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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?

schematic

simulate this circuit – Schematic created using CircuitLab

thanks

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  • \$\begingroup\$ The current through the LED is roughly equal to: I_LEDx = hfe*(Vpinx - 0.7)/Rx. Transistors act like current controlled current sources in this configuration. The current going through the LED is roughly equal to the current flowing into the base*hfe. (that's why you need really high vlaie base resistors) \$\endgroup\$ – Sam Dec 22 '16 at 23:49
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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.

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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).

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