LED Driving Issue

Hi there,

We are trying to control a string of 3W led's off of an arduino, but are having issues doing it. We originally tried using a meanwell ldd-1000h but we (somehow, we are unsure how) fried it. Now we are trying to use a NPN transistor (BD139) on PWM. The circuit goes as follows, the + from our 16v 3.5A Power Supply goes straight to the LED string +. The ground of the power supply goes to the collector, as well as the ground from the arduino. The emitter is connected to D10 (We are just running the fade code). And the negative of the LED string is connected to the base. This setup works with just one standard 5mm LED, but is not working with our LED string. We desperately need to get this working ASAP so any and all help is appreciated.



D10 to 150 ohm resistor to base (limits current from Arduino pin).
Emitter to Gnd.

LED anode to +V, LED cathode to collector.
Do the LEDs need a current limit resistor too? BD139 is only good for 1.5A.

he emitter is connected to D10

I sincerely hope you have muddled your terminology there.

The only part of your transistor that should be connected to a data pin is the base. Even this should have a current limiting resistor on it.

Hi there,

thanks for the quick reply. The LEDs are 3w each and have the correct length of resistance wire connected to the positive input. We only have a ohm resistor or a 220 will these work? Regarding KenF's post, we copied another circuit which had D10 connected to the Emitter without a Current limiting resistor, this was working with a small LED. We will change that now and add the current limiting resistor



Thank you so much everyone we know have it working

And it only cost you one LDD? You got off lightly :slight_smile:

The only time you would connect the emitter of a transistor to an I/O pin is if you're using it for INPUT of a very weak signal.

Haha no, it was a misinterpretation from me, I thought the NPN transistor we were using had the base in the middle, not as pin 3. And we weren't using the transistor with the LDD, we are really not sure what happened with the LDD...


Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png or pdf?

Tom..... :slight_smile:

Hi there,

I made this quick Fritzing diagram to show you :slight_smile: Pretend the 3W LED's are wired in series, and the little coil at the end of one of the strings is meant to be some resistance wire we used as a current limiting resistor, the 2nd string didn't need it. Also pretend the 220 ohm reisistors attached to the base of the transistors are 150 ohm. The power input was 16v 3.5A hence the 5v regulator for the arduino.