This is the circuit diagram of a portable mobile charger and it takes a 9V battery and gives 5.12 to 5.40 volt as output. It output voltage fluctuates. But that is not the concerned matter. I wanted to know why the two capacitors are placed there- what is the exact work of the two capacitors there? Any other components could be used there or not? Thank you!
The 100 uf capacitor is used to reduce voltage transients on the input of the regulator and improve the output voltage stability. It may not be necessary when using a battery as a power source. The 0.1uf capacitor reduces transient voltage fluctuations on the output side caused by rapid changes in load current. Download the data sheet for the 7805 regulator if you wish to understand more about what external components are needed for that part and their effect on performance.
Also consider that the 7805 is a linear voltage regulator, which means that it converts nearly half of the energy in the battery to waste heat. Recom and others make switching regulators that are a drop-in replacement for the 7805. They have a claimed efficiency of 95%. Most of the energy in the battery is converted to output power and very little is lost as waste heat.
They are input and output decoupling capacitors for the 7805 voltage regulator. They provide a low-impedance high-frequency path (virtually a short circuit) at the input and at the output of the regulator. Both are needed and recommended to suppress high frequency noise from the source and/or from the load (e.g. peak currents absorbed by digital circuits), to avoid bounce on the regulated voltage and to achieve EMC. The output capacitor can be also required for frequency compensation (i.e. to avoid instability) of the regulator IC.
Charge and voltage regulation at the time of switching in my view.
As David Sarraf said "The 100 uf capacitor is used to reduce voltage transients on the input of the regulator and improve the output voltage stability." Moreover the 0.1uF capacitor is used to decouple EMI noise from USB Port. That noise could have frequencies greater tan 100 MHz and the 0.1uF capacitor provides a low impedance path to high frequencies.
As David Sarraf already statet: "download the data sheet". Adding to that: find a good data sheet (as data sheet quality varies between manufacturers.
Th make things short: the 100 uF cap is to buffer the input. Could be a smaller value when supplied from a battery or another stabilized source.
The 100 nF is to prevent oscillations. Keep in mind that even linear oscillators depend on a control loop where loop stability matters.
What I'm missing: a 100 nF cap on the input side! IIRC this is necessary as well to prevent HF oscillations.
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