LED driving and controlling methods AuthorGiorgos LazaridisFebruary 5, 2012

PAGE 5 of 6 - Injecting PWM pulses to control the brightness of the LEDs Injecting PWM pulses to control the brightness of the LEDs

The title of this theory page is "LED driving and controlling methods". So far we've seen some simple and some not so simple methods to drive LEDs.

For the first method - A linear voltage supply is enough
Starting with the first method described, the LED voltage control with limiting resistor, the brightness of the LED can be controlled with a simple linear supply circuit. A circuit like this Simple Linear Fan Controller, and since the LED is a pure ohm load, the capacitors C1 and C2 can be omitted.

For the second method - Convert the voltage divider into a variable voltage divider

For the second method (Single Transistor Constant Current Driver), R2 form the voltage divider can simply be replaced with a potentiometer and a limiting resistor in series. This way the ration of the voltage divider can vary, and therefore the base voltage can vary as well. The result is a variable emitter voltage which eventually leads to a variable current set for the LED. This is by far the simplest method of all and most straight-forward to understand.

For example, let's convert the example circuit that was implemented in page 2. The circuit was an LED driver with single transistor biased with voltage divider. For the voltage divider, we calculated R2 at 750 Ohms. I will now replace this resistor with an 1K potentiometer. Let's calculate first the result.

When the potentiometer is at lowest position, the expected voltage is:
R1 = 4700 Ohms
R2 = 0
VB = VDD x R2 / (R1+R2) = 0
VE = 0
IC = 0

And when the potentiometer is at maximum:
R1 = 4700 Ohms
R2 = 1000
VB = VDD x R2 / (R1+R2) = 2.1 volts
VE = 2.1 - 0.7 = 1.4
IC = VE / RE = 1.4 / 47 = 29.7 mA

So, the LED current can be adjusted from 0 to 29.7 mA. At 0.02mA (20uA) the LED barely lights The brightness is proportional to the current. At 20mA the LED is bright enough Full brightness is at 30mA.

One method to control them all - Injecting PWM pulses to control the brightness

Controlling the brightness with PWM pulses is the most efficient way to do it. Linear controllers dissipate the excessive power in the form of heat to deliver the required power, instead, PWM drivers do not deliver the power at all (or better i should say the "energy").

To inject PWM pulses to the previous circuits, you need first a PWM oscillator. There are numerous different PWM generators. The following one is my favorite one for two reasons: It is dead-simple to make and (most important) it acts as a current sink and not as a current source. The oscillation frequency (for the current setup) is about 4.5 KHz. This means that the designer selects the pulse amplitude with a pull-up resistor: The 555 timer has pin 3 for output, but this circuit uses pin 7 instead. Read the 555 timer theory and you'll understand what exactly happens. Pin 7 is labeled as "Discharge", and it is simply an open collector transistor which grounds the capacitor through a resistor. The capacitor and the resistor are external components, so pin 7 is actually directly connected to the collector of the internal transistor of the 555. Therefore, the output acts as current sink: To get pulses out of this pin, you need to add a pull-up resistor. This has a great advantage: The output pulses can have different amplitude (voltage) than the operating voltage of the 555 chip! From now on, i will use the symbol shown on the right side for this circuit.

Injecting PWM pulses to the resistor LED driver

This is how you must connect the previous PWM generator to control the brightness of the simple resistor LED driver: The PWM pulses pull down the Anode of the LED and thus the LED turns off. When the 555 internal transistor is switched off, all the current runs through the LED thus turning it on. The circuit's resistor plays the role of the pull-up resistor as mentioned before. Continue reading. Click here to go to the next page >>>. OR click here to view the presentation.

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