My LED string Test Setup

Discussion in 'Handy Tools & Equipment' started by aussiexmas, May 10, 2010.

  1. aussiexmas

    aussiexmas Sinnamon Lights

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    After being frustrated with the required voltages for different commercial LED strings, I built myself a test rig to measure the voltage and currents required to drive the strings from DC after cutting off the controllers.

    First step was to identify the number of channels in the controller and identify the common (ie +ve) wire. Number of separate channels in a string is 1 fewer than the number of wires connecting the controller to the string, (ie 2, 3 or 4). Next step is to work out the number of parallel circuits in each channel and the bulbs/circuit. This can usually be done by checking along the string for the locations where there are the same number of wires between bulbs as there are form the controller to the first bulb. Count the bulbs/ segment and divide by the number of channels.

    My test box consists of a voltage supply (24 or 36V selectable). I wired a selected series of 1 W resistors between banana sockets and then use clip leads to connect the required channel of a string to the resistors on 1 side and the +ve supply voltage. I have measured the total resistance from each terminal to the string connection point (yellow label in pic). I then use a banana jumper lead to connect the supply volts to the required resistance point. I then use 2 multimeters to measure the voltage across the string and the voltage across the resistors. Total current / channel is then a simple calc from the voltage across the known resistors - I = V/R. The individual LED currents are then obtained from this current by dividing by the number of circuits/channel.

    By starting at a high resistance and working to lower values, you can quickly evaluate a current/brightness relationship, and thus what voltage to run the string at, or what resistance to add to run at a different voltage.

    I have found that for blue, green and white LEDs, a bulb current of 8-12 mA provides adequate brightness, but for red and yellow, a higher current of 12-18 mA is required to achieve a comparable level of brightness. (My opinion)
     

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  2. AussiePhil

    AussiePhil Dedicated Elf Administrator

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    Geoff

    very nice, professional looking, reminds me of bench test equipment from Amateur Radio/CB Radio days.

    I keep promoting the need to measure LED strings for the voltage you plan to use and either add resistors as needed or accept the under driven string as is or modify it.

    My personal preference is just to set the string for 20mA per section regardless of colour as i have found this to give me the best dimming performance across all strings.

    Measuring and testing provides good baselines so you know your expected current draw per controller.

    When i get a chance i will post up my testing rig and explanation as i have an alternative method that i worked out.

    Cheers and thanks for posting this.
     
  3. JPB

    JPB Full Time Elf

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    In the post Christmas sales I picked up a few box of LED icicle strings from Kmart. They are only 2 channel but I am planning to convert them to 4 and as the first step, I put a DMM into the string.
    Each section of the icicle is only pulling 4.5mA which I have found surprisingly low.
    Using a resistance substitution wheel I have worked out the values I will need to maintain the 4.5mA current when I cut each section into two.

    Jon
     
  4. aussiexmas

    aussiexmas Sinnamon Lights

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    Hi Jon,
    I presume you are intending to use these as chasing strings. I have found it takes a lot of time to reconfigure string wiring. I always spend 2-4 times longer on wiring projects than I expect at the start.

    As you have several strings, an easy way to convert 2 to 4 channel is to zip tie 2 strings together taking care to match the correct bulbs. You can then control the 4 channels as required. The only difference is that the bulbs are closeer together. I recently did something similar with some C7 LED strings after a friend blew their controllers (& some bulbs) by connecting nominal 12V strings to 24V transformers. These were originally 2 channel, 4 colour strings which I have now converted by replugging bulbs & paralleling strings to give me a 4 channel chase capable string with each channel being a different colour.

    By the way, be VERY cautious on relying on a DMM to get reliable (or even useful) measurements of DC voltages or currents in an AC powered controller string. While the controller typically includes a full wave unfiltered rectifier, the LEDs actually respond to the instantaneous voltage which resembles a half sine wave. They typically only light at the higher voltage period and are off for the regions near the zero crossings. If you swing a lit string around, you can see the varying light intensity (on-off) - see attached pic. The instantaneous LED currents will of course vary also.
     

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  5. dmoore

    dmoore http://www.HolidayCoro.com

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    I'm guessing a TrueRMS meter helps out here?
     
  6. dmoore

    dmoore http://www.HolidayCoro.com

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  7. aussiexmas

    aussiexmas Sinnamon Lights

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    Depends on what you are looking for -If (as is most likely) you are looking for LED string voltage/current relationships to determine resistor values or DC supply voltage limits, then you really need to go the DC route, ie DC supply and DC measurements. LEDs have a highly non-linear voltage/current relationship, so the voltage and current waveforms will be very different in shape. Using a true rms measurement of each to try to evaluate resistor values or LED characteristics will give erroneous results.

    Thank you for the compliment. Last year's display was not synchronised - music added to video after recording. Planning to make that NEXT step this year.
     
  8. AussiePhil

    AussiePhil Dedicated Elf Administrator

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