BJT Transistor as a Switch, Saturation Calculator

A BJT transistor can be used as an electronic switch when it is driven into saturation, or alternatively driven to the cut off region. Calculating the base resistor is a common engineering task, which this calculator automates.

Rc (Collector resistor) (K ohms)
VP (Positive Voltage) (V)
Beta(min) (Current gain)
VBE (Base to emitter drop) (V)
Rb (Base resistor) (K ohms)


The current through the load at saturations is Ic= VP/Rc. The base current must be Ib= Ic/Beta. Transistors have a varying gain so we want to use the minimum beta value, to ensure saturation.

Rb= (VP-VBE)/Ib= (VP-VBE)*Beta/Ic= (VP-VBE)*Beta*Rc/VP

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