M317 is a three-terminal adjustable regulator IC that is very familiar to electronic enthusiasts. When making linear adjustable power supply, I like to use LM317 to make it. As long as the peripheral circuit of the LM317 is slightly modified, the LM317 can also be used as an LED constant current driver with adjustable drive current. Compared with the dedicated LED constant current driver IC, the LM317 consists of a constant current driver circuit with a simple operating voltage range. Wide (8 ~ 40V), large output current, and low cost.

The LM317 has a maximum operating voltage of 40V, a maximum output current of 1.5A, and an internal overvoltage, overcurrent, and short-circuit protection circuit. In this circuit, the LM317 is connected to a constant current source circuit, and its output constant current (ie, the driving current of the LED) is Ih=1.25V/R. The 1.25V in the equation is the fixed voltage between the LM317 output and the regulation terminal.
Calculation of the resistance and power of the resistor R <br> It is assumed that the LM317 is driven by a 1W white LED lamp bead. The operating current of this lamp bead is about 330mA, then we can calculate R=1.25V/0.33 according to the above formula. A = 3.79 Ω. Since there is no nominal value of 3.79 Ω in the ordinary resistor, a resistor with a nominal value of 3.9 Ω is used. The power of R is 1.25V x 0.33A=0.41W, so the resistor R can be selected from a metal film resistor of 3.9Ω/?W. If you want to adjust the brightness of the LED bead, just adjust the resistance of R.

Since the minimum differential pressure of the LM317 is 3V, the operating voltage of the white LED lamp bead is generally around 3V, so the LM317 requires a minimum input voltage of 8V when driving a white LED. In addition, when driving high-power LED lamp beads, if the pressure difference of LM317 is too large, an external heat sink should be connected.

The long LED light board shown in the above figure should not be directly illuminated by DC power supply. Generally, an external DC LED constant current driver is required. This constant current driver can use the constant current driver composed of LM317.
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