# NPN BJT in saturation base voltage irrelevant

Posted on Feb 7, 2014

Build as simple a circuit as possible that will pull a logic line low every time an automotive ignition coil `fires`. My first inclination is to simply attach the `negative` terminal of the coil to the base of of an NPN BJT through a current-limiting resistor, with the emitter tied directly to ground. So there would be 5V on the coll

ector and roughly 300V on the base resistor when being triggered. Somehow, though, I feel like I`m missing something fundamental that won`t let this work. Perhaps it`s just me being spooked by the `high` voltage. I suppose I could use a voltage divider at the base, but I still feel like I`m missing something. Looking for confirmation regarding the proposed functionality, or of my idiocy. Thanks. Edit: I don`t intend to pass any current through the transistor, besides what little is required by the device itself. The idea is that the transistor will go into saturation when the ground is removed from the coil (when the counter-EMF brings the coil`s `negative` terminal up to around 300 volts). Here`s a (rather amateurish) schematic of what I`m thinking: Switch `S1` in that schematic is just a placeholder for the IGBT/Points triggering the ignition coil. Sorry again for the confusion, and thank you, everyone, for all the replies! Your description sounds like you are trying to pass the primary coil current through the base-emitter junction via a resistor that would severely limit the primary current and would not work. Please post a circuit diagram. JIm Dearden May 22 `13 at 6:28 A BJT is in essence a current driven device, hence base voltage is, for most purposes, irrelevant, as long as the actual voltage expressed on the base-emitter junction is within device ratings. However, without a schematic, determining whether the...

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