# Using Transistor to dump excessive voltage?

I'm way over my head with this and need a poke in the right direction.

I'm building a small hobby LiPo charger (I'm aware of the risks with a LiPo battery packs regarding usage, charging and discharging). And I figured, why not build in a >4.1v dumper circuit that grounds out any voltage above 4.1v but still feeds the 4.1v through the rest of the circuit.

I tried to do this with a transistor, hoping I could utilize a gate voltage of ~4.1v and drain the source. This would (obviously?) "break" the monitor output to the right seeing as there's less resistance to ground through the transistor, thus cutting off the monitor? Even that I would be fine with. But running a simulator (NI Multisim) I'm getting a weird voltage combination instead of a over-voltage dump.

This is the monitoring part in the LiPo charger, where each Vs# is the individual cell-stack in the LiPo (5 Series LiPo pack in the example).

How can I feed excessive voltage to ground, but still feed 0-4.1v to the monitor gate to the right?

My end goal is also to combine all these voltage monitoring gates into 1 serial/digital mechanism so that I can feed the 5 voltages to a single Arduino port, would this be possible in any way shape or form?

• Where is the charger itself? – Ale..chenski Aug 10 '17 at 18:18
• @AliChen It's on two leads on both ends of the cells in series. Which is controlled by a transistor to cut off the positive lead once the voltage of the entire battery is at it's max combined value. Problem is I can't meassure the combined total voltage because one cell might be slightly off in relation to the others, and thus I need to drain the cells that's ahead of the "slow" cell. But what about it? Is it important? – Torxed Aug 10 '17 at 18:43
• It is apparent that you should familiarize yourself with the concept of "balanced multi-cell charger" , en.wikipedia.org/wiki/Battery_balancing – Ale..chenski Aug 10 '17 at 18:52
• @AliChen That's exactly what you're looking at above? Or have I missed something completely? – Torxed Aug 10 '17 at 18:54
• Hardware design is not like programming: there is no guaranteed "context free grammar" to help you analyze code in isolation. Although you can break down almost any circuit into functional blocks, and analyze them in isolation, often this is not sufficient to understand the bigger picture when they are put together, especially when troubleshooting. Parasitics and PCB layout come to mind. Therefore the "minimal working example" approach, typical of SO, often is hampering troubleshooting when you deal with real circuits! – Lorenzo Donati Aug 10 '17 at 19:35