PWM Controlled LED Driver (First Post!)

For my first official post on this new site, I thought that I would keep it simple. Going through some of my older projects, I came across a simple, but powerful circuit when one needs to power a higher-wattage LED. All schematics, PCBs and further project details can be found on my circuit maker project space.

This project allows the driving of higher power LEDs. The circuit is rather simple and only requires jellybean parts that most people should have in their part drawers. This project is meant as a demonstration of the circuit, and either halves of the circuit can be changed out to suite to projects needs. The circuit consists of two main parts: the PWM generator and the constant current source.

The PWM generator uses the standard 555 timer IC, two resistors, a diode, a capacitor and a potentiometer to create a PWM signal with the duty cycle proportional to the potentiometers position. This half of the circuit can be modified in many ways. It could be replaced with an MCU if more logic is needed. And I might do that in the future to demonstrate how to do so.

The current source is even simpler, requiring a N-Mosfet, NPN transistor and two resistors. The three resistors, R5, R6, and R7 are used to set the max allowable current through the LEDs. The current source works by using the properties of the BJT transistor. The base-emitter voltage drop is around 0.5V to 0.7V. Since this is in parallel with the three resistors, the current being set is as follows:

I = Vb-e / R

Where Vb-e is the voltage drop between the base and emitter of the BJT and R is the value of the resistance at the base of the BJT and source of the Mosfet.

The downside to this setup is that the Mosfet will drop all unused power on itself, so make sure the Mosfet has proper heatsinking. To reduce this power loss on the Mosfet, Vcc should be chosen so the the LED voltage drop(s) + Vb-e drop are as close to the power supply voltage as possible. As well, the resistors drop power proportional to the max current. To help combat this, multiple resistors in series or parallel to spread the power drop.

When the pulse going into the current driver is at the high point, the current through the LEDs is the max set current, and when the pulse is at the low point, the current is 0. Using the PWM, the duty cycle sets the brightness of the LEDs.