I'm bit exciting about my new plan here, my goal is to make a small and simple 5W LED driver with Arduino. I know about current limiting resistor in series with led or in emitter pin of transistor, but just curious, can i driving LED with only transistor and base resistor to limit current going from collector to emitter?
simulate this circuit – Schematic created using CircuitLab
thanks
Short answer, no. You need to either have a resistor in series with the LED (easy), or design the driving transistor to deliver a constant current (harder).
Long answer, well yes, if you want to go to that trouble. If you use a large base resistor (I'd expect to see more like 10k/100k rather than 250 ohms) then a limited base current, multiplied by the hFE of the transistor, should give you a constant collector current. BUT hFE is not a well specified parameter. If you buy a cooking grade transistor, there might be a 6:1 spread of hFE. If you buy binned versions, so a -A or -B versions, then you might get down to 2:1 variation. Unfortunately, hFE varies with temperature, collector voltage, and base current as well.
Given that LEDs have quite a wide current operating range, a LED that's happy with 10mA can still look fairly bright at 1mA. If you select a resistor for each transistor to get your target current, then you could get reasonable subjective performance. You're trading selection time and consistency for ease of build.
This design would rely on transistor hFE wich is not consistant between transistors. As stated in The art of electronics : a design that imply a specific value for hFE is generally a bad design.
You might want to add resistors in series with your LEDs and use the transistors as switches (in saturated mode).
