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Winding coils
Hi all, I need to wind 180nH inductor for a parallel tuned circuit I'm building, since the 180nH factory-made chokes I have don't look up to the job power-handling wise. This coil needs to handle about 90mA p-p/ 500mW maximum dissipation sine current and I've allowed 3 ohms for series resistance. Can anyone give me some steer on dimensions, number of turns, core type and so on? Thanks, p. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Paul Burridge wrote:
Hi all, I need to wind 180nH inductor for a parallel tuned circuit I'm building, since the 180nH factory-made chokes I have don't look up to the job power-handling wise. This coil needs to handle about 90mA p-p/ 500mW maximum dissipation sine current and I've allowed 3 ohms for series resistance. Can anyone give me some steer on dimensions, number of turns, core type and so on? Thanks, Here are the basic formulas for air core coils. 3 ohms sounds pretty high for such a small inductance. -- John Popelish |
Paul Burridge wrote:
Hi all, I need to wind 180nH inductor for a parallel tuned circuit I'm building, since the 180nH factory-made chokes I have don't look up to the job power-handling wise. This coil needs to handle about 90mA p-p/ 500mW maximum dissipation sine current and I've allowed 3 ohms for series resistance. Can anyone give me some steer on dimensions, number of turns, core type and so on? Thanks, Here are the basic formulas for air core coils. 3 ohms sounds pretty high for such a small inductance. -- John Popelish |
For a very complete analysis of performance, including power handling
capacity, of solenoid coils of all proportions, dimensions and number of turns, download in a few seconds and run immediately program SOLNOID3 from website below. ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
For a very complete analysis of performance, including power handling
capacity, of solenoid coils of all proportions, dimensions and number of turns, download in a few seconds and run immediately program SOLNOID3 from website below. ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
Paul Burridge wrote...
I need to wind 180nH inductor for a parallel tuned circuit I'm building, since the 180nH factory-made chokes I have don't look up to the job power-handling wise. This coil needs to handle about 90mA p-p / 500mW maximum dissipation sine current and I've allowed 3 ohms for series resistance. Can anyone give me some steer on dimensions, number of turns, core type and so on? Thanks, As John has said, that's a very low inductance that should not present any problems at such a low power level. But perhaps for a more detailed answer you can tell us the frequencies your coil will experience. At high frequencies skin and proximity effects dominate, and these can be evaluated with an Rac/Rdc ratio. If a ferrite is used its high-frequency core loss can also be modeled as an inductor resistance. Do you have any special size constraints? Unless you really need a miniature size, an air core may be best for 180nH. You can use the Wheeler equation to experiment with different coil designs. I'll add some new grist for the mill, with a copy of a portion of a posting I made 28 Dec 1997, about air-coil inductance equations. ------------------------------------------------------------------- Throughout the discussion we'll use the same dimensional system, based on the drawing below. Here and in the 14 formulas below, N = turns, a = mean radius, b = length, and c = winding thickness, and all are in inches, unless otherwise stated. length |------ b --------| --- ,-----------------, c | cross section | ------------ a = winding mean radius --- '-----------------' a | __________________________ | axis D = 2a ,-----------------, | | cross section | -------- '------------\----' \ solenoid coil layout N turns -------- [ snip five formulas and discussion ] To simplify our lives, Wheeler empirically derived his popular single-layer solenoid equation, using Nagaoka's equation and tables. Wheeler's equation is shown below in two different ways. a N^2 a^2 N^2 / 10 b (6) L = ---------- = -------------- uH / inch 9 + 10 b/a 1 + 0.9 a/b Wheeler says this equation is accurate to about 1% for long coils, or any coils with (b/a 0.8). [Confirmed with extensive measurements I made and posted on s.e.d.] It's easy to solve this equation for N. A simple re-arrangement adds the concept of winding pitch. This can be very useful, in part because a low-winding-height multilayer coil can be treated as a single-layer coil with a higher winding pitch. a^2 p N 1 (7) L = -------- * ---------------- uH / inch 10 1 + 0.9 a p / N Here p is my turn-density pitch parameter, in turns/inch. Incidentally, this makes clear that for long coils, once you pick a coil-winding pitch, the inductance scales by N, rather than by N^2. Of course, the length scales as well. Now solving for N isn't as easy. I get, 10 b (8) N =~ ----- ( 1 + 9 a^3 p^2 / 100 L ) turns p a^2 Alan Fowler pointed out a version of Wheeler's equation, claimed more accurate, in F. Langford-Smith's "The Radiotron Designer's Handbook," 1942. In the 3rd edition only, the work of Esnault-Pelterie is detailed, a Frenchman who followed the "des savants japonais" (i.e. Nagaoaka) for his derivation of a simple Wheeler-like formula with a claimed accuracy of 0.1% for values of diameter/length between 0.2 and 1.5. Rearranging, a^2 N^2 / 9.972 b (9) L = -------------------- uH / inch 0.9949 + 0.9144 (a/b) [ snip more formulas and stuff ] ------------------------------------------------------------------- OK, there you have a small panaply of equation forms to select from. (7) is easier to use than it appears at first glance. Let's design a coil for you. We'll pick wire size #22, which has a diameter of 0.020 inches, prompting us to pick a winding-spacing of 0.04 inches, or a 25 turns/in pitch. Inspired by a small art brush in my pencil cup, we'll pick a coil diameter of 0.2", so equation (7) reduces to .. 0.01 25 N 1 .. L = --------- * ------------------ uH / inch .. 10 1 + 0.9 0.1 25 / N .. .. 1 .. = 0.025" N * ------------ uH / in .. 1 + 2.25/N This formula is more simple than it appears, because the second term approaches unity for coils of more than 10 - 20 turns. The first term says a 180nH coil requires about 180/25 = 7 turns, so we'll try N = 9, and get L = 225nH * 0.8 = 180nH, right on the money. That's a 9-turn coil 0.2" in diameter and 0.36" long. It uses less than six inches of wire, has a DC resistance of about 0.008 ohms, and can handle very high DC currents. Plugging our coil into equation (6) as a test, we have a = 0.1" and b = 0.36" and N = 9, so we get L = 8.1 / (9 + 36) = 0.180 uH, bingo. Thanks, - Win whill_at_picovolt-dot-com |
Paul Burridge wrote...
I need to wind 180nH inductor for a parallel tuned circuit I'm building, since the 180nH factory-made chokes I have don't look up to the job power-handling wise. This coil needs to handle about 90mA p-p / 500mW maximum dissipation sine current and I've allowed 3 ohms for series resistance. Can anyone give me some steer on dimensions, number of turns, core type and so on? Thanks, As John has said, that's a very low inductance that should not present any problems at such a low power level. But perhaps for a more detailed answer you can tell us the frequencies your coil will experience. At high frequencies skin and proximity effects dominate, and these can be evaluated with an Rac/Rdc ratio. If a ferrite is used its high-frequency core loss can also be modeled as an inductor resistance. Do you have any special size constraints? Unless you really need a miniature size, an air core may be best for 180nH. You can use the Wheeler equation to experiment with different coil designs. I'll add some new grist for the mill, with a copy of a portion of a posting I made 28 Dec 1997, about air-coil inductance equations. ------------------------------------------------------------------- Throughout the discussion we'll use the same dimensional system, based on the drawing below. Here and in the 14 formulas below, N = turns, a = mean radius, b = length, and c = winding thickness, and all are in inches, unless otherwise stated. length |------ b --------| --- ,-----------------, c | cross section | ------------ a = winding mean radius --- '-----------------' a | __________________________ | axis D = 2a ,-----------------, | | cross section | -------- '------------\----' \ solenoid coil layout N turns -------- [ snip five formulas and discussion ] To simplify our lives, Wheeler empirically derived his popular single-layer solenoid equation, using Nagaoka's equation and tables. Wheeler's equation is shown below in two different ways. a N^2 a^2 N^2 / 10 b (6) L = ---------- = -------------- uH / inch 9 + 10 b/a 1 + 0.9 a/b Wheeler says this equation is accurate to about 1% for long coils, or any coils with (b/a 0.8). [Confirmed with extensive measurements I made and posted on s.e.d.] It's easy to solve this equation for N. A simple re-arrangement adds the concept of winding pitch. This can be very useful, in part because a low-winding-height multilayer coil can be treated as a single-layer coil with a higher winding pitch. a^2 p N 1 (7) L = -------- * ---------------- uH / inch 10 1 + 0.9 a p / N Here p is my turn-density pitch parameter, in turns/inch. Incidentally, this makes clear that for long coils, once you pick a coil-winding pitch, the inductance scales by N, rather than by N^2. Of course, the length scales as well. Now solving for N isn't as easy. I get, 10 b (8) N =~ ----- ( 1 + 9 a^3 p^2 / 100 L ) turns p a^2 Alan Fowler pointed out a version of Wheeler's equation, claimed more accurate, in F. Langford-Smith's "The Radiotron Designer's Handbook," 1942. In the 3rd edition only, the work of Esnault-Pelterie is detailed, a Frenchman who followed the "des savants japonais" (i.e. Nagaoaka) for his derivation of a simple Wheeler-like formula with a claimed accuracy of 0.1% for values of diameter/length between 0.2 and 1.5. Rearranging, a^2 N^2 / 9.972 b (9) L = -------------------- uH / inch 0.9949 + 0.9144 (a/b) [ snip more formulas and stuff ] ------------------------------------------------------------------- OK, there you have a small panaply of equation forms to select from. (7) is easier to use than it appears at first glance. Let's design a coil for you. We'll pick wire size #22, which has a diameter of 0.020 inches, prompting us to pick a winding-spacing of 0.04 inches, or a 25 turns/in pitch. Inspired by a small art brush in my pencil cup, we'll pick a coil diameter of 0.2", so equation (7) reduces to .. 0.01 25 N 1 .. L = --------- * ------------------ uH / inch .. 10 1 + 0.9 0.1 25 / N .. .. 1 .. = 0.025" N * ------------ uH / in .. 1 + 2.25/N This formula is more simple than it appears, because the second term approaches unity for coils of more than 10 - 20 turns. The first term says a 180nH coil requires about 180/25 = 7 turns, so we'll try N = 9, and get L = 225nH * 0.8 = 180nH, right on the money. That's a 9-turn coil 0.2" in diameter and 0.36" long. It uses less than six inches of wire, has a DC resistance of about 0.008 ohms, and can handle very high DC currents. Plugging our coil into equation (6) as a test, we have a = 0.1" and b = 0.36" and N = 9, so we get L = 8.1 / (9 + 36) = 0.180 uH, bingo. Thanks, - Win whill_at_picovolt-dot-com |
http://w1.859.telia.com/~u85920178/begin/calc-00.htm
About 1/2 way down the pageare calculators that may be of interest. |
http://w1.859.telia.com/~u85920178/begin/calc-00.htm
About 1/2 way down the pageare calculators that may be of interest. |
On 4 Dec 2003 17:30:27 -0800, Winfield Hill
wrote: As John has said, that's a very low inductance that should not present any problems at such a low power level. But perhaps for a more detailed answer you can tell us the frequencies your coil will experience. At high frequencies skin and proximity effects dominate, and these can be evaluated with an Rac/Rdc ratio. If a ferrite is used its high-frequency core loss can also be modeled as an inductor resistance. Thanks, Win! You're a diamond. John's formulae didn't appear on his post for some reason, but you've given me the info I need to start winding and be slap in the ball park right away. Great. BTW, the factory inductors I have already are only about the size of the newest half-watt resistors, so I was reluctant to chance it. I know things keep getting smaller and somehow seem to defy the laws of physics, but just call me old fashioned. :-) Thanks again. P. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On 4 Dec 2003 17:30:27 -0800, Winfield Hill
wrote: As John has said, that's a very low inductance that should not present any problems at such a low power level. But perhaps for a more detailed answer you can tell us the frequencies your coil will experience. At high frequencies skin and proximity effects dominate, and these can be evaluated with an Rac/Rdc ratio. If a ferrite is used its high-frequency core loss can also be modeled as an inductor resistance. Thanks, Win! You're a diamond. John's formulae didn't appear on his post for some reason, but you've given me the info I need to start winding and be slap in the ball park right away. Great. BTW, the factory inductors I have already are only about the size of the newest half-watt resistors, so I was reluctant to chance it. I know things keep getting smaller and somehow seem to defy the laws of physics, but just call me old fashioned. :-) Thanks again. P. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Paul Burridge wrote:
...John's formulae didn't appear on his post for some reason, I hate it when I find a page and forget to paste the link: http://www.qsl.net/wa7zcz/area2/page34.html from (remember this old book of electronic data?): http://www.qsl.net/wa7zcz/area2/t_of_c.html -- John Popelish |
Paul Burridge wrote:
...John's formulae didn't appear on his post for some reason, I hate it when I find a page and forget to paste the link: http://www.qsl.net/wa7zcz/area2/page34.html from (remember this old book of electronic data?): http://www.qsl.net/wa7zcz/area2/t_of_c.html -- John Popelish |
"Paul Burridge" wrote in message ... On 4 Dec 2003 17:30:27 -0800, Winfield Hill wrote: As John has said, that's a very low inductance that should not present any problems at such a low power level. But perhaps for a more detailed answer you can tell us the frequencies your coil will experience. At high frequencies skin and proximity effects dominate, and these can be evaluated with an Rac/Rdc ratio. If a ferrite is used its high-frequency core loss can also be modeled as an inductor resistance. Thanks, Win! You're a diamond. John's formulae didn't appear on his post for some reason, but you've given me the info I need to start winding and be slap in the ball park right away. Great. BTW, the factory inductors I have already are only about the size of the newest half-watt resistors, so I was reluctant to chance it. I know things keep getting smaller and somehow seem to defy the laws of physics, but just call me old fashioned. :-) Thanks again. P. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill Paul, I really don't think you need to wind your own. For example, a Coilcraft 1008CS 180nH part is rated at 620mA rms: http://www.coi1craft.com/1008cs.cfm The "trick" is that the dissipation is a function of the series resistance, which is very low (0.77 ohms) as long as the core doesn't saturate. If you want to get really silly, their "Spring" inductors are rated at 3A rms. Regards Ian |
"Paul Burridge" wrote in message ... On 4 Dec 2003 17:30:27 -0800, Winfield Hill wrote: As John has said, that's a very low inductance that should not present any problems at such a low power level. But perhaps for a more detailed answer you can tell us the frequencies your coil will experience. At high frequencies skin and proximity effects dominate, and these can be evaluated with an Rac/Rdc ratio. If a ferrite is used its high-frequency core loss can also be modeled as an inductor resistance. Thanks, Win! You're a diamond. John's formulae didn't appear on his post for some reason, but you've given me the info I need to start winding and be slap in the ball park right away. Great. BTW, the factory inductors I have already are only about the size of the newest half-watt resistors, so I was reluctant to chance it. I know things keep getting smaller and somehow seem to defy the laws of physics, but just call me old fashioned. :-) Thanks again. P. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill Paul, I really don't think you need to wind your own. For example, a Coilcraft 1008CS 180nH part is rated at 620mA rms: http://www.coi1craft.com/1008cs.cfm The "trick" is that the dissipation is a function of the series resistance, which is very low (0.77 ohms) as long as the core doesn't saturate. If you want to get really silly, their "Spring" inductors are rated at 3A rms. Regards Ian |
I need to wind 180nH inductor for a parallel tuned circuit I'm
building, Paul- I suggest you find some stiff wire, wind about five turns using a pencil as a form, and stretch or compress it to tune the circuit. If you want a more stable inductor, then wind it on a high value, one or two watt resistor. Once the desired inductance is set, put some kind of varnish (coil dope) on it to hold the winding in place. 73, Fred, K4DII |
I need to wind 180nH inductor for a parallel tuned circuit I'm
building, Paul- I suggest you find some stiff wire, wind about five turns using a pencil as a form, and stretch or compress it to tune the circuit. If you want a more stable inductor, then wind it on a high value, one or two watt resistor. Once the desired inductance is set, put some kind of varnish (coil dope) on it to hold the winding in place. 73, Fred, K4DII |
On Fri, 5 Dec 2003 15:37:15 -0000, "Ian Buckner"
wrote: Paul, I really don't think you need to wind your own. For example, a Coilcraft 1008CS 180nH part is rated at 620mA rms: http://www.coi1craft.com/1008cs.cfm The "trick" is that the dissipation is a function of the series resistance, which is very low (0.77 ohms) as long as the core doesn't saturate. If you want to get really silly, their "Spring" inductors are rated at 3A rms. Thanks, Ian, but it only took about 15 seconds to wind the coil according to Win's spec and more importantly, winding my own enables me to take a tap off it, which I believe may be necessary in this app. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Fri, 5 Dec 2003 15:37:15 -0000, "Ian Buckner"
wrote: Paul, I really don't think you need to wind your own. For example, a Coilcraft 1008CS 180nH part is rated at 620mA rms: http://www.coi1craft.com/1008cs.cfm The "trick" is that the dissipation is a function of the series resistance, which is very low (0.77 ohms) as long as the core doesn't saturate. If you want to get really silly, their "Spring" inductors are rated at 3A rms. Thanks, Ian, but it only took about 15 seconds to wind the coil according to Win's spec and more importantly, winding my own enables me to take a tap off it, which I believe may be necessary in this app. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Fri, 05 Dec 2003 08:38:18 -0800, Bill Turner
wrote: As you may know, the inductance of a coil is not a fixed value, but varies dramatically with frequency. Er, you mean *reactance* of a coil varies dramatically with frequency, don't you? -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Fri, 05 Dec 2003 08:38:18 -0800, Bill Turner
wrote: As you may know, the inductance of a coil is not a fixed value, but varies dramatically with frequency. Er, you mean *reactance* of a coil varies dramatically with frequency, don't you? -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Sat, 06 Dec 2003 13:09:02 -0800, Bill Turner wrote:
On Sat, 06 Dec 2003 18:39:18 +0000, Paul Burridge wrote: As you may know, the inductance of a coil is not a fixed value, but varies dramatically with frequency. Er, you mean *reactance* of a coil varies dramatically with frequency, don't you? _________________________________________________ ________ Er, no I don't. They both vary with frequency. If the inductance did NOT vary with frequency, the X sub L vs F plot would be linear. In reality, it is anything but linear. X sub L is inductive reactance, yes that varies with freq. L is the fixed value inductance. though there is some parasitic capacitance between windings that does effect the resonant freq. a bit. www.coilcraft.com gives guidelines to this. Remove "HeadFromButt", before replying by email. |
On Sat, 06 Dec 2003 13:09:02 -0800, Bill Turner wrote:
On Sat, 06 Dec 2003 18:39:18 +0000, Paul Burridge wrote: As you may know, the inductance of a coil is not a fixed value, but varies dramatically with frequency. Er, you mean *reactance* of a coil varies dramatically with frequency, don't you? _________________________________________________ ________ Er, no I don't. They both vary with frequency. If the inductance did NOT vary with frequency, the X sub L vs F plot would be linear. In reality, it is anything but linear. X sub L is inductive reactance, yes that varies with freq. L is the fixed value inductance. though there is some parasitic capacitance between windings that does effect the resonant freq. a bit. www.coilcraft.com gives guidelines to this. Remove "HeadFromButt", before replying by email. |
Bill Turner wrote...
Er, no I don't. They both vary with frequency. If the inductance did NOT vary with frequency, the X sub L vs F plot would be linear. In reality, it is anything but linear. We're talking a small air-coil here. As SRF is approached the reactance does depart from the expected linear plot, but that's because one should have considered X_C as well. Say, you're not talking about an exotic high-frequency region where the physical diameter of a coil may drop slightly due to proximity effect? Sheesh! Thanks, - Win whill_at_picovolt-dot-com |
Bill Turner wrote...
Er, no I don't. They both vary with frequency. If the inductance did NOT vary with frequency, the X sub L vs F plot would be linear. In reality, it is anything but linear. We're talking a small air-coil here. As SRF is approached the reactance does depart from the expected linear plot, but that's because one should have considered X_C as well. Say, you're not talking about an exotic high-frequency region where the physical diameter of a coil may drop slightly due to proximity effect? Sheesh! Thanks, - Win whill_at_picovolt-dot-com |
On Sat, 06 Dec 2003 13:09:02 -0800, Bill Turner
wrote: On Sat, 06 Dec 2003 18:39:18 +0000, Paul Burridge wrote: As you may know, the inductance of a coil is not a fixed value, but varies dramatically with frequency. Er, you mean *reactance* of a coil varies dramatically with frequency, don't you? _________________________________________________ ________ Er, no I don't. They both vary with frequency. If the inductance did NOT vary with frequency, the X sub L vs F plot would be linear. In reality, it is anything but linear. I'm still none the wiser. Unless you're taking into account stray inductance from the leads, of course. But the *body* of the inductor by itself must surely be of a fixed inductance. One does not come across coils rated at "3uH @ 150Mhz.", for example! Are you talking about the impact of stray L from the lead-lengths? -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Sat, 06 Dec 2003 13:09:02 -0800, Bill Turner
wrote: On Sat, 06 Dec 2003 18:39:18 +0000, Paul Burridge wrote: As you may know, the inductance of a coil is not a fixed value, but varies dramatically with frequency. Er, you mean *reactance* of a coil varies dramatically with frequency, don't you? _________________________________________________ ________ Er, no I don't. They both vary with frequency. If the inductance did NOT vary with frequency, the X sub L vs F plot would be linear. In reality, it is anything but linear. I'm still none the wiser. Unless you're taking into account stray inductance from the leads, of course. But the *body* of the inductor by itself must surely be of a fixed inductance. One does not come across coils rated at "3uH @ 150Mhz.", for example! Are you talking about the impact of stray L from the lead-lengths? -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Bill Turner writes:
On 6 Dec 2003 13:39:51 -0800, Winfield Hill wrote: We're talking a small air-coil here. Doesn't matter what kind of coil; all coils have a non-linear plot of either inductance vs frequency OR reactance vs frequency. ALL coils. Well, just about anything is "non-linear" if you measure it accurately enough! But is it really true that the *inductance* of a "small air coil" is "dramatically" non-linear with frequency as you stated? -- John Devereux |
Bill Turner writes:
On 6 Dec 2003 13:39:51 -0800, Winfield Hill wrote: We're talking a small air-coil here. Doesn't matter what kind of coil; all coils have a non-linear plot of either inductance vs frequency OR reactance vs frequency. ALL coils. Well, just about anything is "non-linear" if you measure it accurately enough! But is it really true that the *inductance* of a "small air coil" is "dramatically" non-linear with frequency as you stated? -- John Devereux |
John Devereux wrote:
Bill Turner writes: On 6 Dec 2003 13:39:51 -0800, Winfield Hill wrote: We're talking a small air-coil here. Doesn't matter what kind of coil; all coils have a non-linear plot of either inductance vs frequency OR reactance vs frequency. ALL coils. Well, just about anything is "non-linear" if you measure it accurately enough! But is it really true that the *inductance* of a "small air coil" is "dramatically" non-linear with frequency as you stated? Intuitively I'd have thought the answer was plainly No, but I'm certainly not technically savvy enough to be confident about that. But I strongly suspect that the thread is already ovedue an unambiguous definition of 'inductance'. Where's John Woodgate when you really need him...g. -- Terry Pinnell Hobbyist, West Sussex, UK |
John Devereux wrote:
Bill Turner writes: On 6 Dec 2003 13:39:51 -0800, Winfield Hill wrote: We're talking a small air-coil here. Doesn't matter what kind of coil; all coils have a non-linear plot of either inductance vs frequency OR reactance vs frequency. ALL coils. Well, just about anything is "non-linear" if you measure it accurately enough! But is it really true that the *inductance* of a "small air coil" is "dramatically" non-linear with frequency as you stated? Intuitively I'd have thought the answer was plainly No, but I'm certainly not technically savvy enough to be confident about that. But I strongly suspect that the thread is already ovedue an unambiguous definition of 'inductance'. Where's John Woodgate when you really need him...g. -- Terry Pinnell Hobbyist, West Sussex, UK |
On Sat, 06 Dec 2003 17:32:58 -0800, Bill Turner
wrote: Actually, one does come across such coils. All coils have a frequency where they become a parallel resonant circuit, due to the capacitance between windings. And oddly enough, *above* that parallel resonant frequency, they become capacitive. Yes, you read that right, they actually act like a capacitor, believe it or not. This is only an artefact if you try to determine the inductance of an inductor by measuring the reactance of that component at some specified frequency. The inductive reactance (Xl=2*pi*f*L) will grow in a linear way towards a positive value depending of the frequency. Since the parasitic capacitances are present, the negative capacitive reactance (Xc=-1/(2*pi*f*C) will complicate the situation. When approaching resonance in a parallel resonant circuit, the reactance goes to +infinity, switching rapidly to -infinity as the resonance frequency has been passed and slowly approach the linear drop of the negative capacitance at frequencies far above resonance. One can still argue that the inductance and inductive reactance are as well as the capacitance and the capacitive reactance are still there as separate entities, but we can not measure them separately from terminals of the coil. Thus, this is an artefact of the measurement method. Thus, the inductance should be measured at a low frequency to avoid the capacitive reactance. On the other hand the capacitance should be measured at a high frequency well above resonance to avoid the effects of the inductance. Or just measure the inductance at a low frequency and determine the capacitance from the resonance frequency and inductance. While inductance and capacitance are frequency independent, the resistance of a coil will vary with frequency due to the skin effect, since at higher frequencies, the conductivity of the inner part of the conductor is not used. Paul OH3LWR |
On Sat, 06 Dec 2003 17:32:58 -0800, Bill Turner
wrote: Actually, one does come across such coils. All coils have a frequency where they become a parallel resonant circuit, due to the capacitance between windings. And oddly enough, *above* that parallel resonant frequency, they become capacitive. Yes, you read that right, they actually act like a capacitor, believe it or not. This is only an artefact if you try to determine the inductance of an inductor by measuring the reactance of that component at some specified frequency. The inductive reactance (Xl=2*pi*f*L) will grow in a linear way towards a positive value depending of the frequency. Since the parasitic capacitances are present, the negative capacitive reactance (Xc=-1/(2*pi*f*C) will complicate the situation. When approaching resonance in a parallel resonant circuit, the reactance goes to +infinity, switching rapidly to -infinity as the resonance frequency has been passed and slowly approach the linear drop of the negative capacitance at frequencies far above resonance. One can still argue that the inductance and inductive reactance are as well as the capacitance and the capacitive reactance are still there as separate entities, but we can not measure them separately from terminals of the coil. Thus, this is an artefact of the measurement method. Thus, the inductance should be measured at a low frequency to avoid the capacitive reactance. On the other hand the capacitance should be measured at a high frequency well above resonance to avoid the effects of the inductance. Or just measure the inductance at a low frequency and determine the capacitance from the resonance frequency and inductance. While inductance and capacitance are frequency independent, the resistance of a coil will vary with frequency due to the skin effect, since at higher frequencies, the conductivity of the inner part of the conductor is not used. Paul OH3LWR |
Bill Turner wrote...
Yes, it really is true. If you graph the reactance vs frequency of any coil, starting just above DC, it will rise in a near-linear fashion for a while, but will begin to steepen and when approaching the self-resonant frequency, will quickly rise to maximum, and at that point will suddenly drop to the opposite (negative, or capacitive) extreme and then diminish back to near zero as the frequency continues to increase. At that self-resonant frequency, the coil is behaving like a parallel resonant circuit, which of course it is, due to the parasitic capacitance between each winding. This parasitic capacitance is unavoidable and ALL coils exhibit this characteristic. The truly strange thing is that above the self-resonant frequency, the coil actually behaves exactly like a capacitor, believe it or not. Bill, it's one thing to say a coil's reactance is non-linear, but it's another to assert its inductance varies with frequency. As I responded before, the inductance of air coils varies very little with frequency. I know this having made many types of air coils to verify the standard inductance formulas, and precisely measured them over a 60Hz to 50MHz range. Earlier in the thread I pointed out the effects of SRF (self- resonant frequency), due to the coil's parallel capacitance. It's not useful to my thinking to characterize those two components as one part, and it's little surprise one gets into trouble when attempting to do so. A similar statement can be made at very low frequencies where the dc resistance exceeds the reactance, and the coil is best considered as two separate parts in series. The capacitance and dc resistance are both simple and rather obvious considerations, with straightforward solutions. In contrast, a subtle and difficult issue in air coils is modeling Q or loss vs frequency. The concept of ac resistance is often used for loss, and is expressed as a ratio to the dc resistance, Rac/Rdc. Predicting that ratio is the tough part, including not only the well-understood skin effect, but also the relatively obscure and often larger proximity effect. Further complications enter if one uses multiple wires, and how they are wound, or if one uses any of the many types of litz wire. Thanks, - Win whill_at_picovolt-dot-com |
Bill Turner wrote...
Yes, it really is true. If you graph the reactance vs frequency of any coil, starting just above DC, it will rise in a near-linear fashion for a while, but will begin to steepen and when approaching the self-resonant frequency, will quickly rise to maximum, and at that point will suddenly drop to the opposite (negative, or capacitive) extreme and then diminish back to near zero as the frequency continues to increase. At that self-resonant frequency, the coil is behaving like a parallel resonant circuit, which of course it is, due to the parasitic capacitance between each winding. This parasitic capacitance is unavoidable and ALL coils exhibit this characteristic. The truly strange thing is that above the self-resonant frequency, the coil actually behaves exactly like a capacitor, believe it or not. Bill, it's one thing to say a coil's reactance is non-linear, but it's another to assert its inductance varies with frequency. As I responded before, the inductance of air coils varies very little with frequency. I know this having made many types of air coils to verify the standard inductance formulas, and precisely measured them over a 60Hz to 50MHz range. Earlier in the thread I pointed out the effects of SRF (self- resonant frequency), due to the coil's parallel capacitance. It's not useful to my thinking to characterize those two components as one part, and it's little surprise one gets into trouble when attempting to do so. A similar statement can be made at very low frequencies where the dc resistance exceeds the reactance, and the coil is best considered as two separate parts in series. The capacitance and dc resistance are both simple and rather obvious considerations, with straightforward solutions. In contrast, a subtle and difficult issue in air coils is modeling Q or loss vs frequency. The concept of ac resistance is often used for loss, and is expressed as a ratio to the dc resistance, Rac/Rdc. Predicting that ratio is the tough part, including not only the well-understood skin effect, but also the relatively obscure and often larger proximity effect. Further complications enter if one uses multiple wires, and how they are wound, or if one uses any of the many types of litz wire. Thanks, - Win whill_at_picovolt-dot-com |
Paul Keinanen wrote...
While inductance and capacitance are frequency independent, the resistance of a coil will vary with frequency due to the skin effect, since at higher frequencies, the conductivity of the inner part of the conductor is not used. Skin effect applies equally around the periphery of each wire, what you've described above is the more serious proximity effect. Thanks, - Win whill_at_picovolt-dot-com |
Paul Keinanen wrote...
While inductance and capacitance are frequency independent, the resistance of a coil will vary with frequency due to the skin effect, since at higher frequencies, the conductivity of the inner part of the conductor is not used. Skin effect applies equally around the periphery of each wire, what you've described above is the more serious proximity effect. Thanks, - Win whill_at_picovolt-dot-com |
On Sat, 06 Dec 2003 17:32:58 -0800, Bill Turner
wrote: Actually, one does come across such coils. All coils have a frequency where they become a parallel resonant circuit, due to the capacitance between windings. And oddly enough, *above* that parallel resonant frequency, they become capacitive. Yes, you read that right, they actually act like a capacitor, believe it or not. Yes, I'm sure no one here disputes that coils behave like capacitors above their SRF and capacitors behave like coils above the SRF. That's not news. And it's to do with the *reactance* of the part, not its inductance. AIUI, inductance is pretty much stable over the frequency spectrum. You appear to be the only person here who claims otherwise. Now, if you are always working with relatively small coils at relatively low frequencies, you will probably never see this effect. But if you ever have access to a $10,000 HP sweep impedance meter, hook up your favorite coil and see just what I'm talking about. You will never look at coils the same way again. :-) That's *reactance* giving rise to that effect, not inductance! -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Sat, 06 Dec 2003 17:32:58 -0800, Bill Turner
wrote: Actually, one does come across such coils. All coils have a frequency where they become a parallel resonant circuit, due to the capacitance between windings. And oddly enough, *above* that parallel resonant frequency, they become capacitive. Yes, you read that right, they actually act like a capacitor, believe it or not. Yes, I'm sure no one here disputes that coils behave like capacitors above their SRF and capacitors behave like coils above the SRF. That's not news. And it's to do with the *reactance* of the part, not its inductance. AIUI, inductance is pretty much stable over the frequency spectrum. You appear to be the only person here who claims otherwise. Now, if you are always working with relatively small coils at relatively low frequencies, you will probably never see this effect. But if you ever have access to a $10,000 HP sweep impedance meter, hook up your favorite coil and see just what I'm talking about. You will never look at coils the same way again. :-) That's *reactance* giving rise to that effect, not inductance! -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Bill, it's one thing to say a coil's reactance is non-linear, but it's another to assert its inductance varies with frequency. Both statements are true and easily provable. A simple air core coil which measures one microhenry at a low frequency may have an inductance of several millihenries (or even henries) when near its self resonant frequency. It's a simple law of physics; there is no way around it. And *above* the self-resonant frequency, the choke actually behaves like a capacitor, believe it or not. As I responded before, the inductance of air coils varies very little with frequency. That statement is true only at relatively low frequencies. Get near the self-resonant frequency of an air core coil and you'll find otherwise. Designers using relatively large coils over a wide frequency range run into this problem all the time. As I mentioned in another post, the classic example for Amateur Radio is the plate choke in a tube type amplifier. Designing such a choke that has enough inductance to work over the entire HF spectrum without self-resonances is nearly impossible. Many amplifier designers don't even try; they just switch inductance in and out of the choke depending on frequency. Youall seem to be hitting all around the 'problem'. A coil has 3 components, the resistance of the wire, the inductance, and the stray capacitance. As the frequency is changed from DC to low AC to RF each component has more or a less effect on how it acts in a circuit. The actual value of each does not change, just the effect on an external circuit. For small coils at DC the reisitance is the major item that will be seen by an external circuit. At low to medium frequencies the inductance will be the major factor. At very high frequencies the capacitance may be the major factor. At self resonant frequencies , the tuned circuit effect takes over. |
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