LED lamp dimmer questions: Flickering, LED lifetime, power consumption



  • Hi All,

    The LED dimmer solution listed in the Build section has very little information and I am not very knowledgeable about LED dimming. So, here I ask:

    LED Flickering
    Is this solution flicker free? What is the PWM frequency? (I don't see any pwm reference in the sketch)
    I have read it that lower PWM frequencies are disturbing for people and can have adverse health effect.

    LED Lifetime
    Do this dimming solution reduce the expected lifetime of the LEDs?

    Power consumption
    The primary target of my LED dimming project is a fixture which we inadvertently, massively oversized (5000lm) so I would like to run it at 50% brightness, most of the time. (We liked the fixture itself and it looked much dimmer in the store).
    At full brightness (as it is without dimming) the fixture is rated for 50W power input. Will the dimming solution lower the power consumption to near ~25W if we run them at 50%? Where will the dimming circuit have a lot of losses (if any)? The Arduino consumes next to nothing. Will the MOSFET consume a lot?

    Input power
    The drawing notes says this for the power source "12VDC 6-9A input". The 12V converter in my fixture is only capable of ~5A. Is this difference important? I suppose this was written there for the recommended LED strip which consumes more than the LEDs in my fixture.

    MOSFET cooling
    The description mentions that cooling is needed for the MOSFET but there is nothing relevant in the shopping list.
    Would this cooler be suitable?

    Any insight is appreciated!
    Andras


  • Mod

    @sola said in LED lamp dimmer questions: Flickering, LED lifetime, power consumption:

    LED Flickering
    Is this solution flicker free? What is the PWM frequency? (I don't see any pwm reference in the sketch)
    I have read it that lower PWM frequencies are disturbing for people and can have adverse health effect.

    I guess by "flicker free" you mean that the leds are flickering faster than humans are able to notice?
    From Arduino's documentation:

    The frequency of the PWM signal on most pins is approximately 490 Hz.

    I haven't noticed any problems with my dimmed LEDs, but it could just be that I'm not receptive enough. Do you have a reference to the research paper you read?

    I have not read all the pdfs linked in this source but I found this part interesting:

    1. Risks of seizures due to flicker in frequencies within the range ~3- ~70Hz;
    2. Health concerns due to invisible (not perceivable) flicker at frequencies below ~165Hz including, but not limited to, headaches, migraines, impaired ocular motor control, and impaired visual performance;
      ...
      As noted in the table below, much of the literature might suggest that ~160Hz – ~200Hz may be a sufficient limit for flicker to have negligible biological effects in some lighting applications,

    This suggests that 490Hz would be enough.

    LED Lifetime
    Do this dimming solution reduce the expected lifetime of the LEDs?

    The main factor in LED lifetimes is their temperature. Dimming the leds will consume less power, and therefore lead to lower temperature and longer lifetime.

    Power consumption
    The primary target of my LED dimming project is a fixture which we inadvertently, massively oversized (5000lm) so I would like to run it at 50% brightness, most of the time. (We liked the fixture itself and it looked much dimmer in the store).
    At full brightness (as it is without dimming) the fixture is rated for 50W power input. Will the dimming solution lower the power consumption to near ~25W if we run them at 50%? Where will the dimming circuit have a lot of losses (if any)? The Arduino consumes next to nothing.

    The leds will not consume 50% power at 50% perceived brightness. Our eyes don't experience brightness linearly, and the brightness of leds doesn't change linearly with the led power consumption. But at 50% duty cycle, the leds will consume about half of what they do at 100% duty cycle.

    Will the MOSFET consume a lot?

    According to the irlz44n datasheet linked on build page, the mosfet has a resistance of the 0.035 ohm. At 5A the power consumption will be 0.035 * 5 * 5 = 0.875watt. So no, the mosfet will not consume a lot 🙂

    Input power
    The drawing notes says this for the power source "12VDC 6-9A input". The 12V converter in my fixture is only capable of ~5A. Is this difference important? I suppose this was written there for the recommended LED strip which consumes more than the LEDs in my fixture.

    Yes, that's correct. As long as your power source can handle your leds at max brightness you will be fine.

    MOSFET cooling
    The description mentions that cooling is needed for the MOSFET but there is nothing relevant in the shopping list.
    Would this cooler be suitable?

    The mosfet is in a TO-220 package. The one you linked probably works, but I suggest a heatsink that is made for TO-220, for example https://www.aliexpress.com/item/10-Lots-Sets-Heatsink-Heat-Sink-With-Screw-Sets-For-TO-220-FW1S/32799922266.html



  • I have similar solution so I can tell
    a) Flickering is not visible at all. Not even close to low frequency monitors we had before LCD come (maybe you remember that times)
    b ) lifetime of led should not be afected, at least could be ignored (if you will replace LED after 5 or 7 years ....it doesn't matter much, since it will last much more than traditional bulbs)
    c) Power consumption is current times voltage. Voltage is constant, and current if at 100% is for maximum brigtness. If you use arduino PWM then 50% means that at 50% you have current, and vice versa. Effectively it means half the current. And that means half the power. So your assumption is correct.
    MOSFET has incredibly low resistance, so almost nothing is spent on MOSFET (comparing with traditional transistors as your question might imply)
    d) Input power must cover your LEDs. If you use 50W LED than you should have at least 50W/12V = 4,16 A for maximum load. With some reserve i would probably take at least 5A supply. Real answer is more - how long you will really have 100% power.
    In my project I usually pick some color so even if red is 100, and blue is 100, maybe green is 0%, or .... So in my case average is ~50% of maximum calculated power most of the time.
    TO be on safe side pick 8A 12V supply (conservative) or take 5A 12V supply which in my case is more than enough because I almost never use white collor (all collors at maximum).

    1. MOSFET cooling. It is incredibly low heat on MOSFET. In fact my power converter 12V to 5V for electronic is hot, while MOSFETS are incredibly cold. Again the fact that internal restistance is very low, helps a lot. I created a board with place for coolers. and I bought passive coolers (similar to these on link you provided) . But after testing I decided that that makes no sense. So I didn't put them on!

    I am talking about my project expirience, where I use 5m 5050 RGB LED, with 8266 microcontroller and 3 MOSFETs - 1 for each color. I use 5A 12V power suply, and swithching module for adapting power for my 8266 microcontroler (otherwise power converter overheats).

    I hope my expirience helps.



  • @Igor-Antolić @mfalkvidd

    Great answers, thank you both.



  • Power = Current squared x Resistance, so for your example of 5 A and 0.035 Ron the power dissipated is 0.875 W. It's still not a lot but it's substantially more than was stated.


  • Mod

    @Carywin thanks. I've corrected my calculation.


 

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