Wednesday 20 May 2015

Constant current LED driver

LEDs have become ubiquitous these days you can find them in different roles like being used as indicators, home and office lightning solutions, in music synced lightening systems at clubs, signboards  etc. Today i will share about a constant current driving circuitry to make some LEDs glow with DC power sources.

Why do we need a driver circuit?

The first thing that might come to your mind is why do we need a driver circuit when you can just simply connect a coin cell battery to make an LED glow and  for higher voltage sources you can easily drop the voltage to desired value by using a resistor.
The use of a dedicated circuit is necessary because using the above mentioned method won’t provide a consistent and robust solution. The current through the LED is sensitive to even small changes in voltage across the LED and the temperature of LED so using the above mentioned way may provide different LED performance in different environmental conditions. The inefficiency using a resistor to drop the voltage would even be magnified while driving a high wattage LED.

Constant current LED circuit built around IRFZ44

The circuit below shows a reliable way of lighting an LED. This circuit allows us to decide a particular value of current that will flow through the LED by adjusting the value of R2 resistor.Depending upon how much current is allowed to pass through the LED the intensity of LED is adjusted.

How does the Circuit works and deciding the values of  current setting resistance?

when the value of current flowing through the LED to the R2 becomes high enough to provide a drop of 0.7 V across the base emitter junction of the transistor the diode gets turned on and the extra current goes to ground through the diode. This feedback mechanism leads to a constant current flowing through the LED.

Deciding the value of  R2
Value of current through the LED = 0.5/R2
value of power dissipated across =0.25/R2
R1 hould be a high value quater watt resistance say 100k.

Benefits of using this circuit
  • consistent LED performance over large range of supply voltage
  • low dropout ie the output voltage can be just 0.6 volt higher than the input voltage
  • cheap circuit
Important Points

  • Because the transistor T1 is thermal sensitive the current set point is also thermal sensitive so choosing a transistor with low thermal senstivity will give more accurate results like 2N5088BU
  • Care should be taken that the power dissipated by Q1 is within the range specified in data sheet power = voltage across Q1 x current through the LED
  • For maximum efficiency the LED should be configured such that the voltage is near the supply voltage.

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