LED PWM causes PSU noise

Discussion in 'The Projects Forum' started by reefduino, Sep 18, 2013.

  1. reefduino

    Thread Starter New Member

    Sep 17, 2013
    3
    0
    Hello everyone,

    I bought my Arduino Mega a year ago and never started doing something with it. As a child I drew close to electronics but I never went past the basic principles and rules. Now I want to combine two passions. Aquariums and electronics just to have simple tasks automated. Nothing fancy, yet.

    I saw there are a lot of people using Arduino to automate water changes, lights, pumps and whatnot to their aquariums but there is little explanation on how to build the circuits. Started out with the wealth of information on the Arduino site and experimented with different circuits until I got the basics right. Now I want to venture further with my project which for the moment is targeting at controlling the lights above a 80 gallon (300L) reef aquarium. The fixture is made by me out of 36 3W LEDs and 5 10W LEDs.

    I'm planning on adding more light so I bought a MeanWell switching power supply of 12V 16.7A for my project.

    I designed the circuit so the LEDs are grouped in 4 channels all of them hooked to the PWM outputs from my Arduino. To control them I used a fairly popular and cheap power transistor (TIP122). The LEDs are glued to a large heatsink with thermal compound glue and also the transistors so that I can keep them cool.

    The transistors are wired through a 1k resistor directly to the Arduino out pin and they are wired in the circuit on the -Vcc rail for each LED group.

    The problem I think I'm facing now is circuit noise due to the PWM frequency. However, I tried putting decoupling capacitors with no success. The power supply emits a sharp noise that waves when the duty cycle is less than 75%. At full duty cycle everything seems to be working okay. The 10W LEDs tend to flicker a bit at about 10-15% duty cycle.

    I have attached the schematic so far, bare in mind each LED represents an array of LEDs but for the sake of keeping things simple I've put just one for each channel.

    The 3W LEDs are wired 3 in series and each "rail" is wired in parallel to the +Vcc and -Vcc. The 10W LEDs are all wired in parallel.

    Any help on what I might be doing wrong and how I can do this correctly is greatly appreciated. Also, note I don't have an oscilloscope.
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,131
    267
    I assume that you mean audible noise?
    The noise you describe is due to:
    1) Magnetostriction of the core in the switcher power transformer.
    2) Piezoelectric effects in the capacitors on the output of the supply- ceramic caps can really sing when an audio frequency current flows through them- 2-3 Khz is the worst.

    Raising the PWM frequency is the best idea, out of the audible- 20 Khz is good.
    This also reduces the ripple voltage on the caps to some degree. You can load up the power rails with more capacitors, but this will never completely solve the problem.
     
  3. reefduino

    Thread Starter New Member

    Sep 17, 2013
    3
    0
    Yes, audible noise from the PSU. Is there a way to raise the PWM frequency with components outside the Arduino board?
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
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    Looking at your schematic it doesn't look like you have anything to limit the current thru the LEDs (like a series resistor). This could make the current very high.
     
  5. reefduino

    Thread Starter New Member

    Sep 17, 2013
    3
    0
    Can I replace the current limiting resistors with power MOSFETs? I have a couple of IRF530 which I put together in a circuit to replace the TIP122 transistors.

    Would that work better or do I still need current limiting resistors?
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
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    You still need some form of current limiting or your LEDs probably won't last long. Do you have a link to the specs of the LEDs? When you say the 10 watts are in parallel do you mean each one is across your 12 volt supply with only the transistor in series with them?
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,770
    970
    The way most of us in the "reef" world are doing it is simply buying proper constant current (CC) drivers that have a dimming input and using the arduino to output the proper dimming signal.. Like 10V PWM and changing duty cycle to change dimming levels.

    This way the CC drivers are setting the current limits properly and the arduino is only dimming up/down as needed.
     
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