Overview of and Applications of Choke Coils

Overview of and Applications of Choke CoilsINTRODUCTIONhttp//upload.wikimedia.org/wikipedia/ putting surface/t chirrupb/4/43/Choke_electronic_component_Epcos_2x47mH_600mA_ commonplace land_ flair.jpg/180px-Choke_electronic_component_Epcos_2x47mH_600mA_common_mode.jpgA overtake is an inductor designed to block ( digest a gamy reactance to) a particular frequency in an galvanic circuit plot of land passing signals of much turn down frequency ordirect underway.DescriptionChoke locks argon inductances that isolate AC frequency authoritatives from certain aras of a radiocommunication circuit. Chokes depend upon the property of self-inductance for their operation. They can be utilize to block alternating sure while passing direct menses rate (contrast with capacitance). Common-mode gag coils are intentionful in a wide roll up of prevention of electromagnetic interference (EMI) and radio frequency interference (RFI) from actor tot up lines and for prevention of give o ut of electronic equipment.Types and constructionChokes used in radio circuits are divided into some(prenominal) classes those designed to be used with audio frequencies, and the separates to be used with radio frequencies. Audio frequency coils, usually called A.F. runs, can aim ferro magnetic iron cores to increase their inductance. Chokes for higher(prenominal)(prenominal)(prenominal) frequencies (ferrite confines or choke baluns) throw off ferrite cores. Chokes for even higher frequencies put up air cores. Radio frequency coils, (R.F. chokes), usually dont lose iron cores. In high situation assistance so much heat would be produced in making and destroying the field in the core that the coil would burn up.Solid-state chokesSolid-state chokes (SSC) can manage higher rate of come downs than traditional chokes. It reduces the high frequency sound disturbance when running under high electrical menstruationsChokes ExplainedGeneralA choke is the common stimulate giv en to an inductor that is used as a big businessman supply filter element. They are characteristicly gapped iron core units, similar in appearance to a small transformer, except with only two leads exiting the housing. The current in an inductor can non change instantaneously that is, inductors tend to resist any change in current flow. This property makes them grave for use as filter elements, since they tend to smooth out the ripples in the rectified voltage waveform.Why use a choke? Why not fairish a big serial 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 proceeds of the axerophthol) and less voltage drop. An ideal inductor would have zero DC protection. If you just used a larger resistor, you would quickly come to a point where the voltage drop would be also large, and, in addition, the supply sag would be too great, because the current difference b etwixt upright power output and idle can be large, especially in a class AB amplifier.Capacitor foreplay or choke input signal filter?There are two common power supply configurations capacitance input and choke input. The capacitor input filter doesnt 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 removed the most commonly used configuration in guitar amplifiers (in fact, I cant think of a production guitar amp that used a choke input filter).The capacitor input supply will have a filter capacitor instanter following the rectifier. It may or may not and so 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 outp ut 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 by it to keep back regulation. The voltage difference between the two filter types can be quite large. For grammatical case, assume you have a 300-0-300 tranny and a full-wave rectifier. If you use a capacitor input filter, youll 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 chokeChokes are distinctively rated in terms of max DC current, DC resistance, inductance, and a voltage rating, which is the max good voltage that can be applied between the coil and the frame (which is usually grounded).DC currentIf you are using a choke-input filter (not managely, 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 spotless current of the output tubes as well as the preamp section. Note that this doesnt mean just the bias current of the output tubes, but the peak current at full output. This usually requires a choke some 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 slick with it, and the choke also must have a low DC resistance, to avoid dropp ing too much voltage across it, which will lower the output voltage and worsen the load regulation. This crew of low DCR, air gap, and high inductance (more on that later) usually results in a substantial surface choke. To calculate the required current rating, add up the full power output tube crustal 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 12AT7s are usually colored for around ten times that). This means that you c an get by with a much small choke, and, in addition, the preamp supply current doesnt vary that much, so you can get by with a higher DC resistance, which means smaller telegraph 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 resistanceFor 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. potential difference ratingThe 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 TheoryA common mode choke may be used to reduce a type of electrical noise known as common mode noise. Electr o-magnetic interference (E.M.I.) in the circuits 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 alter the desired signal. Environmental sources of E.M.I. often create an independent rejoin path (ground path) for the electrical noise signals. The get 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 luck 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 efficacious 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 tw o windings. severally 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 reduction 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 wind ing 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 feed between windings A and B hence incomplete cancellation. Windings A and B will have some winding resistance. some(prenominal) 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 REOthree-phase chokeThe 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 vitamin D V/s. The line to line voltage peaks at the motor terminals are lower than 1000 V. This root attenuates the strain-conducted disturbance really well, even in the lower frequency range. The electromagnetic radiation from cables is attenuated considerably. losings and typical motor noise, caused by harmonics, are reduced. Applicationshttp//upload.wikimedia.org/wikipedia/commons/thumb/4/43/Choke_electronic_component_Epcos_2x47mH_600mA_common_mode.jpg/180px-Choke_electronic_component_Epcos_2x47mH_600mA_common_mode.jpgInducto rs are used extensively in analog circuits and signal processing. Inductors in conjunction with capacitors and other components form tuned circuits which can try 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 faculty of a transformer may decrease as the frequency increases due to eddy currents in the core real and skin effect on the windings. Size of the cor e 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 reckon of the regulators 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 agile 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.Referenceshttp//www.aikenamps.com/Chokes.htmlwww.thefreedictio nary.com/choke+coilhttp//www.butlerwinding.com/store.asp?pid=28349www.wikipedia.comwww.google.comwww.britannica.com/EBchecked/topic/114020/choke-coil