# Capacitors

#### horsebox

Joined Jun 9, 2007
32
Im trying to learn electronics but im stuck on capacitors and its driving me insane. I've spend about 2 weeks reading trying to figure out capacitors but since I cant understand what the purposes for them are im having a hard time.

I've read about the primary functions of them but I dont know enough about electronics to know what they even mean when constructing a circuit. Thats why im stuck.

Heres one of the functions which I have a slight idea what it might mean
where a
fluctuating DC voltage needs to be smoothed, or, have the ripple taken out.
Lets say I have a basic circuit with a power source and a light bulb. My power source gives 20V but by the time it gets to the light bulb its only 15V due to the wires resistance. Lets say 15V is insufficient. If I was to put a capacitor which held 20V before the light bulb that would solve the problem since it would charge to 20V. Is that what they mean when they say "smooth out fluctuating currents" or am I getting the idea?

Another question

Lets say I have a circuit like this

Im guessing the power supply would cause the light to flash on and off in the short space of time it takes to charge the capacitor. Once the capacitor is fully charged will it just stay like that as long the switch is closed? How would you go about discharging it in a simple circuit like that? Would I have to make the circuit much more complex in order to make the capacitor instantly discharge after it reaches it capacitance?

#### beenthere

Joined Apr 20, 2004
15,819
I believe that using a capacitor to remove ripple is a function of the filter following a rectifier. If the Ebook's explanation wasn't clear, you can try this one - http://www.play-hookey.com/ac_theory/ps_filters.html - for more explanation.

Your example of the capacitor overcoming the wiring resistance to restore voltage across the lamp is not valid.

You are correct about the brief illumination of the lamp during the time the capacitor is charging. It will stay at full charge as long as the switch is closed. An instant discharge would require a low resistance path across the capacitor, say with another switch.

The proper term is full charge, not full capacitance. The capacitor is manufactured with a certain ability to hold a quantity of charge - it's capacitance. There are several ways to discharge a capacitor once it has charged to a preset voltage. It is very hard to tell when a cap is fully charged by the elements in a circuit, so an arbitrary voltage level is used.

The simplest way to discharge the cap is with a relaxation oscillator. If the cap charges through a high value resistor, and has a neon lamp across it, that is one way to make a relaxation oscillator. Voltage builds on the capacitor until the ionization potential for the neon lamp is reached. The neon lamp suddenly becomes a low resistance , and discharges most of the charge on the capacitor. For lower voltages, a unijunction transistor like a 2N2646 can be used in much the same way.

#### horsebox

Joined Jun 9, 2007
32
Thanks alot.

#### niftydog

Joined Jun 13, 2007
95
When getting right back to basics I use this rule;
Capacitors are short circuit for AC and open circuit for DC.

The value of capacitance alters how short the short circuit is for a given frequency. The bigger the cap, the shorter the short and vice versa.

So, in your ripple filter example the cap (which is in parallel with the voltage source) would appear to shunt the AC component (the ripple) to ground while leaving the DC part unaffected.

Or, say you have a signal with both DC and AC components, but you only want the AC. Put the cap in series with the signal source and the DC gets blocked while the AC passes through.