When to use Capacitors?
When I first started out in electronics I struggled with the same question. The problem is that capacitors are used in a vast number of different ways.
However, as you're just starting out in electronics you probably only need to know about a few of these to start with. The most widely used and basic of these are:
Power Supply Smoothing
This is the easiest and very widely used application of a capacitor. If you stick a big beefy electrolytic capacitor (the bigger the better), it will fill in all the gaps created by rectifying an AC waveform, to create a relatively smooth DC. It works by repeatedly charging during the peaks, and discharging during the gaps. However, the more load you put on it, the quicker it will drain the capacitor and the more ripple you'll get.
Timing
If you supply power to a capacitor through a resistor, it will take time to charge. If you connect a resistive load to a capacitor, it will take time to discharge. The key thing to understand here about timing circuits is that capacitors appear as though they are short circuit while they are charging, but as soon as they are charged, they appear to be open circuit.
Filtering
If you pass DC through a capacitor, it will charge and then block any further current from flowing. However, if you pass AC through a capacitor, it will flow. How much current flows depends on the frequency of the AC, and the value of the capacitor.
Uses:
ac coupling –- blocking -- isolation
timing -- Time for a capacitor to charge or discharge is very roughly RC where R is the resistor in series with the capacitor.
Filter ( often power supply filter )
decoupling
tuned circuits
http://opencircuits.com/Capacitors
How Stuff Works says
Sometimes, capacitors are used to store charge for high-speed use. That's what a flash does. Big lasers use this technique as well to get very bright, instantaneous flashes.
Capacitors can also eliminate ripples. If a line carrying DC voltage has ripples or spikes in it, a big capacitor can even out the voltage by absorbing the peaks and filling in the valleys.
A capacitor can block DC voltage. If you hook a small capacitor to a battery, then no current will flow between the poles of the battery once the capacitor charges. However, any alternating current (AC) signal flows through a capacitor unimpeded. That's because the capacitor will charge and discharge as the alternating current fluctuates, making it appear that the alternating current is flowing.
Wikipedia lists the following applications:
- energy storage
- pulsed power
- power conditioning
- power factor correction
- signal coupling
- decoupling
- noise filters and snubbers
- motor starters
- signal processing
- tuned circuits
- sensing