Current limiting is one of the functions that resistors serve in Electronics Circuits. Every electronic component has a current rating. A Transistor’s Ic maximum is 1A. This means that it can withstand 1A maximum collector current. Let’s assume that the Collector-Emitter resistance of a transistor in saturation mode is zero and that a 10V supply is applied between Collector/Emitter. A resistor between the supply voltage to the collector should be used to ensure that the collector current is not greater than 1A. The resistor value should not exceed 10V/1A = 10 Ohm.
So if the Vcc is 10V and Ic Max of the transistor is 1A then Rc must be more that 10 Ohm.
Let’s take another look at this.
The circuit below will indicate that the maximum current of the LED is 10mA. The LED will be damaged if the current passing through it exceeds 10mA. If the output voltage of the Microcontroller exceeds 5V and the forward voltage of the LED exceeds 2V, the remaining 3V must be dropped across the resistor. The resistor value must not be less than 3V/10mA = 300 Ohm.
The above are two typical example of why resistor are used in Electronics Circuit.
Electronics are nothing but control. Controlling the current electronics allows you to do this. To control the current, resistors can be used. This is why resistors are often used in Electronics Circuits.
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