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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.
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.
The difference between 12.3V and 11.7V is 0.6V, which is exactly the voltage drop of many semiconductors such as diodes and transistors. The TIP120 transistor introduces a voltage drop of 0.6V, so you need to factor this into your calculation for a current-limiting resistor. You are still using a current-limiting resistor, aren't you?
