I'm trying to implement high side switch using P-MOSFET with NPN driver. I will drive the whole switch with AVR GPIO running at 3.3V (
3.3V GPIO label). KT231B9 (it's an English translation from Russian КТ231Б9) NPN have hFE of 110 or greater so feeding 0.1 mA of current should be 10x times enough to turn of
Q1 fully thus effectively turning
Q2 to saturation too.
I've build the prototype on the breadboard and found that if I'm feeding 3.3V to the
R1 then I have full 12V on the drain of
Q2 and if I attach a real load (motorized ball valve) instead of multimeter it works fine. However when no input is given to the
R1 (hanging wire) then I can read about 5.6V on the drain of
Q2 with the multimeter. It have puzzled me and I've attached a 2.2k resistor instead of the multimeter to check whether current flows through it (other side of resistor was connected to the
GND). Interesting enough, voltage drop on the 2.2k was 0V so it seems like no current were flowing through it and the voltage measured with multimeter was some kind of sporadic.
I wonder why this is happening and if I should worry about that. Thank you.