Bjt as a switch10/23/2023 Saturation means there is enough base current to turn on the transistor fully. As a switch, the BJT operates in saturation mode. To make it less of a hassle, we can insert a toggle switch in the circuit so that we can just toggle the lever of the toggle switch if we want to turn ON or OFF the light bulb, instead of connecting or disconnecting the electrical plug. For an NPN BJT, the emitter connects to ground, and the collector attaches to the negative side of the load. So to turn ON the light bulb, we’ll insert the electrical plug to the outlet and to turn it off, we’ll disconnect the plug from the outlet. I hope you can imagine it, in reality, the light bulb is connected to the AC mains or wall outlet through an electrical plug and zip-cord. This allows current to flow (or not) in other parts of a circuit. Using a BJT as a Switch: An Example Cutoff vs Saturated Transistors Why Use a Transistor to Control Current Because a transistor’s collector current is proportionally limited by its base current, it can be used as a sort of current-controlled switch. In figure 3, we can see a schematic diagram of a light bulb connected directly to the AC mains. The transistor can be used as a switch if biased in the saturation and cut-off regions. So basically, an SPST toggle switch is simply an ON-OFF switch with two terminals (A & B) that can be connected together or disconnected from each other by toggling the lever (actuator). In this Physics video in Hindi for class 12 we explained how a Bipolar Junction Transistor (BJT) is used as a Switch. When is a MOSFET more appropriate as a switch than a BJT Answer: 1) a MOSFET is better than a BJT when: When you need really low power. An SPST switch has one pole and one throw, so it can only control one circuit and its pole can be connected to one terminal only. The number of poles in a switch determines how many separate circuits the switch can control while the throw-count of a switch tells us how many positions each of the switch’s poles can be connected to. ![]() In figure 2, you can see a single-pole, single-throw (SPST) toggle switch and the circuit symbol of an SPST switch. A switch is an electrical or electronic device that can open or close a circuit, stopping or allowing the flow of current in a circuit.
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