I am reading up on some reverse polarity options, since I think I busted some hardware recently when incidentally powering my Arduino Uno from the USB and from vin pin at the same time.
GreatScott had a youtube video which helped summarize some useful information which got me started in the right direction. He referenced this blog post and circuit which I am now dissecting and I wanted some confirmation whether or not I am fully understanding it correctly. I am not very good at reading circuits diagrams still. Plus his example uses a 5v BMS and a single cell battery, and in my project I am using a much larger 12v battery so the required mosfets might be subject to change.
The diagram image attached show the circuit in the NPN form. I am choosing this one over the PNP type simply because I already have some NPN mosfets lying around that I want to see if I can use. I have annotated this with how I think the mechanical behavior is functioning. I just wanted to confirm whether or not I got it right.
So after much thought, here is how I see the signal flow:
When in the correct orientation, current would flow in parallel to the collector of the 2N2907A transistor as well as into the positive terminal of the battery cell.
The battery cells (-) terminal would send a negative -3.7v in parallel into the gate of the 2N2907A and to the source of the 2N7000.
The negative voltage would then open the gate of the 2N2907A transistor, allowing the positive voltage to flow in parallel to the base of the 2N7000 mosfet and to the TP4056 BMS.
The positive voltage on the base of the 2N7000 would open the source/emitter path, allowing the negative voltage to flow back into the negative terminal of the TP4056 BMS, completing the circuit.
...and if the battery was the incorrect polarity:
+3.7v would flow from the battery cell to the 2N2907A transistor. Since it is positive, it will remain closed.
The diode prevents the +3.7v current of the TP4056 from flowing to the positive and negative lead of the battery cell, preventing a short from happening. The battery cell is now in a closed loop and there is no sink for the current from the TP4056 to complete the circuit.
I am skeptical that I got this correct. If so, then what is the interaction that happens at the 2N7000 stage as the -3.7v is allowed to pass through the collector/emitter of the 2N7000 while the positive +3.7v TP4056 is flowing the same way in parallel? How does the (-) of the TP4056 allow both a positive and negative current to flow back to the BMS?