JFET constant current LED driver with low voltage operation and low battery cutoff

Discussion in 'General Electronics Chat' started by jmogens, Nov 13, 2015.

  1. jmogens

    Thread Starter New Member

    Nov 11, 2015
    I'm trying to design a battery powered constant current regulator for a LED with a forward voltage of 2.1V and a nominal current of 30 mA.

    I've seen simple current regulators based on a JFET and resistor here and here. I'd like to modify this circuit to minimize the additional voltage overhead above the 2.1V for the LED's forward voltage and to shut off/remove the load when the batteries have discharged low enough that they cannot supply the 2.1V + overhead voltage for the current regulator to maintain it's current.

    Thank you!
  2. dl324

    Distinguished Member

    Mar 30, 2015
    Post a schematic for your idea. State what types of LEDs you plan to use as few will have a forward voltage of less than 2.1V @ 30mA. Also consider that some LEDs may not be spec'ed for DC operation at 30mA.

    Regarding using a JFET; bipolar and MOS transistors are probably less expensive and would work as well.
  3. RichardO

    Well-Known Member

    May 4, 2013
    I don't think you will find a J-FET that has both a 30ma limiting current and a low gate to source threshold voltage. If you find one I would like to get a few for myself. :)

    Is this just a size issue or does it have to be low cost? As dl324 says it is simple to do with a couple of resistors, a bipolar transistor and a MOS-FET.

    An LT1635 and a resistor will almost work. It will still need a MOS-FET if you must have 30 ma through the LED and it will draw about 1/2 ma all the time. :(
  4. DickCappels


    Aug 21, 2008
    You would do well to look for LEDs with a low pinchoff voltage (Vp) and a high Idss. If you are lucky you will get a low pinchoff JFET with an IDSS equal to your desired current so you can skip the resistor and connect the gate directly to the source.