Hey everyone,
I'm struggling to understand the behaviour of a little circuit I'm playing with and was wondering if someone here could help explain it to me. I'm looking to control a set of 5 super bright LED's via PWM from a Raspi Zero W. I envisioned using an NPN transistor (currently using a 2N2222) with the base connected to my PWM pin (3.3v), the Collector connected to the 5VDC power rail on the Raspi, and the Emitter connected to a current-limiting resistor (100 ohm) and subsequently the 5 LEDs in parallel). The forward voltage drop on my LEDs is 3v, with peak brightness at 20mA.
I've mocked up the circuit in the browser simulator here:
http://tinyurl.com/yam4zfhtThe issue I'm having in real life (and in the simulator) is that the voltage on the emitter is capping out at ~2.5/2.7V, less than the forward voltage drop of the LEDs, so they are very dim and are drawing very little current.
It was my understanding that as long as I saturated the B, the voltage drop between C and E should be small (0.2V or so) and I should be able to realise the required 3v to the LEDs without issue. I've checked the PWM signal which is clean and as-expected, the 5V rail is strong (it's tied directly to the 5V USB input, a 2.4A iPad charger), and trying multiple current-limiting Base resistors makes little/no difference to the Emitter voltage.
Does anyone know why this is? The voltage drop between CE seems really high, and I don't understand the relationship between BE voltage and CE voltage (in the simulator, changing the base logic voltage level to 5V yields a better voltage on the emitter, but still higher drop than I would have imagined).
Thanks!