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LED buck drivers and reducing current

Discussion in 'LEDs and Optoelectronics' started by Steveggz, Oct 14, 2017.

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  1. Steveggz

    Steveggz

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    Oct 7, 2017
    I'm building a remote phosphor light for videography. It'll consist of several strings of Cree XT-E royal blue LED's in series driven by several Mean Well LDD-1500H Buck drivers and a dc power supply. I'm planning on using the Coralux storm LED controller for PWM dimming.

    My question is, is it safe to run the LED's rated at 1500mA max on the Mean Well LDD-1500H (1500mA) buck drivers? Should I lower the current to around 1490mA (I need as much light output as possible without damaging the LED's)?

    And if I so, would adding a current limiting resistor in series with the LED's mess with the slew rate of the buck drivers or PWM frequency? I don't want the slew rate or pulse frequency to be in any way effected.
     
  2. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Reducing the current is a good idea and dropping it 10% is unlikely to make a noticeable difference to light output.

    You 'could' use a resistor but since Men Well do many other current ranges and 1400mA seems a nice round number, why not just use the correct device without a resistor?
     
  3. Steveggz

    Steveggz

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    Oct 7, 2017
    Mean Well's next current down is 1200mA for their LDD-H series. Even at 1400mA I would still like to get as much output as possible.

    I'm just worried about the resistor effecting the pulse frequency or slew rate. I'm a novice when it comes to electronics and I don't have much experience when it comes to limiting current with buck regulators.

    If a current limiting resistor in series with the LED's might cause issues what would be the right way to limit the current?
     
  4. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Purely resistive elements will not affect slew rate - unless there's a high reactive element of the resistance itself (wire-wound) but, even then, it would be well outside any operating (switching) frequency range that could make a difference - for normal PWM systems anyway.

    The difference in light output between 1500mA and 1200mA will be unnoticeable without very careful comparison. You're looking at the difference between 4.275 watts and 3.99 watts, less than 10% in 'power' but in terms of light output - actually noticeable by the human eye - practically impossible to discern.
     
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  5. BobK

    BobK

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    Jan 5, 2010
    Your driver is a constant current driver. Adding a resistor in series will not change the current. You could add one in parallel, but better would be to turn doen the current on the driver.

    Bob
     
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  6. Audioguru

    Audioguru

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    Sep 24, 2016
    How will you cool the LEDs? They will need a pretty big finned heatsink or they will quickly melt.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    And be careful with PWM and videography. You might end up with strobing on your recording.
     
  8. Steveggz

    Steveggz

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    Oct 7, 2017
    Thanks (Kellys_eye, Bobk) for your replies!

    @Audioguru I'm going to thermal paste the MCPCB to a massive heatsink with fans following what the reef tank builders use.

    Looking at the datasheet I don't see a way to turn down the current. If I were to add a current limiting resistor in parallel I'm assuming I would have to divide the total resistance by the number of strings of LED's to find the correct Ohm resistor for each string correct?
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    To determine the required resistor, calculate the approximate voltage across the LED string (don't have multiple in parallel). Then divide this voltage by the current you want to bypass the LEDs.

    Let's say the LED voltage is about 27 volts and the current you want to bypass is 100mA. The resistor value is 27/0.1 = 270 ohms.

    It will dissipate 27 * 0.1 = 2.7 W, so use a 5W resistor (about twice the power to ensure it has a long life).

    Adjust the values to match your requirements.
     
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  10. Steveggz

    Steveggz

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    Oct 7, 2017
    Ah, thank you!!

    Yes I'm aware of PWM dimming and videography. The buck drivers and PWM controller I had in mind pulse at frequencies in the kilohertz range which shouldn't be noticeable even at high shutter speeds.
     
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