High Power LED driver using LM317
In this blog I will discuss about one of the easiest led driver circuit. Driving high power led is often the most tricky thing among the electronics hobbyists. Most of our electronics designs are based on voltage regulated but for led we need a circuit to regulate the current. LEDs are current sensitive devices. A slight overcurrent could damage the device completely. So in order to use LED we need a circuit that can control current irrespective of change in voltage. Also LEDs starts to draw more current when they get hot while in use. That’s another reason why we need to regulate the current and not voltage.
There are many LED driver ICs available but those are mostly in SMD packages and are quite hard to find sellers in local market. You have to order these from online sellers like mouser, digikey, etc. Also these driver ICs are pretty expensive and requires additional components like inductors and current regulating low value resistors. Finding all these components together is bit of a pain for hobbyists. But of course these are the ultimate solutions to LED driver.
The most simple method that I could find to drive a high power LED is by using LM317 as a precision current regulator. There are only two components required to make this circuit. You only need a resistor whose value we will calculate, and LM317 IC. Link to the datasheet is here.
Schematic of LED driver
Calculation of current controlling resistor value
The reference voltage of LM317 is 1.25V. Now the current limit Ilim could be calculated using the folmula
Ilim = 1.25 / R1; where R1 is the current controlling resistor.
In order to drive 1 watt led I will be limiting the current to 300mA or 0.3A. So in my case R1 = 4.16 ohms. But there is no resistor of 4.16ohms so I will be using a 4 ohms 1 watt resistor.
Finally connect the components as per the circuit diagram and you are done. Congratulations! We have successfully made an ultra low cost LED driver that can regulate the current.