Voltage divider with transistor switch

Fascinating conversation about voltage direction

FYI, The discussion you refer to was about the convention for CURRENT direction (not voltage ) and the bottom line was that the accepted convention now is the direction of the positively charged holes as opposed to the negatively charge electrons. The positively charged holes travel from pos it neg , hence the convention for that direction. I hope your next question isn't "what's a hole ?"... :grin:

raschemmel: FYI, The discussion you refer to was about the convention for CURRENT direction (not voltage ) and the bottom line was that the accepted convention now is the direction of the positively charged holes as opposed to the negatively charge electrons. The positively charged holes travel from pos it neg , hence the convention for that direction. I hope your next question isn't "what's a hole ?"... :grin:

I meant current of course.

CrossRoads: P-channel MOSFET can have a much lower Rds (on-state resistance) and thus have lower voltage loss (= to Rds x current) vs a fixed voltage drop (0.5V to 0.7V) across a PNP transistor.

What is the battery voltage you are using? It's possible you won't need the NPN BC547 at all if the MCU can handle the battery voltage on its IO pins.

Thanks for the info! I'm actually monitoring voltage of external device (DC/AC inverter's data port, technically it's LED status light), which varies between 7V and 14V depending if inverter is in sleep mode or on so MCU won't be able to handle voltage. BC547 is just an example from discussion on the topic I found online, I'm actually using 2N3904. I already ordered IRF9640 MOSFET (before I saw your post), I think I should be fine, datasheet says Vgs is 2-4V...

CrossRoads:
P-channel MOSFET can have a much lower Rds (on-state resistance) and thus have lower voltage loss (= to Rds x current) vs a fixed voltage drop (0.5V to 0.7V) across a PNP transistor.

Oopsie!

You are confusing the Vbe of the transistor with the VceSat which is in fact much lower and plausibly comparable with the FET at the single mA current level considered here. Suggest you go look at some curves.

CrossRoads:
What is the battery voltage you are using? It’s possible you won’t need the NPN BC547 at all if the MCU can handle the battery voltage on its IO pins.

It does help to read the discussion first. This was revealed in reply #9.