electricked said:
I'll keep reading trying to understand the practical uses for a cap.
Thanks Jeff!
--Viktor
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