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Topic: Transistor Newb needs help. (Read 1 time) previous topic - next topic


Howdy all. I'd like an Arduino to monitor the charging of 2 3-packs of NiMH cells by reading their voltages independently.
When the charger is connected to a steady supply of coulombs (i.e., the wall wart is plugged in) the atmega (a 328 in my case, though that's probably immaterial) will join the packs together, and an LM317 will juice up the 6 cells.
To read the voltages, I'd like to split the battery of 6 cells into two batteries of 3 cells, and then read the voltages directly on an analog pin.
Here's the circuit (which doesn't work) that I designed.

When the digital signal goes high, the not gate drops to like 38 milivolts.
What I'd like to happen is this:
Digital Signal goes High, Ground Splitter stops conducting, Ground joiner starts conducting, and A5 Protector starts conducting.
But that doesn't happen.

I don't understand transistors very well.


Oh, the transistors are all 3055 NPN in TO220 packages, if that makes any difference.


I don't understand transistors very well.

Yep, and having looked at that circuit I am not sure I do either any more!

A transistor is not a switch, it can look like a switch only under certain wiring conditions. The idea behind Q3 switching the voltage to A4 will simply not work. When a transistor turns on the collector / emitter has about 1V across it anyway.
Transistors control the flow of current.
What you are trying to do basically won't work. You can't split the ground and switch like this unless you use relay contacts.


I guess I could put a servo on an SPDT...



You would not do it with a SPDT you need a DPDT switch, because you need to switch the ground as well. You can get DPDT relays, it's much easier than putting a servo to a switch..

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