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Thread: Driving an LED with a LD1084V but not able to get more than 3A...

  1. #1
    Unenlightened
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    Default Driving an LED with a LD1084V but not able to get more than 3A...

    Hi, I just wrote my entire post and pressed the wrong damn button and it disappeared in a puff of frustration so please excuse me if I come across a bit cranky!

    I am trying to get a red LED from a C*s*o projector, a PT54, to work in a flashlight build... a small project of mine.

    OK so I have a beefy bench PSU, connected to a breadboard with a classic DDL circuit, using a LD1084V, but even when the ADJ resistance is dropped to almost zero, the current drawn by the LED does not exceed 3 amps. Doesn't matter how much I turn up the input volts.

    Why is this? There are plenty of volts. There is no shortage of current... it seems like the LED is simply not drawing more current. But it's a diode! Surely it should take as much as it can get? So why stop at 3 amps? Is it a limitation of the regulator? Why 3A though, when it's supposed to handle 5?

    If I'm doing something wrong, any ideas what? And if I am not and this is simply a limitation of the regulator, can I wire more than one LD1084V in parallel to get more amps out of it? I wanna drive this baby with like 6 or 7 amps! But there does not seem to be much info on how to wire two or more together to increase current... or if there is, I can't find it.

    Thanks if you can help,
    Phil



  2. #2
    *Flashaholic* wquiles's Avatar
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    Default Re: Driving an LED with a LD1084V but not able to get more than 3A...

    Phil,

    I would start by sharing with us exactly the "classic DDL circuit unit a LD1084V" you are using. Even if a hand drawn of the schematic is all you have, please share it with us.

    Will
    Please no PM/Visitor Msg's. Email for questions/Paypal: wquiles [at] gmail {dot} com. Please visit my new website.

  3. #3
    Flashaholic
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    Default Re: Driving an LED with a LD1084V but not able to get more than 3A...

    Quote Originally Posted by wquiles View Post
    Phil,

    I would start by sharing with us exactly the "classic DDL circuit unit a LD1084V" you are using. Even if a hand drawn of the schematic is all you have, please share it with us.

    Will
    I agree .....

    On a side note I just googled LD1084V and have to ask if you attached a heat sink to it? 3amp and above I bet it is getting quite hot?!

  4. #4
    Unenlightened
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    Default Re: Driving an LED with a LD1084V but not able to get more than 3A...

    Hi Jason and Will, thanks for your replies. The circuit I'm using is this one, with a couple of minor changes. The LD1084V is a drop-in replacement for the LM317.


    CHANGES

    My capacitor is 47uF but I don't think that's relevant.

    My 2x10 ohms in parallel (making 5 ohms) is actually 2x1 ohms in SERIES making 2 ohms... in fact when they are in circuit, even when the pot is adjusted to zero, meaning the total resistance in the 'feedback' loop is 2 ohms, the maximum current I can get the circuit to draw is 0.45 A, at which point the voltage drop across the LED is 2.14 V. Only when I short the resistors, making the feedback path zero ohms, does the current to the LED increase to 3A, and the voltage drop is 4V.

    The regulator is heatsinked! The LED is not, but I am only running it for very short periods while I work out what is going on.

    I think I am making some rookie mistake but it won't be the first time

    Cheers!
    Phil

  5. #5
    *Flashaholic* wquiles's Avatar
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    Default Re: Driving an LED with a LD1084V but not able to get more than 3A...

    Looking at the data sheet for the LD1084, Section 4, Figure 3, the adjusting arm of the circuit is referenced to GND. In your schematic, it is not. That would be the first thing I would check into.

    http://www.st.com/internet/com/TECHN...CD00003086.pdf


    Will
    Please no PM/Visitor Msg's. Email for questions/Paypal: wquiles [at] gmail {dot} com. Please visit my new website.

  6. #6
    Unenlightened
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    Default Re: Driving an LED with a LD1084V but not able to get more than 3A...

    omg I'm such a moron. The max current of the bench PSU was set to ~3A.

    So sorry to waste your time... I really appreciate everyone's help, thank you.

    I will post some pics if I ever finish this build. I can't believe the sheer amount of light from this LED! I think it must be the reddest red thing I've ever seen

    Best
    Phil

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