LED Driver Circuit

[jisoccer]

Hi all,

I need help getting started with my LED driver circuit.  I have built a working circuit using a LM 317 to keep my current constant at/around 20 mA; however, when doing this, my voltage drop is so great the the power, and therefor lumens, take a severe hit to where the LEDs will not be bright enough for what I need them to do.  So, is there a better way to wire the circuit, or should I just add more strings of LEDs to compensate for the loss of power?  Any help would be greatly appreciated!

[GGoodwin]

Hi jisoccer,

The voltage drops in the LEDs are not so great, in fact, they are very low. This is because Multisim Live does not automatically provide the correct device characteristics when you pick LED from the component palette. The default characteristics for LED is close to a silicon diode, changing the color does not change any of the device parameters.

Even if you have the correct device model, your circuit will not work as you thought. The LM317 is wired as a constant current source but you cannot expect that its output current will divide equally in the three branches. In simulation you obtained ≈20 mA in each branch because all LEDs have the same (incorrect) model. In a real circuit one branch will be bright while another will be faint or off, a worse case is when LED(s) get damaged. The red, green, and blue branches should not be wired directly in parallel.

There are several ways of driving the LEDs at essentially equal currents, if you prefer using the LM317 you can try any of these:

• Use three LM317 constant current source, one for red branch, one for green, and one for blue branch. Increase the value of the current-setting resistor so that the LM317 output current is 20 mA.
• Wire all LEDs in a single series string, this will require a single LM317 20mA current source. The input voltage should be increased because the single LED string will need higher voltage.

Best regards,

G. Goodwin