If you are an electronic hobbyist you would very well know about transistors and how they work. Transistors basically have three leads viz. the base, the emitter and the collector. The emitter is the reference lead, or the common exit point for all potentials applied over the base and the collector. The base forms the trigger lead, while the collector becomes the load handling lead of the device. Transistors are primarily of two types, NPN and PNP, both behave similarly but with opposite polarities.
Using a transistor in electronic circuits is quite easy. If it’s a NPN transistor, connect the
A small voltage and current applied across base/emitter of the transistor will allow a relatively higher voltage and current to flow across collector/emitter. This voltage may be anywhere near or above 0.7 volts, which shows how minute can be the voltage for triggering a transistor. However this may not be true for the current parameter. The
Therefore if the current required for operating the load at the collector of the transistor is known, the base current drive may be easily determined.
A new hobbyist may feel confused regarding the current which may be required for switching a particular load at the specified current. This may be easily found and confirmed either by measuring the resistance of the load or by practically connecting the load and an ammeter in series across the rated supply voltage.
Using an ammeter will provide you with a direct reading of the consumed current or the optimal current requirement of the load. If you want to identify this through the resistance of the load then you may to use the formula presented HERE.
The above link provides a comprehensive explanation regarding how to calculate the base resistor transistors while using in electronic circuits.