PLEASE NOTE: The schematic is slightly wrong. It is a MOSFET in my circuit, not the BC850 BJT. The actual part is: BVSS138LT1G
Problem Summary: In the circuit below, I'm turning on the relay by driving the gate of the MOSFET with a microcontroller GPIO pin - I believe it's 3.3V/12mA driving signal. However, when I do this my VDD_5V line becomes about 2.7V.
This seems like a fairly common relay driving circuit, so I'm mystified as to what is going on here!
My initial thought is add a resistor somewhere. I tried adding a resistor between the source of the MOS and GND but I can't get enough voltage for the relay to close (it needs 3.5V at least to open). I might try adding a resistor between pin 2 of the relay and the drain of the MOS, but this is on a PCB and it is tedious to cut traces so I want to be more certain of the approach to take.
The relay coil current from the specs is 74.9 mA (however when I turn it on with a power supply I get less than that... closer to 40mA).