Switched Mode Power Supplies - risk of flicker for LED lights?

Discussion in 'The Projects Forum' started by rogue_uk, Jun 6, 2015.

  1. rogue_uk

    Thread Starter New Member

    Jun 6, 2015
    3
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    Hello :)

    Background

    I am being provided with 5 LED lights - the type of lighting used with video cameras, so each is basically an array of 300-ish LEDs in a single package.

    I believe they are this model: http://www.neewer.com/10069627.html

    I've been told they want 7.2 - 12V, and consume 18.24W - which is consistent with the link above.

    Situation

    To avoid previous issues with tangled spiderwebs of extension cords and excessive battery use with previously rented lights (these new ones are being purchased instead of rented), I have been asked if I can put together a single power supply box that we can plug all the lights into, and use some nicely weighted coilable cable. Ideally this would fit in (or could even be integrated into) the case we use to pack and transport the lights.

    Building a PSU isn't so much of an issue (I've built a few toroidal PSUs for my CNC equipment) but for the sake of ease, cost, and a short timescale, I've been looking at picking up a pre-made powersupply.

    Based on the requirements given, I reckon that I'll need something that can support 12V@7.2A (92.1W)

    I can get my hands on the following SMPS within price and time constraints:
    http://www.maplin.co.uk/p/power-pax-150w-switch-mode-acdc-multi-voltage-power-supply-n52kh

    The SMPS provides 12V@12.5A (150W).

    This should give me scope to support the five lights needed, and potentially a few more in the future. I'd rather run it under capacity than close to capacity, however.

    Question(s)


    LED flicker is an ongoing concern with video, especially as it's not always noticeable until you get to the editing stage. I've also always heard that SMPS are not the cleanest of power sources (which is why I build toroidal PSUs for my CNC kit).

    So, my questions are:

    1. Would the SMPS electrical noise issue be likely to translate into flicker in the kind of LED lights we are using?
    2. If so, could this be mitigated in a way that is still easier than building a specific PSU, ie additional smoothing caps or other regulation?

    All thoughts, comments, and answers are welcome, thank you :)
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,328
    890
    I suspect you have a non-problem here. The electrical noise from these devices is going to be at the switching frequency (probably somewhere between 20 and 200 kilohertz) and its harmonics, up to several megahertz, and in a well-designed SMPS is unlikely to be more than a few hundred millivolts peak-to-peak. At such high frequencies, and at such low amplitude, I can't see it creating any noticeable flicker in the LED brightness. I would expect little or no noise at the mains frequency.

    Whatever the noise level from the SMPS, it can always be reduced by putting an LC (or LCLC) filter on its output.
     
    rogue_uk likes this.
  3. rogue_uk

    Thread Starter New Member

    Jun 6, 2015
    3
    0
    Thanks, that makes the project a lot simpler.

    I'll do a trial run to see if I can detect any flicker and then look at a filter if necessary.

    I just didn't want to pay good money for the power supply if there was no hope of it working as planned. As long as there's some hope, I'm good!

    Edited to clarify: The flicker is rarely noticeable to the eye, but it sticks out like a sore thumb when reviewing and editing footage on the computer later, which is my primary concern. As long as there are possible options for cleaning up any noise, it makes it worth buying the power supply to test it.
     
  4. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,328
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    I suspect that even when reviewing and editing, you would have a tough time detecting any flicker. Even if you are filming at 60 FPS, each frame would be integrating at least several hundred SMPS switching cycles, further diminishing the effect of the already-small voltage variations from the switching noise.

    Thinking about this, however, I've one added concern (unrelated to light intensity flicker): SMPS of this type usually contain a ventilation fan which comes on when the internal temperature becomes too high; and when that fan comes on, in some models it is EXTREMELY loud and annoying.

    I've no idea whether this fan noise could interfere with your video shoots but if it could, you might consider using a more powerful SMPS (say, one of 30 amp capacity or so) that would be running so far below its maximum rating that it would be less likely to trigger its cooling fan.

    Just a thought...
     
  5. DickCappels

    Moderator

    Aug 21, 2008
    2,659
    632
    With the specified 180mVp-p ripple, you won't get much flicker at all.
     
  6. rogue_uk

    Thread Starter New Member

    Jun 6, 2015
    3
    0
    That thought had occurred to me as well, with errant noises being one of the many banes of my existence. The description for the SMPS I've picked out states that it is an "air cooled design for noise and vibration free, efficient operation", which I'm rather hoping means that it doesn't include a fan!

    Failing that, I'm working on the basis of 10m leads (voltage drop for 12V@1.6A over 10m using 1.5mm wire should be 3.08% or 0.37V; given that these lights take between 7.2V-12V, that should be absolutely fine) so I should be able to get enough distance from the mic to limit the impact of the noise with some careful shielding.
     
  7. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    The item he posted has a frequency of 25 kHz.
     
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