Driving High Power LEDs with this circuit

driverledpowerresistance

I've just completed building this circuit (thanks alexan), times 3 for each RGB channel.

R1 = 4.7k
R2 = 1K
R3 = 10K

Q1 = BD140

Q2 = BD139

enter image description here

This circuit is used to power 4x20mA RGB LEDs in parallel and am happy with the result. They're using 510/470/470 resistors for the RGB channels respectively.

Now I'd like to do the same but power a much higher power LED but I have a few questions before I go ahead and purchase it.

  1. What's the reason for not having a common cathode, instead having +/- for each channel? Can the cathode go to a common ground in my circuit, while each of the anodes go to the emitter of each Q1?

  2. Would the schematic above be sufficient to drive these LEDs? (remember the schematic represents a single channel)

  3. How do I calculate the resistor values for each channel for a 12VDC power supply? Is the 350mA current for all 3 channels or is it per channel?

If it's per channel, then each channel would need 166.67mA of current. The calculation I'm using is:

Typical Forward Voltage R/G/B (datasheet) = 2.4/3.5/3.4

Red = (12V - 2.4V) / 166.67 = 57 Ohm (1.6W) 
Green (12V - 3.5) / 166.67 = 50 Ohm (1.4W) 
Blue (12V - 3.4) / 166.67 = 51 Ohm (1.4W)

This doesn't seem right to me at all, because the total power comes to 4.4 Watts.

So, where am I going wrong?

Best Answer

What's the reason for not having a common cathode, instead having +/- for each channel? Can the cathode go to a common ground in my circuit, while each of the anodes go to the emitter of each Q1?

But the cathode is common (assuming the led resistor is connected to the anode side), all cathodes are connected to the ground so they are connected to each other too.
If you have connected the resistors to the cathode side of the leds then you can't connect them together.
I'm not sure why you mention the emitter, the leds are connected between the collector and ground

enter image description here

Would the schematic above be sufficient to drive these LEDs? (remember the schematic represents a single channel)

If you increase the base current to be sufficient then it can drive higher power leds too.

How do I calculate the resistor values for each channel for a 12VDC power supply? Is the 350mA current for all 3 channels or is it per channel?

The base current should be about 1/10 to 1/20 of the output current to saturate the transistor and have a low voltage drop across the emitter-collector.

When you are using one color (the others are off), you can use 350mA for sure but I'm in doubt of the total max current when all three are on.

This doesn't seem right to me at all, because the total power comes to 4.4 Watts.

There is nothing wrong in the calculation. What you get is the dissipated power in each resistor for the given input voltage/output current.

Related Topic