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

Question about Non-Linear LED Loading

  1. May 6, 2016 #1
    I often create LED arrays, and have encountered a common issue where, as the LED load is increased, the total current requirement is not a linear factor as each branch (LED strip) is added to the circuit.

    It is common to have many branches of LED strips on a 12VDC 60W power supply.

    LED Strip Non-linear Loading Diagram.png

    Power source: 12VDC 60W power supply

    Each led strip is rated and measured at 3W, individually.

    Here is the phenomenon in question:

    As each LED strip (branch) is added to the circuit, the load is not a factor of the each branches power (3W) by the number of branches. The diagram shows 24 branches as an example. If one branch is connected, then it is in fact, measured to be 3W. Using a simple linear calculation, 24 branches x 3W is 72W. However, the actual 24 branch circuit draws only 3.88A which is 46.56W (at 12V).

    I connected and measured from 1 to 58 strips and plotted the results. The resulting function is definitely very non-linear. I could place 58 of the LED strips on the circuit before I reached the 60W limit.

    Does anyone know the Theorem or equation explaining this non-linear loading phenomenon?

    I would greatly appreciate anyone's feedback on this. Thank you!
     
  2. jcsd
  3. May 6, 2016 #2

    anorlunda

    User Avatar
    Science Advisor
    Gold Member

    Can you tell us more about the 12V 60w suppply, and about the wiring. One suspect is that all or some of the 3w strips are not getting the full 12v as total load is increased.
     
  4. May 6, 2016 #3
    Check for resistance in the wiring leading up to the strips, and from strips to ground.

    Alternatively, If strips dim as others are added, then you are approaching the limits of your power supply.
     
  5. May 6, 2016 #4
    The power supply is a 12VDC 60W (5A max), commercial grade, constant voltage power supply. It is made specifically as an LED driver.
    The wiring is 20awg. The length between each LED strip is about three inches. The wire from the power supply is 20AWG and is eight inches. Note that I kept the lengths short for testing purposes to minimize resistance from the wire.
     
  6. May 6, 2016 #5

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    EDIT i see others have posted, so this may be redundant...

    It's important to understand one's test equipment lest he be fooled by it.

    What is the nature of your power supply ? Link to its manual ?
    What is the nature of your Watt measurement ? Link to the instructions for the instrument you use ?
    Can you scan and upload your plot ?

    Otherwise your question is well stated, so probably it's half answered..
    You expected 6 amps and only got 3.88 ?

    Just a UnScientificWildA**Guess here -
    3.88/6 = 0.647
    That's pretty close to the ratio of peak to average for sinewaves (0.636). Is your supply well filtered?
    Could be as simple as a bad filter cap in your supply.
    To check ---
    Measure current or voltage (or both) with your DMM set to AC instead of DC
    and plot AC reading as % of DC reading with 1 string, 12 strings and 24 strings .
     
    Last edited: May 6, 2016
  7. May 6, 2016 #6
    Thanks for the responses.

    This phenomenon happens with all power supplies such as many clean (Agilent) lab power supplies and with LED drivers. Also, I have tried to use large 16AWG gauge wire from the power supply, but still get the same results. I must use 20AWG to each strip because that is as large as I can put on the contacts. Being such, I have assumed the phenomenon is a characteristic of the load, not the equipment.

    Consider that I have measured with multiple type of equipment from precise Fluke meters through lab bench Agilent DMM's.

    Also, please consider that the power is "clean" DC power as I see virtually no ripple (evaluated on an o-scope) on many of the power supplies I have tried.
     
  8. May 6, 2016 #7

    berkeman

    User Avatar

    Staff: Mentor

    What is the nature of the 3W LED strips? Are they just series LEDs with a single current-defining resistor? Or do they contain DC-DC converters to drive the LEDs?

    If they contain DC-DC conterters, then as the input voltage droops (because of the increased load on the power supply), they will draw more current at that lower input voltage to supply the same output power.
     
  9. May 6, 2016 #8
    Oh, right. Sorry, I did not mention the composition of the LED strips.

    The Strips are three LED's in series with a ballast resistor per segment. There are 36 LED's (12 segments) per strip. Power is 3W (250mA @ 12VDC) per strip of 36 LED's.
     
  10. May 6, 2016 #9

    berkeman

    User Avatar

    Staff: Mentor

    Can you upload the plot? And sorry if you mentioned it already, but was the 12V supplied to each LED strip steady at 12V as you added more and more strips?
     
  11. May 6, 2016 #10
    LED Strip Non-linear Loading Data Plot.png

    Here is the plot.

    As for the voltage, there is a voltage drop that increases as each strip is added to the circuit.
     
  12. May 6, 2016 #11

    berkeman

    User Avatar

    Staff: Mentor

    Can you plot the distribution voltage on the same graph? Just add it as another column (if you are using Excel) and re-plot, and then click on the voltage plot and right-click, Properties and select "separate axis".
     
  13. May 6, 2016 #12
    Unfortunately, I do not have the voltage data. I made the set-up a little while back and stored the data in an excel file. If I remember correctly, the total voltage drop was down to about 10.5V, measured on any of the strips. Does that help?
     
  14. May 6, 2016 #13

    berkeman

    User Avatar

    Staff: Mentor

    Did you notice whether they were dimmer? They should have been if the input voltage was drooping that much. The voltage drop across the LEDs wouldn't change much as the current varied, but the voltage drop across the current-setting resistors was changing, so the total current per strip was dropping. If you were able to re-take the data and record the strip voltages, I'm guessing that you would be able to explain this effect.
     
  15. May 6, 2016 #14

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    well, YEAH. Berkeman explained it with instructions how to put a number on it.

    LED's have a roughly constant voltage drop
    so the voltage across your current setting resistor dropped by a larger percentage of power supply voltage than you think.

    Just assume 1.9V per led. Red is about there, white may be 3V.

    1.9 X 3 = 5,7 volts across red LED's, 12 - 5.7 = 6.3 volts across current setting resistor.
    When supply drops to 10.5 you have only 10.5 - 5.7 = 4.8 across current setting resistor
    so current drops to 4.8/6.3 = 76% of what it was . Power decreases even more because it's V X I and both went down..

    Flimsy power supply, as others have noticed, explains your symptom.
     
  16. May 6, 2016 #15
    Well, if I leave the array as a simple daisy-chain, then each strip is progressively more dim due to the stacked voltage drop. What I do to manage this, is connect both the leading end and the trailing wire ends of the LED strip array to the power supply lead wire so as to balance the load. That makes all LED's have perceptibly equal output.
     
  17. May 6, 2016 #16
    Jim,

    Thank you for the detailed response. However, I do not understand what you mean about a sub-standard power supply. Please consider that this phenomenon happens will all power supplies, including robust lab grade power supplies. (I have about 15 different lab supplies, including some well respected ones such as Agilent, B&K, etc.)

    Considering this, it is hard to imagine that it is a sub-standard power supply. Clearly you are very talented in this field. Do you have any other thoughts as to why this is occurring, aside from the power supply?

    Berkeman, if you have any thoughts as well, I would appreciate them.
     
  18. May 6, 2016 #17

    berkeman

    User Avatar

    Staff: Mentor

    A lab-grade power supply will not droop its output from 12V to 10.5V if the output power spec is being met. If you try to draw more current from it than it is rated for, you will likely get a drop in output voltage (or a full current-limit behavior, depending on the type of power supply). You are saying that all of the supplies in your lab were not able to deliver their rated output current without the 12V output drooping to 10.5V?
     
  19. May 6, 2016 #18

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Attention to wording is the price of clear communication.
    A voltmeter has two probes. It reports voltage between them.
    10.5 volts measured between what two points "on any of the strips" ? What was voltage between power supply output terminals ?

    Well, you said you saw 10.5 volts. Where did you see it ? Why was it not 12 ?
    Go back and take a look. One observation is worth a thousand expert opinions.
     
  20. May 6, 2016 #19
    Well, if I leave the array as a simple daisy-chain, then each strip is progressively more dim due to the stacked voltage drop. What I do to manage this, is connect both the leading end and the trailing wire ends of the LED strip array to the power supply lead wire so as to balance the load. That makes all LED's have perceptably equal output.
     
  21. May 7, 2016 #20
    Berkeman,

    Thank you. Clearly, I need to observe more about the circuit. (It is a lot of work as you can imagine, but I need to get this phenomenon figured out, so I will make the configuration again.) Sometime early next week will be my next chance to set it up and observe. Once I do so, I will put my findings here.

    Jim and Berkeman, thank you much for your persistence it trying to get this figured out.

    Have a good weekend.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted