Re: Capacitors & Capacitance

From: Jamie (jamie_5_not_valid_after_5_Please_at_charter.net)
Date: 01/25/05 

Date: Tue, 25 Jan 2005 13:10:19 -0800

Richard Harris wrote:

> Hi,
> Just been reading about capacitors and understand the property of
> capacitance. The problem is I don't see how this property is of use, many
> circuits use capacitors but I don't understand what there role is.
>
> I know capacitors don't allow current to flow through them and that they can
> be charged and discharged and that the time taken to charge and dischage can
> be calculated. But how are these devices off any real use?
>
> Thanks for ya time guys.
>
>
let me give you an example.
  lets assume that you have the need for a negative pulse from a circuit
that only has a common and + supply of voltage.
    using a 100Uf capacitor to test with, connect the - side to your +
input of your DM ( multimeter Voltage Scale). the - lead of your DM to
the common side of lets say a 9 volt battery which would be the -
terminal in this example.
   using a simple thing like a double throw switch, connect the + side
of the capacitor to the center pole of the switch, one side of the switch
to common and the other side to the + terminal on the 9 v battery.
  watch you DM as you switch from one side to the other.
   you will noticed that you get a momentary pulse of + voltage when the
center pole of the switch is connected to the + terminal of the Battery.
  now switching the center pole of the switch to the common side (which
would be the - side of your battery) yields a - voltage in your meter.
   if you were to short the capacitor with a jumper and repeat the action
of connecting the center pole of the switch to + terminal of the battery
and then the common side (- terminal of the batter), you will see only a
+ reading on your meter and then only a 0 zero, you will not see a -
voltage develop.
     just think of a battery's terminals being switched to give reverse
polarity.
   the capacitor is in effect a cell (battery being 2 or more cells); 

--
  now lets apply this to a simple class A amplifier.
  i will only talk about the basics here.
  imagen using a resistor from the + terminal of your 9 volt battery to
supply current to the collector of a transistor.
  now when the circuit is properly biasing the transistor (setting is 
current state), you can measure the voltage at the collector and lets 
assume that it is for now 50% of the supply voltage, in this case
using your 9 volt batter = 4.5 volts.
   if you were to connect a speaker to this same point, the speak would
push to one side and stay there due to the DC , there for you would only 
get a half cycle movement in the speaker. this is not good.
   if you were to now decouple the connection with a capacitor of lets 
say around 1000 uf or more (incase you want to experiment with this), 
the variation of voltage at the collector and resistor will give you a
+ and - (true -) voltages. When the DC stabilizes, the cap will become
fully charged and thus no current will no longer flow leaving the 
speaker cone at it's natural resting place.
    So you can simply think of capacitors in circuits like alternate the 
direction of flow to give you real AC voltage effects, and
also caps are used to suppress the variations voltages and for making 
time constant effect circuits etc. ( the list goes on)
   if you would look at a LM555 timer and some examples, you could a 
good idea how they could be applied in timers, oscillators (tone 
generators) etc.
   you mite want to get your hands on one of those 101 kits. they are 
very intuitive for introducing people to electronics.
   I hope long winded message didn't bore you.
:)

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