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

Thread Starter

jmogens

Joined Nov 11, 2015
2
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!
 

dl324

Joined Mar 30, 2015
8,869
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.
 

RichardO

Joined May 4, 2013
2,273
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. :(
 

DickCappels

Joined Aug 21, 2008
5,876
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.
 
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