arduinoled
I'm trying to control a 7W 560MA 12-14V COB LED Strip using Arduino. When I connect the LED directly to a power supply at 14V (using 1 ohm limiting resistor) I have 0.56A / 12.3V on my LED. But if I connect it through TIP120 transistor to control it by Arduino (using 14V power supply) I have only 0.38A / 11.7V on my LED. So I have a significant drop of LED power. Did I choose the wrong transistor? If so, please help me to choose the right one.
Your setup is quite different from the standard TLC5940 use, but can be done. The key points are that the TCL5940 is open-drain, and that the actual current and high voltage will be driven by the TIP120.
Logic will be reversed, from the TLC5940's standpoint. Motor will be On, when the TLC Out is OFF. When the OUTx is set to Off, the line will be brought up to 5V through the R Pullup, which also limits the current to 2.4mA (5V - TIP120's 1.4v drop = 3.6V through the resistor, 3.6 / 1500 = 2.4mA). Based on the TIP120's gain, this should (2.4mA * 1000) 2.4 Amps. You can adjust the resistor for the needed gain. When the OUTx is set to On, the line is shorted to ground, turning off the Tip120.
I.E. Setting the TLC5940 to 65% ON, will result in an actual 35% ON, at the motor. Setting it to 25% ON will result in 75% ON at the motor.
Some things to watch out for here:
Try it out at DC, swapping out various parts and observing with your voltmeter.
Test your LEDs (but with a current-limiting resistor). What if they are backwards?
Test your transistor, driving the base from 5V with a 1K ohm (or so) resistor. When the LEDs turn on, note the voltages on the base and collector of the transistor.
Finally, ensure your Arduino output can actually drive the base of the transistor. Read it with the voltmeter.
Best Answer
A Darlington has great current amplification but doesn't switch on with a zero volt drop: -
You can put a decent drive voltage on the base that will make the input transistor turn on and only drop 0.2 (ish) volts. This is recognized as the saturation limit for a BJT generally. Some are lower of course.
So, the input transistor is turned on and the voltage on its emitter is 0.2V below the voltage on the common collector point. The final transistor needs about 0.7V across its base and emitter but it can only get this voltage via the 1st transistor hence, the best volt drop across the final stage is 0.7V + 0.2V - you lose about a volt of drive.
This is the basic problem and maybe consider using a MOSFET instead.