# Capacitors

Discussion in 'Electronic Basics' started by Jeffrey Turner, Mar 5, 2004.

1. ### Jeffrey TurnerGuest

First, those caps aren't in parallel there's the 5V regulator between
them. Second, paralleling caps is done either to allow more charge
storage (the capacitances add) or because of the frequency
characteristics of the capacitors (a large value electrolytic for
smoothing out the voltage and a small value cap for shorting high
frequency spikes). Putting capacitors in series is rarely done as it
gives you an overall smaller capacitor value, but it may be used when
the full voltage is above the rating of either capacitor or if a
specific value of capacitance must be achieved. Putting capacitors in
series gives an overall capacitance of C1*C2/(C1 + C2) [similar to
resistors in parallel].

--Jeff

--
A man, a plan, a cat, a canal - Panama!

Ho, ho, ho, hee, hee, hee
and a couple of ha, ha, has;
That's how we pass the day away,
in the merry old land of Oz.

2. ### electrickedGuest

Hi all,

I'm looking at the following website:
http://electronics.howstuffworks.com/digital-electronics4.htm

The two caps are connected in parallel. I'd like to know what's the
difference in terms of practical applications of connecting the caps in
parallel and series and how the circuit works.

TIA!

--Viktor

3. ### Jeffrey TurnerGuest

The first cap is across (in parallel with) the input of the 7805
and the second is across (in parallel with) the output. The caps
would only be in parallel if there was only wire connecting both of
their legs.
They cannot mean a simple transformer. The input to the 7805 has to
be a DC voltage. The capacitor is just to smooth the input.
The capacitor on the output helps smooth out noise from the 7805. If
your load might draw more current than the 7805 can supply you should
use a different regulator or build your own. This is an easy to build
switching regulator:

http://www.romanblack.com/smps.htm
At high frequencies, an ideal capacitor behaves like a short circuit.
Electrolytics don't have very good response to spikes but ceramic caps
work well in those applications. If there are really severe spikes
then there are products like transorbs (and other things) that are
designed to short them effectively. I designed with such things when
I was designing burglar alarm systems that had to survive nearby
lightning strikes, but those are beyond what most electronics require
in normal operation. But it is not uncommon to put a small capacitor
across the power supply pin of a chip to eliminate problems with RF.

--Jeff

--
A man, a plan, a cat, a canal - Panama!

Ho, ho, ho, hee, hee, hee
and a couple of ha, ha, has;
That's how we pass the day away,
in the merry old land of Oz.

4. ### electrickedGuest

First, those caps aren't in parallel there's the 5V regulator between
Thanks for the fast reply, Jeff!

Hmm. They are not in parallel? I'm seeing it like this. The first cap is the
main circuit coming from transformer. Then we have the regulator in parallel
with the first cap and the second cap in parallel with the regulator. Am I
reading this right? So since the first cap is in parallel with the regulator
then it would smoothen out the voltage before it reaches the regulator. If
the transformer (let's say 12V AC transformer) connection doesn't have a
bridge rectifier, wouldn't the voltage be close to +12V in half cycle
and -12V in the other half of the cycle with the smoothening cap in place?

As far as the second cap which is in parallel with the regulator, I'm
thinking if the voltage goes below 5 volts for some reason and the second
cap has charged up, it would drive the voltage output back to 5V. Is this
correct?

One more question having to do with one of your questions. If there's a
voltage spike and I use a small cap value, how would the cap short the
spike? I'm interested in what the cap would do to get rid of the spike.

Thanks!

--Viktor

5. ### electrickedGuest

I'll keep reading trying to understand the practical uses for a cap.

Thanks Jeff!

--Viktor

6. ### Robert C MonsenGuest

The name 'capacitor' derives from the word capacity. It 'stores' energy,
acting like a little, weak, quickly discharged, quickly recharged battery.

For your little circuit, the capacitor on the left of the regulator is used
to store energy, since the power line is AC, and so 'rectifying' it leaves
gaps when there is no energy being fed into the regulator. The capacitor
supplies energy during those gaps.

There are several reasons for the capacitor on the right side of the
regulator. The first reason is that, again, capacitors are charge storage
devices, and can thus respond to requirements of the circuit for charge much
more quickly than the regulator (although not for as long, obviously.)

The second reason is that the 7805 is really an amplifier, and amplifiers
will oscillate in particular situations where some of the energy they are
directing gets back into their input. Capacitors will soak up some of the
energy by storing it, thus decreasing the feedback and preventing
oscillations. This is particularly true of energy at higher frequencies,
which turns out to be more problematic.

On the input of integrated circuits, capacitors are again used as storage
devices; digital ICs can suddenly draw large amounts of current, and if
there isn't a capacitor across their power supply, the voltage of the power
supply would droop in the region near the IC because of this. A capacitor
will supply this relatively shortlived requirement for extra current, and
thus prevent the power supply from drooping. This is good, because a droop
might cause effects in nearby circuit elements.

Another reason capacitors are used is in timing circuits. If you have a
capacitor that is charged up to a voltage V, and you discharge it through a
resistor of value R, then you can predict how long it will take for the volt
age to drop to a given value using a simple formula. Thus, capacitors, in
conjunction with resistors, are useful for creating timers of various kinds.

Yet another reason folks use capacitors has to do with a phenomena called
resonance. It turns out that there are devices called 'inductors', which,
when put in a circuit with a capacitor, will 'resonate' at a frequency that
can be predicted based on the values of the capacitor and inductor. This
means that oscillators (called LC oscillators) with an extremely stable
frequency can be built using capacitors.

Capacitors, in circuits with resistors and inductors, can also be used to
'filter' signals, allowing certain frequencies through, and supressing
others. There is a whole science devoted to designing these 'passive
filters'. Radios (used to) consist of passive filters, in which the
frequency you want to listen to is selected using one of these filters, then
passed through other circuitry to pick out the information from this
frequency.

Anyway, these are some of the uses of capacitors. They also make nice
jewelry, and my youngest daughter used to call ceramic caps 'lolly pops' due
to their shape.

Regards,
Bob Monsen

7. ### electrickedGuest

The name 'capacitor' derives from the word capacity. It 'stores' energy,
Thanks Bob! Your post was definitely informative! I wish more people would
explain what you just did when they explain capacitors before they start any
of the math. If I were to write a tutorial on caps (and I WILL once I know
more about them) I would explain certain applications as you just did, I
would then show an example how to make a circuit using caps and then explain
their values and math behind it; i.e. In series you use C1*C2/C1+C2 and in
parallel you simply find the capacitances to get the final capacitance, or
energy to be stored.

Once again, thanks Bob! great info...

--Viktor