The scenario
An NTC thermistor in a voltage divider drives an NPN transistor. As temperature rises, NTC resistance drops, base voltage increases, turning on the transistor to pull the open-collector output low, signaling an overheat condition.
What is in this drawing
Read the decisions behind it.
01
Use a common-emitter NPN transistor for sufficient gain to switch with the divider output.
02
Open-collector output allows wired-OR connections and compatibility with various logic levels.
Suitable for thermostat controllers, over-temperature protection in power supplies, and alarm systems where the output must interface with microcontrollers or relays.
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