Design

CHAPTER 4: LEDs 63

Quantity Names Item 3 R1 4.7Ω resistor Appendix Code K2

1 1 Battery clip (to destroy) H2 PP3 9V battery Jumper wires T6

Design

Figure 4-10 shows the schematic diagram for regulating the current to a high-power LED like the one shown in Figure 4-9.

The LM317 is very easy to use in a constant current mode. It will always strive to keep its output voltage at exactly 1.25V above whatever voltage the Adj (adjust) pin is at.

The LED we are going to use is a 1W white light LED. It has an If (forward current) of 300mA and a Vf (forward voltage) of 3.4V.

The formula for calculating the right value for R1 for use with the LM317 is:

R = 1.25V / I

so in this case, R = 1.25 / 0.3 = 4.2Ω

If we used a standard resistor value of 4.7Ω, then this would reduce the current to:

I = 1.25V / 4.7Ω = 266 mA

Checking the power rating for the resistor, the LM317 will always have 1.25V between Out and Adj. So:

P = V × I = 1.25V × 266mA = 0.33W

Figure 4-10 An LM317 constant current LED driver schematic