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Current Regulator

Discussion in 'Beginner Electronics' started by Farticus, Sep 18, 2006.

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  1. Farticus

    Farticus Guest

    I have a requirement for a constant current supply to drive 7 high power
    LEDs wired in series.
    The voltage applied to the LED series string must be adjustable between 25
    to 33 volts and the current regulated at 300mA.
    Any circuit ideas, chips etc?
  2. Farticus

    Farticus Guest

    Thanks for that info John.

    I can't quite see how you derive the constant 300mA with that LM317 cct

    The LEDs are 4.1 volt 1W devices. I made up a PSU using 6 x LM317T regs,
    one for each bank of 7 LEDs. This drove the LEDs "sort of" OK, not quite
    getting to their required 300mA, a bit short on voltage.

    The customer now insists that the LEDs be "current driven" and produced some
    chips suggesting I build a PSU using them. They are DD311 - I don't know if
    you know them. Quite interesting devices, except I can't seem to get the
    buggers to work as I would expect. (Data sheet available at ).

    The way I see it is (refer to page 11 of the .pdf using the diagram at the
    top of the page) if the cct was set to provide a constant current of 300mA,
    then if load ILED was increased, no additional current should flow, it would
    be limited to 300mA.

    However when I hook it up using a stiff voltage souce for VLED/REXT and a
    REXT resistor as specified using the chart on page 6 it doesn't work out. If
    I increase the load then the current in the path VLED (pin 2) also
    increases? I would expect it to stay at 300mA?

    I use load resistors in place of LEDs for the test - could this be the
    incorrect way of testing and be the cause of this result?

    Your comments would be appreciated.
  3. Farticus

    Farticus Guest

    Should be

    .... I missed out the *=* before 3302

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