Regulated current in LM317 power supply - Ammeter

Discussion in 'Power Electronics' started by Standisher, Jul 27, 2016.

  1. Standisher

    Thread Starter Member

    Jan 16, 2015
    47
    3
    I'm putting together a simple low power supply (15V incoming supply input via AC/DC Adaptor - delivering 5 - 9V DC) based on the LM317 (similar to this design: http://2.bp.blogspot.com/-Z19R6d6cLJA/U95KVfry-ZI/AAAAAAAAH10/4F6t_u_Z47I/s1600/steven.png ). I was planning to have current regulation but having variable, potentiometer controlled operation seems to end up in a lot of meaningless guess work. I understand that I could fix the maximum allowable current by simply using one resistor calculated using the formula 1.25/R; so using a resistor value 2.2 Ohms would give a maximum current of approx 570MA?

    It seems to me however that introducing a potentiometer, for a variable maximum current setting ,would leave me not knowing how many Ohms I have applied and therefore what current I am setting. Having an Ammeter in series on the output would surely only tell me how much current is being drawn by the load so would be no help in determining the maximum current set.

    Am I missing something here? Is there a way of having some visual indication of the regulated current (in this particular circuit or with some addons) or would I be be better just setting an absolute maximum with a fixed resistor and merely using the ammeter to see what current is being drawn by a device? In reality, I am unlikely to be applying power to a load requiring more than 500MA but current regulation would be a 'nice to have' if there was a practical way of seeing what current had been set.
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,308
    884
    No, you're not missing anything.

    I think my choice for a hobby power supply would be to use a rotary switch to select among a set of fixed resistors to give current limits of, say, 50, 100, 200 and 500 mA. For me, anyway, that would cover most situations.
     
    Standisher likes this.
  3. Standisher

    Thread Starter Member

    Jan 16, 2015
    47
    3
    Excellent, thanks for confirming my understanding. Funnily enough I am already using one rotary switch to select desired voltages. I think I will just follow your suggestion and add second rotary for fixed currents. Regards.
     
    OBW0549 likes this.
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