The circuit used for the constant current driver for the LED's was sourced from an article by Giorgos Lazaridis
(5). With reference to the details in (5) and the components labelled on the diagram in the Schematic Diagram section below, when switch SW1 is closed, voltage as applied to the gate of MOSFET Q2 and current flows through the LED's and the sense resistor (formed by the combination of R2 and RV1).
As the current increases through the LED's and the sense resistor, the voltage drop across the sense resistor also increases. When this voltage reaches ~0.7V transistor Q1 starts to conduct, pulling the MOSFET gate to ground, and hence turning off the MOSFET. Therefore, current through the LED chain is regulated by the value of sense resistor.
To calculate the value of the sense resistor use the following formula:
be = base-emitter voltage transistor Q1 (~0.7V) I
led = limiting current through LED (amps from datasheet)
The white LED's used in the project have a maximum continous current of 300mA, therefore, the sense resistor value is 0.7/0.3 = 2.3 ohm. In the constructed circuit, the sense resistor is composed of the total resistance of R1 plus RV1 . The potentiometer RV1 allows varying the resistance and hence the current and therefore the brightness of the LED's.
A value of 1ohm was selected for R2 (which gives the minimum sense resistance even if the potentiometer is wound to zero resistance) which with RV1 at minimum resistance still gave a measured resistance of ~2.5ohm (due to manufacturing tolerance of RV1).
The value of R1 is selected to enable ~1mA of current to flow to the gate of the MOSFET
Each of the 1W 350mA white LED's have a voltage drop of ~2.8V as measured for the particular items used. From the collection of scrounged/saved 'wall warts' from various laptops and other electrical equipment, I had a 'wall wart' that produced nominally 12V DC output at 1.5A from 240V input AC.
The actual measured output of the 'wall wart' was 12.8V and the voltage drop across a string of 4 x white LED's was 11.8V. Therefore, the voltage drop across the MOSFET Vm is:
m = V dd - V leds - V rs
i = supply voltage from wall wart V
leds = voltage drop across the 4 x LED's V
rs = voltage drop across transistor Q1 base-emitter ie. 0.7V
Which for the values measured in the actual test circuit are Vm = 12.8V - 11.8V - 0.7V = 0.3V. Therefore, the power disspated in the MOSFET equals (Vm x I
led) which is 0.3V x 0.3A = 90mW. This is well within the MOSFET max power disspation of 65W from the datasheet.
Also, the MOSFET datasheet reports 0.43 W/
oC which gives an expected neglible temperature rise for the MOSFET, and hence no extra heat sink required. As reported in the Testing/Experimental Results section this was exactly what was observed (nice!)