009 PNP Bipolar Junction Transistor BJT Switch including Capacitor Timed Fade Off

PNP Bipolar Junction Transistors (BJTs) need a small amount of Emitter to Base current to allow a lot of current to flow through Emitter to Collector. A capacitor can store enough charge to maintain base current for a while.

Basic electronics circuit fragments and simple circuits 7 PNP Bipolar Junction Transistor BJT Switch and capacitor fade off diagram by electronzap electronzapdotcom
Basic electronics circuit fragments and simple circuits 7 PNP Bipolar Junction Transistor BJT Switch and capacitor fade off diagram by electronzap electronzapdotcom
  • Closing the switch instantly charges the capacitor while allowing Emitter to Base current. The Base resistor primarily sets the amount of Emitter to Base current.
  • Releasing the switch makes it so that the capacitor is what provides Emitter to Base current while it discharges. Base resistor  and capacitor value is what sets current base current based on the voltage the capacitor was charged to. Don’t forget though, that the Emitter to Base drops approx. 0.6V from the capacitor voltage. So, the capacitor won’t be able to provide anymore base current once it’s voltage drops to about 0.6V.

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Remember that NPN BJTs behave like PNP, but with opposite polarities. The voltages are applied, and the current flows, in the opposite direction of a PNP BJT.

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