Originally I wanted to switch a 5V relay with a 3.3V output of a microcontroller.
It didn’t work, and I tried to isolate the cause, so I ended up with the attached circuit.
Okay, maybe I could use a 3V relay without a transistor, I know. But I really want to understand what’s going on here:
The regulator works, it has 3.25V on it’s Vout pin. The relay works, I can switch it with 5V. I checked the polarity. The transistor works (the multimeter sais it’s ok) and I tried different ones (also a 2N2222).
R1 was 1K but I also tried 100R and no resistor at all.
I guess it’s the 3.3V which cause the problem. I can see max 50µA flowing through R1, which is too little for the transistor to go low resistance.
What can I do?
After staring at the drawing, I realize, the coil limits the flow from the transistor’s base to ground.
I’m sorry for the post…