Electronic – Getting more current through a BJT

bjtled

I'm trying to alternate between two 3W LEDs (forward current of 750mA, forward voltage of 3.6V). I've got a 555 set up in astable mode, and am driving the LEDs via npn and pnp transistors. In practice, the LED that's run via the pnp transistor gets more current when it's switched on compared to the LED connected to the npn, and I'm not sure why. I'd just do away with the transistors all together and let the 555 sink/source the current for me, but I believe it can only handle up to 200mA, and the LEDs need more. FWIW, I'm running this circuit off of 2 3V coin cell batteries, and when connected directly to the LEDs they are extremely bright.

So my questions are:
1. Is there a configuration change I can make to get more current through the transistors? (more base current maybe?)

  1. Is there a better or more efficient setup to perform essentially the same function?

Thanks!
Imgur Schematic

Best Answer

Coin cells or no coin cells, the base resistors R3 and R4 are much to big. You will need to chose transistors that can sink more current and feed them with a base current of 15mA or so (divide your desired LED current by 50, typically). Your base resistors need to be a few hundred ohms.

Also, you shouldn't use a PNP as a low side switch. It will only act as an emitter follower and never reach saturation. Move the PNP to the high side...that is, between the anode of D2 and the supply voltage.