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LED flickering with CC driver

Discussion in 'LEDs and Optoelectronics' started by john2k, Dec 2, 2012.

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  1. john2k

    john2k

    185
    3
    Jun 13, 2012
    I have a constant current LED driver that delivers 300mA to my LED lights. It's used on a car and uses the cars 12V supply. It's been working fine until now when the weather has turned cold the LED's are flickering and it seems to stop once it has warmed up.

    I dont think it is the drivers themselves because i have the exact same driver powering another LED light on the car with a ready made voltage regulator. Whereas the one that is flickering, I have a low drop-out LM2940CT 12v regulator with a 22uF Low ESR 105°C capacitor as the datasheet says. I am wondering if this low ESR capacitor could be causing the problem? The datasheet of the LM2940CT says that ESR operating temperature will drop significantly when cold.
     
  2. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    The TI data sheet covers this, the ESR rises as temperature goes down, particularly with aluminium electrolytics, you can add as much capacitance as you like. They suggest adding a tantalum in parallel which is less influenced by temperature.
     
  3. john2k

    john2k

    185
    3
    Jun 13, 2012
    thanks for your reply. I have a IN4007 rectifier diode on the input, i had issues with the IN4007 dying out before and caused the input current to be limited and dim. just wondering can a dying IN4007 on the input of a CC driver cause the flickering?
     
  4. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    I do not do a vast amount of construction but I have never known of the case of a diode dying. In my experience, they are either living or dead.

    Diodes can be killed by switching them on into a large capacitor but the 1N4007 is robust and should stand this. Just don't try it with a 1N4148.

    Have you tried more capacitance?
     
  5. cjdelphi

    cjdelphi

    1,096
    104
    Oct 26, 2011
    I've had a dying diode, intermittently it would work and stop working, it behaved like a crack in a wire, depending on how you moved it it would occasionally work, but this diode was conducting on and off at random with no touching, it was weird i replaced it in the end all was good again.
     
  6. john2k

    john2k

    185
    3
    Jun 13, 2012
    thanks for your replies.

    duek37, i havent tried more capacitance yet. What would you recommend? I currently have a 22uF Low ESR Electrolytic Capacitor on the output of the LM2940CT. Would you recommend changing this with a higher uF rating Low ESR Electrolytic Capacitor? i would prefer to replace than add extra components have a good capacitor in the first place.

    Also, having a look at the datasheet of the LM2940CT it says that the operating temperature range is 0°C ≤ TJ ≤ 125°C does this mean that if the if the temperate drops below 0°C it would not work? i've looked at most of the 12V regulators and most of the seem to have a operating temperature starting from 0°C but in winter the temperature drops below this. Is it a matter of the regulator warming up and then working or will it not work at 0°C or below?
     
  7. cjdelphi

    cjdelphi

    1,096
    104
    Oct 26, 2011
    sounds like it's the vreg at fault..

    easy enough to prove though, you have 2 choices... use a different regulator or....

    use a low ohm resistor and mount it onto the regulator.. if it's cold hit a button which warms up the regulator for a few seconds, a 555 one shot then have the led's switch on... id get a different v reg.
     
  8. john2k

    john2k

    185
    3
    Jun 13, 2012
    but most of the regulators out there seem to have a temperature rating of 0°C ≤ TJ ≤ 125°C but once you turn them on they do kind of warm up. If the temperature was to drop below the 0°C will it not just be a matter of minutes before it powers up as the vreg will start warming up?
     
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