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Lm317 help?

Discussion in 'General Electronics Discussion' started by kgthomas, May 16, 2013.

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  1. kgthomas

    kgthomas

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    May 16, 2013
    Hi I am currently in the process of going to build a led light for my marine fish tank. I am going to be using 48 cree LEDs fed from 4 meanwell eln-60-48d drivers.

    The drivers output 48 volts with a slight adjustment. They also output 62 watts. The current drive is max 1.3A. The units are 0-10 volt dimmable. Looking at the units they are 0 output with dimmer voltages below 1.5 volt.

    Therefore I am planning on using lm317 chips with a potentiometer to vary the dimmer voltage. I am wondering I am looking to vary the voltage from 1.25 to 10 volts. I do know that the R1 is usually around 240 ohm I am looking at the possibility of using R1 at 680 ohm with a 4k7 ohm potentiometer to achieve the voltage shift I am looking. Or I can use R1 at 160 ohm with a 1k potentiometer. I am wondering which setup would work best with the lm317?

    I was thinking of using a 12 volt dc supply to supply the power for the dimmer circuit and I'm wondering if it is possible to use the same power supply for all 4 dimmer circuits and if so what is the best way to wire it up? I am not from an electronics background but can usually work most things out.

    Also is it possible to use the output from the drivers to power the dimmer circuit? I know the lm317 chips can accept upto 37 volt and my drivers output more than this but wondering if it can be stepped down to suit?

    Eventually I plan to rework the dimmer to run from an arduino output. This will allow me to stimulate a sunrise and sunset in the tank. However at present the dimmer circuit will simply be for adjusting the colour blend of the 4 strings of LEDs. I plan to control the on and off with timers. If I do use a separate 12 volt dc power supply do I simply have this switched on and off with the first timer?

    Finally if anyone has any other suggestions or better ways of doing this the input would be really appreciated.

    Thanks in advance for any replys.

    Cheers for taking the time to read this.
    Keith
     
  2. BobK

    BobK

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    Jan 5, 2010
    I doubt that the dimming control needs enough current to require an LM317. A simple potentiometer should do it. Unfortunately, the specifications do not mention an impedance or current requirement for the dimming control, so you would have to measure it to make sure.

    Bob
     
  3. kgthomas

    kgthomas

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    May 16, 2013
    Hi Bob

    Thanks for getting back to me. I will see if I can find the impedance required for the dimming control. Am I thinking correctly that if the lm317 is not required then I could just use a dc power supply and a potentiometer to vary the voltage and if so then the potentiometer would work in reverse meaning as I increase the resistance of the potentiometer then the voltage fed to the dimming would decrease? Also could I use the 48 volt output from the driver to feed the dimming circuit?

    Cheers Keith
     
  4. BobK

    BobK

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    Jan 5, 2010
    A potentiometer is a 3 terminal device. You connect one side to ground, and other to 10V and then the center terminal will vary from 0 to 10V as you turn the pot.

    To size the pot you have to know the impedance of the input. You can measure this by applying 10V and measuring the current in. The impedance is then 10 divided by the current. The resistance of the potentiometer should be about 1/10th of that. If the impedance is less than about 1000 Ohms, i.e. the current is 10mA or more, maybe your LM317 idea is a better idea.

    Bob
     
  5. kgthomas

    kgthomas

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    May 16, 2013
    Hi Bob

    I have yet to purchase the drivers and LEDs just making sure first I understand how to control the dimming. I have had a look online for impedance info but I have been unable to find this.

    What I have found is that if the dimming input goes above 12 volt then it will result in irrepairable damage to the driver. Also the pot recommended by people currently using this setup is a 10k pot. I plan to use 4 drivers in total each driving 12 LEDs. What I am wondering is can I run four 10k pots from one 10 volt power supply with each at different outputs? And if so what current would I need from the power supply?

    Also as I have not purchased any drivers yet is there any other drivers you would recommend looking at before I would buy the meanwell drivers?

    Cheers Keith
     
  6. BobK

    BobK

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    Jan 5, 2010
  7. kgthomas

    kgthomas

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    May 16, 2013
    Hi Bob

    Thanks for all your help with this. Just one more stupid question. The 10 volt drop out voltage do I just use one for all four pots or do I use one for each pot?

    Thanks again for all your help

    Cheers Keith
     
  8. BobK

    BobK

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    Jan 5, 2010
    One regulator is good for all four.

    Bob
     
  9. BobK

    BobK

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    1,684
    Jan 5, 2010
    If you 12V is regulated you could dispense with the regulator and use an additional resistor on each pot (between the pot and 12V) to drop the voltage across the pot to 10V. For a 10K pot, the resistor would be 2K.

    Bob
     
  10. kgthomas

    kgthomas

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    0
    May 16, 2013
    Hi Bob

    Thanks for all your help. I will probably go with the 10 volt regulator to be on the safe side for all the extra cost of it. Really appreciate all your help and advice with this problem.

    Cheers Keith
     
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