My circuit (which for now is just a uC and a switching transistor) has a problem
if I connect the main transistor switch in a way that the load is on collector, and a digital pin goes to the base trough an optoisolator, so that upon initial turn-on, it can sustain itself until shutdown, I have problems with the + side (which is obviously always connected) floating on pins of the microcontroller.
The circuit will stay always on unless I put a very low pulldown on the base of the power transistor. This is why I have issues with off-state power consumption. I either have a power draw because the transistor is always slightly on, or I have too low of a pulldown resistance and the base never gets saturated fully, or the pulldown itself is slowly leaking.
Problem is that the "local" ground of the uC is always 0.7v higher than the power ground, meaning that there is always a signal on the transistor and pulling down pins to this local ground will only leave the potential to that arbitrary value. I cannot properly pull down the floating pins without introducing a path to the actual ground - because as it turns out the uC can somewhat receive power trough it's digital pins.
In the other case when I put the load on emitter, there is no voltage gain and the microcontroller won't even start. I put two of them in a darlington cascade but it's still not permeable enough.
What should i do if i want/need to have a common ground connected at all times ?
I know this is why MOSFETs exist but i don't have anything besides a z44, which i believe won't work with 3.7v
Naučil som sa angličtinu, programovanie, elektrotechniku, vedecké smery...
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