INTRODUCTION
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A choke is an inductor designed to block (have a high reactance to) a particular frequency in an electrical circuit while passing signals of much lower frequency ordirect current.
Description
Choke coils are inductances that isolate AC frequency currents from certain areas of a radio circuit. Chokes depend upon the property of self-inductance for their operation. They can be used to block alternating current while passing direct current (contrast with capacitor). Common-mode choke coils are useful in a wide range of prevention of electromagnetic interference (EMI) and radio frequency interference (RFI) from power supply lines and for prevention of malfunctioning of electronic equipment.

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Types and construction
Chokes used in radio circuits are divided into two classes – those designed to be used with audio frequencies, and the others to be used with radio frequencies. Audio frequency coils, usually called A.F. chokes, can have ferro magnetic iron cores to increase their inductance. Chokes for higher frequencies (ferrite chokes or choke baluns) have ferrite cores. Chokes for even higher frequencies have air cores. Radio frequency coils, (R.F. chokes), usually don’t have iron cores. In high power service so much heat would be produced in making and destroying the field in the core that the coil would burn up.
Solid-state chokes
Solid-state chokes (SSC) can manage higher currents than traditional chokes. It reduces the high frequency buzzing noise when running under high electrical currents
Chokes Explained
General
A “choke” is the common name given to an inductor that is used as a power supply filter element. They are typically gapped iron core units, similar in appearance to a small transformer, but with only two leads exiting the housing. The current in an inductor cannot change instantaneously; that is, inductors tend to resist any change in current flow. This property makes them good for use as filter elements, since they tend to “smooth out” the ripples in the rectified voltage waveform.
Why use a choke? Why not just a big series resistor?
A choke is used in place of a series resistor because the choke allows better filtering (less residual AC ripple on the supply, which means less hum in the output of the amp) and less voltage drop. An “ideal” inductor would have zero DC resistance. If you just used a larger resistor, you would quickly come to a point where the voltage drop would be too large, and, in addition, the supply “sag” would be too great, because the current difference between full power output and idle can be large, especially in a class AB amplifier.
Capacitor input or choke input filter?
There are two common power supply configurations: capacitor input and choke input. The capacitor input filter doesn’t necessarily have to have a choke, but it may have one for additional filtering. The choke input supply by definition must have a choke. Capacitor input filters are by far the most commonly used configuration in guitar amplifiers (in fact, I can’t think of a production guitar amp that used a choke input filter).
The capacitor input supply will have a filter capacitor immediately following the rectifier. It may or may not then have a second filter composed of a series resistor or choke followed by another capacitor. The “cap, inductor, cap” network is commonly called a “Pi filter” network. The advantage of the capacitor input filter is higher output voltage, but it has poorer voltage regulation than the choke input filter. The output voltage approaches sqrt(2)*Vrms of the AC voltage. The choke input supply will have a choke immediately following the rectifier. The main advantage of a choke input supply is better voltage regulation, but at the expense of much lower output voltage. The output voltage approaches (2*sqrt(2)/Pi)*Vrms of the AC voltage. The choke input filter must have a certain minimum current drawn through it to maintain regulation. The voltage difference between the two filter types can be quite large. For example, assume you have a 300-0-300 tranny and a full-wave rectifier. If you use a capacitor input filter, you’ll get a no-load max DC voltage of 424 volts, which will sag down to a voltage dependent on the load current and the resistance of the secondary windings. If you use the same transformer with a choke input filter, the peak output DC voltage will be 270V, and will be much more highly regulated than the capactor input filter (less variations in supply voltage with variations in load current).
How to select a choke:
Chokes are typically rated in terms of max DC current, DC resistance, inductance, and a voltage rating, which is the max safe voltage that can be applied between the coil and the frame (which is usually grounded).
DC current
If you are using a choke-input filter (not likely, unless you are trying to convert a class AB amp to true class A and need the lower voltage, or if you are designing an amp from scratch and want better supply regulation), the choke must be capable of handling the entire current of the output tubes as well as the preamp section. Note that this doesn’t mean just the bias current of the output tubes, but the peak current at full output. This usually requires a choke about the size of a standard 30W-50W output transformer, since the choke must have an air gap (just like a single-ended OT) to avoid core saturation due to the offset DC current flowing through it, and the choke also must have a low DC resistance, to avoid dropping too much voltage across it, which will lower the output voltage and worsen the load regulation. This combination of low DCR, air gap, and high inductance (more on that later…) usually results in a substantial sized choke. To calculate the required current rating, add up the full power output tube plate currents, screen currents, and the preamp supply currents, and add in a factor for margin. For a 50W amp, this may be 250mA.
If, on the other hand, you are selecting a choke for a capacitor input supply (such as the typical Marshall or Fender design), then the requirements are relaxed quite a bit. The purpose of the choke in these type supplies is not for filtering and voltage regulation, but just for filtering the DC supply to the screen grids of the output tubes and the preamp section. The screens typically take around 5-10mA each, and the preamp tubes draw about 1-2mA or so (for the typical 12AX7; 12AT7’s are usually biased for around ten times that). This means that you can get by with a much smaller choke, and, in addition, the preamp supply current doesn’t vary that much, so you can get by with a higher DC resistance, which means smaller wire can be used to wind the choke, which means higher inductance for a given size core. Just add up the current requirements of the screens and preamp tubes, and add a bit more for margin. For a 50W amp, a typical value might be 50-60mA.
DC resistance
For a typical choke input supply, you need a choke with no more than 100-200 ohms or so DCR. A capacitor input supply typically might use a choke with a 250 ohm – 1K DCR. The higher the resistance, the more voltage drop and the poorer the regulation, but the cost will be lower.
Voltage rating
The voltage rating must be higher than the supply voltage, or the insulation on the wire may break down, shorting the supply to the frame.
Common Mode Choke Theory
A common mode choke may be used to reduce a type of electrical noise known as common mode noise. Electro-magnetic interference (E.M.I.) in the circuit’s environment is one source of electrical noise. E.M.I. induces or couples unwanted electrical signals into the circuit. It is desirable to filter out the unwanted noise signals without significantly affecting the desired signal. Environmental sources of E.M.I. often create an independent return path (ground path) for the electrical noise signals. The return path of the desired signal is a different path. Because there are two different return paths, a common mode choke can be used to significantly block (hence reduce) the unwanted noise signal (at the load) without significant reduction in the desired signal.
A.C. power lines provide a good example. They are known to carry significant levels of electrical noise. Their long length gives environmental E.M.I. ample opportunity to generate unwanted electrical noise into the power lines. Figure 2 illustrates an application without a common mode choke. The power line voltage, “Vs”, causes current, “Iz”, to flow through the load, “Z”. At any non-zero instance, Current “Iz” flows into “Z” through one power line wire and returns through the other power line wire. E.M.I. voltage, “Vnc1”, causes current “Inc1”, to flow through the load “Z”. Similarly, E.M.I. voltage, Vnc2 causes current “Inc2” to flow through the load “Z”. Because the E.M.I is generating both “Vnc1” and “Vnc2” the two voltages tend to be in phase. There is very little current flow between them. Current “Inc1” does not flow through both power line wires. It flows through one power line wire and through the ground path. Similarly, current “Inc2” does not flow through both power line wires. It flows through one power line wire and through the ground path. In this example only “Vnc1” produces electrical noise across load “Z” because the “Vnc2” end of “Z” is grounded. In practice, the effective ground point could occur somewhere between the two ends of load “Z”.
Figure 3 illustrates the same application with a common mode choke. The common mode choke has two windings. Each winding of the common mode choke is inserted between the end of a power line wire and the load. As in Figure 1, current “Iz” flows through both power line wires and currents “Inc1” and “Inc2” each flow through one power line wire and return through the ground path. Observe that current “Iz” flows through both windings but in opposing winding directions, while currents “Inc1” and Inc2” each flow through only one winding and in the same winding direction. The ground path does not flow through a winding.
The inductance of winding A restricts (reduces) the flow of current “Inc1” (when compared to Figure 1), thereby reducing the noise voltage across “Z”. Similarly the inductance of winding B restricts (hence reduces) the flow of current “Inc2”. Windings A and B have the same number of turns. The ampere-turns created by Current “Iz” (but excluding any “Inc1” current component) flowing through winding A is cancelled by the opposing ampere-turns created by current “Iz” flowing through winding B. Ideally, the cancellation results in zero inductance and no restriction (no reduction) of current “Iz”. “Iz” produces the same voltage across load “Z” as it does in Figure 1. In practice this will not be true. The common mode choke will have some leakage flux between windings A and B hence incomplete cancellation. Windings A and B will have some winding resistance. Both of these will have some effect on “Iz” (reduces “Iz”).
In contrast, the load current “Iz” flowing through both windings A and B of the differential choke shown in Figure 1 do not cancel, hence “Iz” will be restricted (reduced). Differential chokes are useful when the electrical noise frequencies are much higher than the operating frequencies. The higher choke impedance at the high frequencies block the electrical noise while having a tolerable effect at the operating frequencies.
Some common mode chokes are intentionally designed to have significant leakage inductance. The leakage inductance acts in series with the load hence the leakage inductance provides differential noise filtering. One common mode choke functions like the combined chokes shown in Figure 1 but may differ in levels.
Three-phase choke coil REO
three-phase choke
The conventional output-choke has a very good storage capacity. It functions like a typical series inductance and smoothes the symmetrical, effective current and the asymmetrical, parasitic current. The voltage rise is limited to less than 500 V/µs. The line to line voltage peaks at the motor terminals are lower than 1000 V. This solution attenuates the cable-conducted disturbance really well, even in the lower frequency range. The electromagnetic radiation from cables is attenuated considerably. Losses and typical motor noise, caused by harmonics, are reduced. Applications
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Inductors are used extensively in analog circuits and signal processing. Inductors in conjunction with capacitors and other components form tuned circuits which can emphasize or filter out specific signal frequencies. Applications range from the use of large inductors in power supplies, which in conjunction with filter capacitors remove residual hums known as the Mains hum or other fluctuations from the direct current output, to the small inductance of the ferrite bead or torus installed around a cable to prevent radio frequency interference from being transmitted down the wire. Smaller inductor/capacitor combinations provide tuned circuits used in radio reception and broadcasting, for instance.
Two (or more) inductors which have coupled magnetic flux form a transformer, which is a fundamental component of every electric utility power grid. The efficiency of a transformer may decrease as the frequency increases due to eddy currents in the core material and skin effect on the windings. Size of the core can be decreased at higher frequencies and, for this reason, aircraft use 400 hertz alternating current rather than the usual 50 or 60 hertz, allowing a great saving in weight from the use of smaller transformers.
An inductor is used as the energy storage device in some switched-mode power supplies. The inductor is energized for a specific fraction of the regulator’s switching frequency, and de-energized for the remainder of the cycle. This energy transfer ratio determines the input-voltage to output-voltage ratio. This XL is used in complement with an active semiconductor device to maintain very accurate voltage control.
Inductors are also employed in electrical transmission systems, where they are used to depress voltages from lightning strikes and to limit switching currents and fault current. In this field, they are more commonly referred to as reactors.
Larger value inductors may be simulated by use of gyrator circuits.
References

http://www.aikenamps.com/Chokes.html
www.thefreedictionary.com/choke+coil
http://www.butlerwinding.com/store.asp?pid=28349
www.wikipedia.com
www.google.com
www.britannica.com/EBchecked/topic/114020/choke-coil