Explain the working of a transistor as a switch.

Explanation

Transistor as a switch : Both the n-p-n and p-n-p bipolar transistors can be made to operate as an "ON/OFF" type solid state switch for controlling high power devices such as motors, lamps. When transistor is operated in cut off region o saturation region, it works as a switch. The circuit for transistor as a switch is shown in the following figure :

On applying Kirchhoff's voltage law to output part,
`V_("CC") = I_(C)R_(L) + V_(CE)`
or `V_(CE) = V_("CC") - I_(C)R_(L)`
When `V_(L) = 0, I_(B) = 0` and hence, `I_(C) = 0`.
`:. V_(CE) = V_("CC")` (source voltage)
That is, when `V_(i) = 0`, no current flow through transistor and load `R_(L)`. Entire voltage develops across transistor `(V_(CE) = V_("CC"))`. This condition is similar to open switch (i.e, `I_(C) = 0, V_(0) = V_("CC") =` source voltage)
When `V_(i)` is positive and greater than `0.7 V` the base current flows that forces transistor into saturation and large current flows through transistor and load `R_(L)`. The entire source voltage gets developed across the load `R_(L)` and very low voltage drops across transistor `(V_(CE) = 0.2)`
This condition is similar to closed switch.
(i.e, `I_(C) = (V_("CC"))/(R_(L))` , and `V_(0) = V_(CE) = 0.2 V`).
The main idea is that, load `R_(L)` is connected or disconnected from the source `V_("CC")` by transistor switch without any make or brake of circuit.