Re: Why are capacitors used in televison sets/radios?

Greetings:
All capacitors operate in the same way; however, the size of
the capacitor and it's location in the electronic circuit will determine it's function.
The unit of capacitance is called the Farad; however, in
electronics circuits we tend to use capacitors with values in microfarads
or picofarads (a micro farad = one millionth of a farad, a
picofarad = 1/1000 of a microfarad).

Large capacitors in the microfarad size are generally made from two large
sheets (or plates) of metal foil seperated by an insulator such as sheet
of beeswaxed paper or Teflon. The three sheets are the rolled up into a
small cylinder for packaging. Other capacitors are made of metal plates
separated by air or oil and some low voltage capacitors use a chemical film to
make a microscopically thick insulator between conducting sheets. The electrical
size of a capacitor is determined by the area of the sheets or
plates, the spacing between the conducting plates and a property of
the insulating material called the dielectric constant. Larger plates,
smaller spacing and greater dielectric constants all make capacitors have
greater electrical values in Farads. In the Mad Science archives I have discussed
operation of capacitors in detail. You can search for the answer to the following
question:

Re: Though there is insulator inside a capacitor,how current passes through it?

Capacitors are used to perform many functions in electronic circuits. The most
common uses in radio and TV circuits are the following:

Filter capacitors are used for smoothing out the direct current (DC) voltage
variations in the output of power supply circuits. These voltage variations
can cause hum in audio circuits and dark bars in the TV picture at the main power
line frequency and it's second harmonic. The larger the value of the capacitor
(usually in microfarads) the lower will be the hum in the out put of the radio
or TV. The physical size and cost of the capacitors usually determines the
largest values used.

Filter capacitors can also provide DC voltage to memory circuits for a few minutes
during battery replacement. A large value of capacitor in a portable electronic
circuit is placed in parallel with the batteries and is charged to the battery
voltage. When the battery is removed the capacitor will act as a DC power source
for a few minutes until it discharged to a voltage that is to low to save the
values in the memory.

Filter capacitors are often connected to the main power lines. This is
mainly in radio and TV sets that have eliminated power transformers to
reduce the cost of manufacturing. In the transformer less sets all of the
circuits can be floating at the main line power frequency and great
caution must be taken when opening up the cabinet.

Coupling capacitors are used to couple alternating current (AC) audio, video or
radio frequency (RF) signals between transistor amplifier stages while blocking
DC bias voltages. A typical transistor amplifier requires a low DC bias voltage
between ground (earth) and the input electrode (base or gate) of the transistor
and a higher DC bias voltage between ground and the output collector electrode.
A typical radio or TV set requires 5 to 10 transistor amplifiers in series to
boost the AC audio, video or RF signals to useful values. Coupling capacitors
couple these AC signals from the output of each transistor amplifier to the
input of another transistor amplifier while keeping the DC bias voltages on
the transistors at the proper levels.

Tuning capacitors are placed in parallel with inductors (coils) to form resonant
circuits that will pass only a narrow band of frequencies to tune RF amplifier
circuits to a single radio station or a TV channel. A variable air filled
capacitor is usually used with a fixed inductor to tune a radio set. TV sets
switch both inductors and capacitors for channel tuning. Typical tuning
capacitors are in the picofarad range of values.

Timing capacitors are used in series with resistors to time television scanning
circuits for the cathode ray picture tube and other time sensitive control
circuits. When a voltage is applied across a resistor and a capacitor in series
the charging time of the capacitor is set by the time constant of the circuit.
The time constant (TC) in seconds is equal to the value of the resistor (R) in
ohms multiplied by the value of the capacitor (C) in farads (TC = R * C). It
takes about 5 times the TC to completely charge a capacitor to the applied
voltage The charging of a capacitor is not linear, it charges to about 63%
of the applied voltage during the first TC, 86 % after 2 * TC, 95% after 3 * TC,
98% after 4 * TC and 99% after 5 * TC. When the voltage across the series
resistor and capacitor is turned off it takes about 5 * TC for the capacitor
to discharge back to zero voltage. The voltage across the capacitor drops to
37% of the initial charge voltage in one TC, 14% after 2 * TC, 5% after 3 * TC,
2% after 4 * TC and less than 1 % after 5 * TC. By adjusting the values
of R and C, time constants can be set for minutes, seconds, microseconds or
even faster. R * C timing circuits are used extensively in computer logic circuits.

Best regards, Your Mad Scientist
Adrian Popa