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Question

How do I use a JFET to regulate LED current?

Answer

With negative feedback a junction FET (JFET) can be used as a adjustable current regulator. The circuit on the right uses the resistor (R) to set the current through the LED. As long as the drain-to-source voltage (V_DS) is above a minimum value a constant current flows through the LED.

Calculation Example

The value of R is calculated using the "Drain Characteristics" curve available in the JFET datasheet. The graph shown is from the On-Semi BFR30LT1 datasheet.

Since the y-axis is drain current (I_D) horizontal lines are constant current. For each gate-to-source voltage (V_GS) there is a minimum drain-to-source voltage V_DS above which I_D is constant.

For example to maintain a current of 2.5mA a V_GS of -3V is required. The resistance R would be --

The minimum voltage for regulation is Using an LED with a forward voltage (V_F) of 2V and the BFR30LT1 at a V_GS of -3V the minimum voltage for regulation is ≈ 5V. For voltages above 5V constant current is maintained.

A couple of other useful values can be read from the curves. V_GS(OFF) is the value of gate-to-source voltage that forces the JFET drain current to zero (-5.8V for the BFR30LT1). The zero-gate-voltage drain current (I_DSS) is the current that flows when V_GS is zero (≈ 9mA for the BFR30LT1). I_DSS is the maximum current of the JFET.