series run from 12volts and then lots of these 5 LED blocks in parallel.

    Yes, running 200 leds all in parallel would be bad - especialy if one short-circuits!
    Also the 5V led you are using has an integrated resistor, this is wasteful if you are running a lot of them - you are better using the raw parts which will take need around 2.1V for a white led and using a 12V supply (eg a computer PSU).

    See http://www.superbrightleds.com/ for finished systems.
  6. Dec 22, 2007 #5
    The leds I was looking at are 3.5v 30mA with max of 40mA for the blue and 1.9v 30mA with max of 150mA for the red. I was thinking of 150 of the red leds and 50 of the blue.
    The leds were from superbrightleds.com mentioned above. the product codes are RL5-B4630 AND RL5-R1330.
    Sorry for not adding this in my original post.
    Would using other colour leds be helpful in growing plants?
    Are there other companies that supply leds that are made in North America. I like to shop closes to home when I can.
  7. Dec 24, 2007 #6
    You need a 48Watt supply (or50Watts rounded up) as 3.5*0.03*50blue + 1.9*.15*150 Red= 48W

    Now your problem is in what config should you arrange them. Considering your output voltage I would put them in a combo of series and parallel as MGP_PHYS mentioned.

    For blue: If you choose an array of 5 leds in series you have 10 rows (5x10= 50 leds) so the output voltage you need is 3.5Vx5= 17.5Volts. You'll have to select a config to match what supply you have.

    For red: Pretty much the same thing as above

    You need to make sure that you don't exceed the forward current else the LEDs burn out so eithier you have to implement a current sense circuit in your supply or you have to put resistors in each branch to limit the current (this will lead to some losses but is the quicker solution)

    Have fun!

    You can try going to www.digikey.com and search for power supply. You could get something form there.
  8. Dec 25, 2007 #7


    User Avatar
    Science Advisor
    Homework Helper

    If size isn't an issue, PC style ATX power supplies are pretty unbeatable in power/cost.
  9. Dec 26, 2007 #8
    In response to your original question, heck no the computer 5VDC power supply is not designed for the additional load. But I have a question, why are you omitting green? Is it because plants reflect green?
  10. Dec 27, 2007 #9
    Simple, buy an off the shelf 5VDC power supply or have your sissy company buy one for you.
    Last edited: Dec 27, 2007
  11. Dec 30, 2007 #10
    I'm not including green as it does little for plant growth. Photosynthesis is most active in blue (450nm) and red (660nm) light.
    Unplebeian can you give me some more details on what this configuation would look like. As I haven't built anything like this before. Also I picked up 20 cheap white leds to practice with.
    Thanks everybody for your input
  12. Jan 5, 2008 #11
    If the colours aren't important and the leds aren't a part of the experiment, then i'd use a flourescent bulb......
    Its cheaper and less power.

    They have special daylight bulbs that are matched to the suns spectrum....even a decent 'daylight' bulb will give you good growing effects.

    Personally, i think a simple emergency flourescent light from a hardware shop should do.
  13. Jan 6, 2008 #12
    The colours are important. If plants are to grow, they need light energy to create photosynthesis Most plants absorb light in the blue and red end of the spectrum. Some flourescent tubes are okay to use for seedlings and to help on cloudy days, they also emit light in many directions and even with refectors on them a lot of light is not getting to the plants. Led's will emmit light with a smaller foot print and hopefully will be cheaper in the long run. Also this project is just an experiment to see if led's will help to grow my tomatoe plants upside down. If I'm wrong on any of these points can some please tell me.
  14. Jan 12, 2008 #13
    Hey, based on the common voltage which is 12V I would use the following:

    A series and parallel combination.
    Blue: 10 branches with 3 LEDs per branch (3 in series make one branch)
    Red: 25 branches with 6 LEDs per branch (6 in series make one branch)

    I would also put Thermistors in each branch, as if one branch starts drawing more current the thermistor heats up and increases the resistance thus negating the effect.

    If an LED fails from one particular branch then that branch then that whole branch will not be lit up. I don't know how to fix that. But it's been done commonly so take the risk.

    Good luck

    P.S. Try buying a 50W supply with 12V from Digikey.
  15. Jan 18, 2008 #14
    Thanks unplebeian for the help. Will try this and let you Know how it works out.
  16. Jan 30, 2008 #15

    I Keep forgetting how I designed the silly thing, so I've deleted two posts already.
    If you have a transformer of about 40 volts and you aren't going to shock yourself wiring it to the AC!, you can make a very efficient LED power supply.
    No DC power supply is needed.

    Series a nonpolar (not a dialectric capacitor) capacitor to a full wave bridge rectifier's ~~ inputs-- the AC inputs. You could lso series a negative coerricient thermistor with them if you have a fuse somewhere that could blow. Put a string of LEDs right across the DC outputs of the bridge (the ones called (+) and (-)). The size of the capacitor determies the current that flows throw the LEDs. these geniuses, here, could tell you the size of the capacitor.

    For this arrangement you don't need LEDs with built in resistors (LED's that are called
    5V , 'n stuff). You could use white leds (yeah, there are white leds). In fact, don't use the LEDs with the built in resistors; the capacitor replaces then, and without wasting
    power lost.

  17. Feb 3, 2008 #16
    Hi again
    Phrak could you please tell me what type of power supply this is. I would like to read more about it. I need to know about this circuit before I start to assemble it. But it sounds like a nice simply power supply for my needs.
  18. Feb 4, 2008 #17

    I don't know if it has a name; it's just something I dreamed-up as a solution for a simple supply where you don't need isolation ('cause we used it straight off the AC line) and don't have the realestate for anything else. When used as a regulated power supply it is shunt regulated with zenor diodes. But with LEDs, they provide their own regulation. It's prolly been reinvented dozens of times.

    I guess you'll need some parts values, so I'll think about it a bit.

    Last edited: Feb 4, 2008
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook