# 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.