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Capacitor.JPG
Signal coupling
Because capacitors pass AC but block DC signals (when charged up to the applied dc voltage),
they are often used to separate the AC and DC components of a signal. This method is known as AC coupling or "capacitive coupling". Here, a large value of capacitance, whose value need not be accurately controlled, but whose reactance is small at the signal frequency, is employed.
Decoupling
Main article: decoupling capacitor
A decoupling capacitor is a capacitor used to decouple one part of a circuit
from another. Noise caused by other circuit elements is shunted through the
capacitor, reducing the effect they have on the rest of the circuit. It is most
commonly used between the power supply and ground An alternative name
is bypass capacitor as it is used to bypass the power supply or other high
impedance component of a circuit.
Noise filters and snubbers
When an inductive circuit is opened, the current through the inductance
collapses quickly, creating a large voltage across the open circuit of the
switch or relay. If the inductance is large enough, the energy will generate
a spark, causing the contact points to oxidize, deteriorate, or sometimes weld
together, or destroying a solid-state switch. A snubber capacitor across the
newly opened circuit creates a path for this impulse to bypass the contact
points, thereby preserving their life; these were commonly found in contact breaker ignition systems, for instance.
Similarly, in smaller scale circuits, the
spark may not be enough to damage the switch but will still radiate
undesirable radio frequency interference (RFI), which a filter capacitor
absorbs. Snubber capacitors are usually employed with a low-value resistor in
series, to dissipate energy and minimize RFI. Such resistor-capacitor
combinations are available in a single package.
Capacitors are also used in parallel to interrupt units of a high-voltage circuit
breaker in order to equally distribute the voltage between these units. In this case they are called grading capacitors.
In schematic diagrams, a capacitor used primarily for DC charge storage is
often drawn vertically in circuit diagrams with the lower, more negative, plate
drawn as an arc. The straight plate indicates the positive terminal of the device, if it is polarized (see electrolytic capacitor).
เต็มๆ ได้ที่
http://en.wikipedia.org/wiki/Applications_of_capacitors
ข้อแก้ที่วงแดงไว้ตะกี๊ วงเอง งงเอง เหมือนอ่านเองแล้วเข้าใจผิดเอง
Capacitive coupling
Use in analog circuits
In analog circuits, a coupling capacitor is used to connect two circuits such that only the AC signal from
the first circuit can pass through to the next while DC is blocked. This technique helps to
isolate the DC bias settings of the two coupled circuits. Capacitive coupling is also known as AC
coupling and the capacitor used for the purpose is known as a coupling or DC blocking capacitor. Capacitive coupling has the disadvantage of degrading the low frequency performance of a system containing capacitively coupled units.
Each coupling capacitor along with the input electrical impedance of the next stage forms
a high-pass filter and each successive filter results in a cumulative filter with a -3dB frequency that
may be higher than each individual filter. So for adequate low frequency response the capacitors used must
have high capacitance ratings. They should be high enough that the reactance of each is at most a tenth of
the input impedance of each stage, at the lowest frequency of interest. This disadvantage of capacitively coupling DC biased,
transistor amplifier circuits is largely minimized in directly coupled designs.
Decoupling capacitor
A decoupling capacitor is a capacitor used to decouple one part of an electrical network (circuit) from another.
Noise caused by other circuit elements is shunted through the capacitor, reducing the effect they have on the rest of the circuit.
For example, because the voltage level for a device is fixed, changing power demands are manifested as changing current demand.
The power-supply must accommodate these variations in current draw with as little change as possible in the power-supply voltage.
When the current draw in a device changes, the power-supply cannot respond to that change instantaneously. As a consequence,
the voltage at the device changes for a brief period before the power-supply responds. The voltage regulator adjusts the amount of
current it is supplying to keep the output voltage constant but can only effectively maintain the output voltage for events at frequencies
from DC to a few hundred kHz, depending on the regulator (some are effective at regulating in the low MHz). For transient events that
occur at frequencies above this range, there is a time lag before the voltage regulator responds to the new current demand level.
This is where the decoupling capacitor comes in. The decoupling capacitor works as the devices local energy storage. The capacitor
cannot provide DC power because it stores only a small amount of energy but this energy can respond very quickly to changing current
demands. The capacitors effectively maintain power-supply voltage at frequencies from hundreds of kHz to hundreds of MHz
(in the milliseconds to nanoseconds range). Decoupling capacitors are not useful for events occurring above or below this range.
An alternative name is bypass capacitor as it is used to bypass the power supply or other high impedance component of a circuit.
Decoupling
One common kind of decoupling is of a powered circuit from signals in the power supply. Sometimes, for various reasons,
a power supply supplies an AC signal superimposed on the DC power line. Such a signal is often undesirable in the powered circuit.
A decoupling capacitor can prevent the powered circuit from seeing that signal, thus decoupling it from that aspect of the power supply circuit.
Another kind of decoupling is stopping a portion of a circuit from being affected by switching that happens in another portion.
Switching in subcircuit A may cause fluctuations in the power supply or other electrical lines, but you do not want subcircuit B,
which has nothing to do with that switching, to be affected.
A decoupling capacitor can decouple subcircuits A and B so that B doesn't see any effects of the switching.
To decouple a subcircuit from AC signals or voltage spikes on a power supply or other line, a bypass capacitor is often used.
A bypass capacitor is to shunt energy from those signals or transients past the subcircuit to be decoupled, right to the return path.
For a power supply line, a bypass capacitor from the supply voltage line to the power supply return (neutral) would be used.
Doctor Gerald Merckel was a leading researcher in bypass capacitors within AC circuits and power lines.
High frequencies and transient currents flow through a capacitor,
in this case in preference to the harder path through the decoupled circuit, but DC cannot go through the capacitor,
so continues on to the decoupled circuit.
Switching subcircuits
In a switching subcircuit switching noise must be suppressed. When a load is suddenly applied to a voltage source,
the circuit tries to suddenly increase its current, but the inductance in the power supply line acts to oppose that increase.
It opposes it by lowering the voltage across the power line supplies. This is not just the voltage that the load in question sees,
but the voltage that every other subcircuit that shares that power supply line sees. This is only temporary;
the inductance ultimately loses the battle and the voltage comes back to normal. But even a temporary reduction in voltage can disturb other subcircuits.
To decouple other subcircuits from the effect of the sudden current demand, a decoupling capacitor can be placed between
the supply voltage line and its reference (ground) next to the switched load. While the load is switched out, the capacitor charges
up to full power supply voltage and otherwise does nothing. When the load is applied, the capacitor initially supplies demanded current.
By the time the capacitor runs out of charge, the power supply line inductance cannot maintain the previous current,
so the load can draw full current at normal voltage from the power supply (and the capacitor can recharge too).
The voltage dip is reduced but not eliminated; i.e. the decoupling is not perfect.
Capacitor.JPG
Signal coupling
Because capacitors pass AC but block DC signals (when charged up to the applied dc voltage),
they are often used to separate the AC and DC components of a signal. This method is known as AC coupling or "capacitive coupling". Here, a large value of capacitance, whose value need not be accurately controlled, but whose reactance is small at the signal frequency, is employed.
Decoupling
Main article: decoupling capacitor
A decoupling capacitor is a capacitor used to decouple one part of a circuit
from another. Noise caused by other circuit elements is shunted through the
capacitor, reducing the effect they have on the rest of the circuit. It is most
commonly used between the power supply and ground An alternative name
is bypass capacitor as it is used to bypass the power supply or other high
impedance component of a circuit.
Noise filters and snubbers
When an inductive circuit is opened, the current through the inductance
collapses quickly, creating a large voltage across the open circuit of the
switch or relay. If the inductance is large enough, the energy will generate
a spark, causing the contact points to oxidize, deteriorate, or sometimes weld
together, or destroying a solid-state switch. A snubber capacitor across the
newly opened circuit creates a path for this impulse to bypass the contact
points, thereby preserving their life; these were commonly found in contact breaker ignition systems, for instance.
Similarly, in smaller scale circuits, the
spark may not be enough to damage the switch but will still radiate
undesirable radio frequency interference (RFI), which a filter capacitor
absorbs. Snubber capacitors are usually employed with a low-value resistor in
series, to dissipate energy and minimize RFI. Such resistor-capacitor
combinations are available in a single package.
Capacitors are also used in parallel to interrupt units of a high-voltage circuit
breaker in order to equally distribute the voltage between these units. In this case they are called grading capacitors.
In schematic diagrams, a capacitor used primarily for DC charge storage is
often drawn vertically in circuit diagrams with the lower, more negative, plate
drawn as an arc. The straight plate indicates the positive terminal of the device, if it is polarized (see electrolytic capacitor).
เต็มๆ ได้ที่
http://en.wikipedia.org/wiki/Applications_of_capacitors
ข้อแก้ที่วงแดงไว้ตะกี๊ วงเอง งงเอง เหมือนอ่านเองแล้วเข้าใจผิดเอง
Capacitive coupling
Use in analog circuits
In analog circuits, a coupling capacitor is used to connect two circuits such that only the AC signal from
the first circuit can pass through to the next while DC is blocked. This technique helps to
isolate the DC bias settings of the two coupled circuits. Capacitive coupling is also known as AC
coupling and the capacitor used for the purpose is known as a coupling or DC blocking capacitor. Capacitive coupling has the disadvantage of degrading the low frequency performance of a system containing capacitively coupled units.
Each coupling capacitor along with the input electrical impedance of the next stage forms
a high-pass filter and each successive filter results in a cumulative filter with a -3dB frequency that
may be higher than each individual filter. So for adequate low frequency response the capacitors used must
have high capacitance ratings. They should be high enough that the reactance of each is at most a tenth of
the input impedance of each stage, at the lowest frequency of interest. This disadvantage of capacitively coupling DC biased,
transistor amplifier circuits is largely minimized in directly coupled designs.
Decoupling capacitor
A decoupling capacitor is a capacitor used to decouple one part of an electrical network (circuit) from another.
Noise caused by other circuit elements is shunted through the capacitor, reducing the effect they have on the rest of the circuit.
For example, because the voltage level for a device is fixed, changing power demands are manifested as changing current demand.
The power-supply must accommodate these variations in current draw with as little change as possible in the power-supply voltage.
When the current draw in a device changes, the power-supply cannot respond to that change instantaneously. As a consequence,
the voltage at the device changes for a brief period before the power-supply responds. The voltage regulator adjusts the amount of
current it is supplying to keep the output voltage constant but can only effectively maintain the output voltage for events at frequencies
from DC to a few hundred kHz, depending on the regulator (some are effective at regulating in the low MHz). For transient events that
occur at frequencies above this range, there is a time lag before the voltage regulator responds to the new current demand level.
This is where the decoupling capacitor comes in. The decoupling capacitor works as the devices local energy storage. The capacitor
cannot provide DC power because it stores only a small amount of energy but this energy can respond very quickly to changing current
demands. The capacitors effectively maintain power-supply voltage at frequencies from hundreds of kHz to hundreds of MHz
(in the milliseconds to nanoseconds range). Decoupling capacitors are not useful for events occurring above or below this range.
An alternative name is bypass capacitor as it is used to bypass the power supply or other high impedance component of a circuit.
Decoupling
One common kind of decoupling is of a powered circuit from signals in the power supply. Sometimes, for various reasons,
a power supply supplies an AC signal superimposed on the DC power line. Such a signal is often undesirable in the powered circuit.
A decoupling capacitor can prevent the powered circuit from seeing that signal, thus decoupling it from that aspect of the power supply circuit.
Another kind of decoupling is stopping a portion of a circuit from being affected by switching that happens in another portion.
Switching in subcircuit A may cause fluctuations in the power supply or other electrical lines, but you do not want subcircuit B,
which has nothing to do with that switching, to be affected.
A decoupling capacitor can decouple subcircuits A and B so that B doesn't see any effects of the switching.
To decouple a subcircuit from AC signals or voltage spikes on a power supply or other line, a bypass capacitor is often used.
A bypass capacitor is to shunt energy from those signals or transients past the subcircuit to be decoupled, right to the return path.
For a power supply line, a bypass capacitor from the supply voltage line to the power supply return (neutral) would be used.
Doctor Gerald Merckel was a leading researcher in bypass capacitors within AC circuits and power lines.
High frequencies and transient currents flow through a capacitor,
in this case in preference to the harder path through the decoupled circuit, but DC cannot go through the capacitor,
so continues on to the decoupled circuit.
Switching subcircuits
In a switching subcircuit switching noise must be suppressed. When a load is suddenly applied to a voltage source,
the circuit tries to suddenly increase its current, but the inductance in the power supply line acts to oppose that increase.
It opposes it by lowering the voltage across the power line supplies. This is not just the voltage that the load in question sees,
but the voltage that every other subcircuit that shares that power supply line sees. This is only temporary;
the inductance ultimately loses the battle and the voltage comes back to normal. But even a temporary reduction in voltage can disturb other subcircuits.
To decouple other subcircuits from the effect of the sudden current demand, a decoupling capacitor can be placed between
the supply voltage line and its reference (ground) next to the switched load. While the load is switched out, the capacitor charges
up to full power supply voltage and otherwise does nothing. When the load is applied, the capacitor initially supplies demanded current.
By the time the capacitor runs out of charge, the power supply line inductance cannot maintain the previous current,
so the load can draw full current at normal voltage from the power supply (and the capacitor can recharge too).
The voltage dip is reduced but not eliminated; i.e. the decoupling is not perfect.
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