Measuring Inductance with the MFJ-259
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Measuring Inductance with the MFJ-259
 
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As I build my second K2, I want to try to measure the inductance of the
toroids with my MFJ-259. The MFJ manual has instructions on how to do this. Essentially one builds a series LC circuit and measures the resonant frequency and then calculates the inductance. Has anyone tried this and have any tips or tricks to pass on? Am I correct in assuming I can adjust the value of inductance on a toroid slightly by compressing or expanding the spacing between turns? If so what can I use to hold the turns in place. Friction is probably good enough, but I wonder about a few drops of something like Q-Dope. Comments? 73, Darrell -- Darrell Bellerive Amateur Radio Stations VA7TO and VE7CLA Grand Forks, British Columbia, Canada _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
RE: Measuring Inductance with the MFJ-259
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Darrell,
Caution, The toroids should be wound as instructed - you may measure them, but please do not try to make the inductance equal that indicated in the manual/schematic because those inductance values are either what was measured on someone else's inductance meter, or they are approximate values. If you want to calculate exactly the inductance required, you must calculate the entire low pass filter design, and then be certain to measure the inductors at the frequency of use - and with a properly calibrated inductance meter - I would bet the MFJ259 could be off by as much as 20% until proven otherwise by measurement of standard inductors at various frequencies. No Q-Dope or fixatives of any kind should be used for Elecraft kits unless specifically stated - I have never experienced any problems with toroids and have never used anything to hold them in place. One of the K2s I built is being used successfully as a mobile - in an 18 wheeler - check the QRZ.com listing for W4GNS if you have any doubts. 73, Don W3FPR > -----Original Message----- > > As I build my second K2, I want to try to measure the inductance of the > toroids with my MFJ-259. The MFJ manual has instructions on how > to do this. > Essentially one builds a series LC circuit and measures the resonant > frequency and then calculates the inductance. > > Has anyone tried this and have any tips or tricks to pass on? > > Am I correct in assuming I can adjust the value of inductance on a toroid > slightly by compressing or expanding the spacing between turns? > If so what > can I use to hold the turns in place. Friction is probably good > enough, but I > wonder about a few drops of something like Q-Dope. Comments? > > 73, > Darrell > No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.344 / Virus Database: 267.10.16/83 - Release Date: 8/26/2005 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Darrell Bellerive
W3FPR has the answer you need here, but I just wanted to add that you aren't
changing the inductance of a toroid by changing the coil spacing. You are changing the distributed capacitance of the toroid by doing so. Eric KE6US www.ke6us.com -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Darrell Bellerive Sent: Sunday, August 28, 2005 4:32 PM To: [hidden email] Subject: [Elecraft] Measuring Inductance with the MFJ-259 As I build my second K2, I want to try to measure the inductance of the toroids with my MFJ-259. The MFJ manual has instructions on how to do this. Essentially one builds a series LC circuit and measures the resonant frequency and then calculates the inductance. Has anyone tried this and have any tips or tricks to pass on? Am I correct in assuming I can adjust the value of inductance on a toroid slightly by compressing or expanding the spacing between turns? If so what can I use to hold the turns in place. Friction is probably good enough, but I wonder about a few drops of something like Q-Dope. Comments? 73, Darrell -- Darrell Bellerive Amateur Radio Stations VA7TO and VE7CLA Grand Forks, British Columbia, Canada _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Measuring Inductance with the MFJ-259
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In reply to this post by Darrell Bellerive
Thanks to all who responded. I had no plans of deviating from the number of
turns that Elecraft directs. I will follow their instructions as to the best of my ability. I have learned a few things: 1) Adjusting the spacing of the winding on the toroids does not change the inductance, just the distributed capacitance of the toroid. 2) The values specified by Elecraft are dependant on their measurement equipment and techniques, and my measurement equipment and techniques probably wouldn't show the same values. So it is not worth the trouble of trying to measure them. 3) Don't bother with Q-Dope or any other method of attaching the windings to the cores as friction is sufficient even in mobile applications. On August 28, 2005 04:31 pm, Darrell Bellerive wrote: > As I build my second K2, I want to try to measure the inductance of the > toroids with my MFJ-259. The MFJ manual has instructions on how to do this. > Essentially one builds a series LC circuit and measures the resonant > frequency and then calculates the inductance. > > Has anyone tried this and have any tips or tricks to pass on? > > Am I correct in assuming I can adjust the value of inductance on a toroid > slightly by compressing or expanding the spacing between turns? If so what > can I use to hold the turns in place. Friction is probably good enough, but > I wonder about a few drops of something like Q-Dope. Comments? > > 73, > Darrell -- Darrell Bellerive Amateur Radio Stations VA7TO and VE7CLA Grand Forks, British Columbia, Canada _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Measuring Inductance with the MFJ-259
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In reply to this post by EricJ-2
The Elecraft designers have stated in past discussions that Coil Dope is not
needed if you wind your toroids tight. Remember, that the dielectric constant of Coil Dope can change your tuning of the coil. It would be alright to use a few drops on ends of winding as an aid, but a lot could be a problem. Stuart K5KVH _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Measuring Inductance with the MFJ-259
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In reply to this post by Darrell Bellerive
Hello Darrell,
The inductance of the toroids typically used in the K2 can be increased by compressing the turns. This can be done to fine-tune a circuit but for consistent results, it's probably best to do it the way the Elecraft guys recommend and spread out the turns a bit. I've evaluated a toroid, L5, in the KAT2 with both close spacing and loose spacing. The data can be seen here: http://w5big.home.comcast.net/L5Analysis.htm 73/ Bob - W5BIG ==================================== ----- Original Message ----- From: "Darrell Bellerive" <[hidden email]> To: <[hidden email]> Sent: Sunday, August 28, 2005 6:31 PM Subject: [Elecraft] Measuring Inductance with the MFJ-259 > As I build my second K2, I want to try to measure the inductance of the > toroids with my MFJ-259. The MFJ manual has instructions on how to do this. > Essentially one builds a series LC circuit and measures the resonant > frequency and then calculates the inductance. > > Has anyone tried this and have any tips or tricks to pass on? > > Am I correct in assuming I can adjust the value of inductance on a toroid > slightly by compressing or expanding the spacing between turns? If so what > can I use to hold the turns in place. Friction is probably good enough, but I > wonder about a few drops of something like Q-Dope. Comments? > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
OT: Effect of Compression and Expansion on the Inductance of Toroids?
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In reply to this post by Darrell Bellerive
Well, it seems now that I am receiving contradictory information. Specifically
that compressing the turns on a toroid will raise the inductance. What is the real scoop. Does compressing or expanding the spacing of the winding on a toroid change the inductance? Or just the distributed capacitance of the toroid? Or is this much more complex than I would have expected? On August 29, 2005 10:40 am, Darrell Bellerive wrote: > I have learned a few things: > > 1) Adjusting the spacing of the winding on the toroids does not change the > inductance, just the distributed capacitance of the toroid. > -- Darrell Bellerive Amateur Radio Stations VA7TO and VE7CLA Grand Forks, British Columbia, Canada _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: OT: Effect of Compression and Expansion on the Inductance of Toroids?
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The toroid formulas are approximations, but good ones. Here is a
physics-based explanation that gives some of the complexities, but there are literally books on inductors. http://tinyurl.com/9brxb Leigh / WA5ZNU On Mon, 29 Aug 2005 3:52 pm, Darrell Bellerive wrote: > Well, it seems now that I am receiving contradictory information. > Specifically > that compressing the turns on a toroid will raise the inductance. > > What is the real scoop. Does compressing or expanding the spacing of > the > winding on a toroid change the inductance? Or just the distributed > capacitance of the toroid? Or is this much more complex than I would > have > expected? Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
RE: OT: Effect of Compression and Expansion on theInductance of Toroids?
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In reply to this post by Darrell Bellerive
Darrell,
There may be a small apparent change in inductance when adjusting the turns spacing of a toroid, but it would only be a small percentage - look at the formulas for a toroid inductor to verify that fact - there is no term for the turns spacing, and the diameter of the toroid fixes the coil length. Check out http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indtor.html for the detailed formulas. If you move all the turns to one side of the toroid core, it may begin to behave more like a solenoidal inductor and the inductance could possibly change because the effective length of the solenoidal inductor will change, but I have not persued that 'angle'. Elecraft usually specifies that the winding should occupy about 85% of the core, so that 'fixes' the length of the coil question for a properly wound Elecraft toroid. The inter-turn capacity is bound to change with the turns spacing, and that may show up as an apparent change in 'inductance' on many meters because they do measure inductance at a specific frequency. The best way I can think of to prove or disprove it would be to measure the time constant in an LR series circuit. 73, Don W3FPR > -----Original Message----- > > Well, it seems now that I am receiving contradictory information. > Specifically > that compressing the turns on a toroid will raise the inductance. > > What is the real scoop. Does compressing or expanding the spacing of the > winding on a toroid change the inductance? Or just the distributed > capacitance of the toroid? Or is this much more complex than I would have > expected? > > No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.344 / Virus Database: 267.10.16/83 - Release Date: 8/26/2005 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
RE: OT: Effect of Compression and Expansion ontheInductance of Toroids?
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Okay, this is interesting so I approached an answer in my usual way.
I grabbed a loose toroidal inductor that had nicely spaced turns filling 90% of the circumference of the core and stuck it in my L-meter. 3.1 uH. Scrunched all the turns tightly together. They filled about 20% of the circumference. Inductance now 3.3 uH. The L-meter uses a low frequency signal so capacitance shouldn't make much difference in the reading. So fully spaced to totally scrunched, in this case, yielded a 9% change in inductance. It's nice to work the numbers, but I've always tended toward a direct experiment whenever possible. Back in school, I always knew where my soldering iron was but was forever forgetting where I had put my slide rule. I haven't changed. (For the newer readers, a slide rule is the ancestor of the pocket calculator. For comparison, a slide rule is to a calculator as working CW on 40 meters is to making a cell phone call.) Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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I don't have your experience or background, Ron, but the L-meter is
measuring inductance indirectly and ignoring the presence of distributed capacitance. Maybe it isn't as negligible at the frequency of the meter as you think. But in the end, it doesn't really matter. The real test is how a particular toroid reacts in the circuit it was intended for. If adjusting the turns spacing changes circuit resonance, then...it changes circuit resonance. That's the result we would have been looking for. Hi. Eric KE6US www.ke6us.com -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Ron D'Eau Claire Sent: Monday, August 29, 2005 5:29 PM To: [hidden email]; [hidden email]; [hidden email] Subject: RE: [Elecraft] OT: Effect of Compression and ExpansionontheInductance of Toroids? Okay, this is interesting so I approached an answer in my usual way. I grabbed a loose toroidal inductor that had nicely spaced turns filling 90% of the circumference of the core and stuck it in my L-meter. 3.1 uH. Scrunched all the turns tightly together. They filled about 20% of the circumference. Inductance now 3.3 uH. The L-meter uses a low frequency signal so capacitance shouldn't make much difference in the reading. So fully spaced to totally scrunched, in this case, yielded a 9% change in inductance. It's nice to work the numbers, but I've always tended toward a direct experiment whenever possible. Back in school, I always knew where my soldering iron was but was forever forgetting where I had put my slide rule. I haven't changed. (For the newer readers, a slide rule is the ancestor of the pocket calculator. For comparison, a slide rule is to a calculator as working CW on 40 meters is to making a cell phone call.) Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
RE: OT: Effect of Compression and ExpansionontheInductance of Toroids?
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Eric, KE6US wrote:
I don't have your experience or background, Ron, but the L-meter is measuring inductance indirectly and ignoring the presence of distributed capacitance. Maybe it isn't as negligible at the frequency of the meter as you think. But in the end, it doesn't really matter. The real test is how a particular toroid reacts in the circuit it was intended for. If adjusting the turns spacing changes circuit resonance, then...it changes circuit resonance. That's the result we would have been looking for. Hi. -------------------------- I'm fond of saying that the only dumb question is the question not asked, so your question is an excellent one, Eric. As for lots of experience, for me that's too often just an opportunity to remember more things incorrectly. About your question, my L-meter evaluates inductance using a low-frequency square wave at about 60 kHz that goes to a differentiator consisting of a 200 ohm resistor in series with the unknown inductance. The waveform at the junction of the resistor and inductor is a series of spikes at the frequency of the input square wave. The spikes decay at a rate proportional to the time constant of the resistor and inductor. This decay rate is converted into a voltage in a simple logic circuit. The ARRL Handbook has featured this circuit in a number of editions. It's a simple and surprisingly accurate meter, depending upon the quality of the inductors used to calibrate it. The way the circuit works means that any significant capacity in parallel with the inductor would tend to cancel the effect of the inductor, and make the inductance value displayed read lower than it really was, not higher. Again, at the bench I tested that conclusion by adding capacitors in parallel with the inductor. Sure 'nuf, adding capacitance decreased the reading. Recall that squeezing the turns together raised the test inductor value read on the meter from 3.1 to 3.3 uH. Now I added fixed capacitors across the inductor to simulate added distributed capacitance caused by squeezing them together. Adding 10 pf of capacitance across the inductor had zero effect on the reading. Adding 33 pf across the inductor *lowered* the reading by 0.1 uH. We're dealing with a very small toroid in this test whose inter-turn capacitance isn't going to be as much as 2 or 3 pf when squeezed together. So I'm confident that the added capacitance by squeezing the turns together is not what is causing my L-meter to show increased inductance. If anything, the capacitance would tend to cause the L-meter to show lower inductance. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
RE: OT: Effect of Compression and ExpansionontheInductanceof Toroids?
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Ron,
I have the same inductance meter, and it does a good job because it does measure using the LR time constant rather than frequency, but 'physics is physics', and with a true toroidal inductor, there is still no dependency on the turns spacing (the math says so). Now, for the practical side of things, I do believe that your results (and others with similar findings) deviate from the 'classic physics' treatment of the ideal (ignoring practical behavior) - and when the toroid turns are irregular (not evenly spaced around the circumference of the core), there is some part of that coil behaving as a solenoidal inductor where turns spacing IS a factor. Remember that a toroid is simply a solenoidal coil formed into a circle with the ends of the coil meeting. This ideal toroid has equal turns spacing all the way around. So my current conclusion is -- IF the toroid is equally spaced around the core, the inductance is not dependent on the spacing of the turns, BUT, if the turns are compressed anywhere around the core, additional factors rear their ugly heads because the inductor is a combination of a solenoidal coil and a toroid coil, and the math becomes quite complex - just how much of each effect depends on just how much deviation from an ideal toroid shape exists in the configuration at hand. In a frequency dependent tuned circuit, how much of the frequency shift is due to the inter-turn capacitance or a change in inductance is (to me) only a matter of curiosity - the plain fact is that some change in the resonant frequency (or inductance) can be achieved by changing the turns spacing, but we all know that the adjustment range is small - the really big determining factor is simply the number of turns. As an example, I often improve the 10/12 meter Low Pass Filter characteristics of a KPA100 by squeezing the toroid turns "just the right way", but I determine what is the 'right way' by monitoring the impedance with my MFJ259B as I do it. Sometimes it is 'this way', while other times it is 'that way' - the batch to batch change in the permeability of the cores likely accounts for more variation than the turns spacing itself. BTW, this change does not really help the KPA100 output, but it improves the base K2 10 meter efficiency at 10 watts or lower when the KPA100 is installed. The overall inductance change that I have experienced is about 10%, so that is within the normal design tolerances using 10% resistors and capacitors - so except for satisfaction of the curiosity factor, I would say just to wind the toroids with the proper number of turns, and 'tweak' them in-circuit as required and as close as is permitted by your measurement capability. 73, Don W3FPR > -----Original Message----- > > Eric, KE6US wrote: > > I don't have your experience or background, Ron, but the L-meter is > measuring inductance indirectly and ignoring the presence of distributed > capacitance. Maybe it isn't as negligible at the frequency of the meter as > you think. > > But in the end, it doesn't really matter. The real test is how a > particular > toroid reacts in the circuit it was intended for. If adjusting the turns > spacing changes circuit resonance, then...it changes circuit resonance. > That's the result we would have been looking for. Hi. > > -------------------------- > > I'm fond of saying that the only dumb question is the question > not asked, so > your question is an excellent one, Eric. As for lots of experience, for me > that's too often just an opportunity to remember more things incorrectly. > > About your question, my L-meter evaluates inductance using a low-frequency > square wave at about 60 kHz that goes to a differentiator consisting of a > 200 ohm resistor in series with the unknown inductance. The > waveform at the > junction of the resistor and inductor is a series of spikes at > the frequency > of the input square wave. The spikes decay at a rate proportional to the > time constant of the resistor and inductor. This decay rate is converted > into a voltage in a simple logic circuit. > > The ARRL Handbook has featured this circuit in a number of > editions. It's a > simple and surprisingly accurate meter, depending upon the quality of the > inductors used to calibrate it. > > The way the circuit works means that any significant capacity in parallel > with the inductor would tend to cancel the effect of the > inductor, and make > the inductance value displayed read lower than it really was, not higher. > > Again, at the bench I tested that conclusion by adding capacitors in > parallel with the inductor. Sure 'nuf, adding capacitance decreased the > reading. Recall that squeezing the turns together raised the test inductor > value read on the meter from 3.1 to 3.3 uH. Now I added fixed capacitors > across the inductor to simulate added distributed capacitance caused by > squeezing them together. Adding 10 pf of capacitance across the > inductor had > zero effect on the reading. Adding 33 pf across the inductor *lowered* the > reading by 0.1 uH. We're dealing with a very small toroid in this > test whose > inter-turn capacitance isn't going to be as much as 2 or 3 pf > when squeezed > together. > > So I'm confident that the added capacitance by squeezing the > turns together > is not what is causing my L-meter to show increased inductance. > If anything, > the capacitance would tend to cause the L-meter to show lower inductance. > > Ron AC7AC > > No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.344 / Virus Database: 267.10.16/83 - Release Date: 8/26/2005 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
RE: OT: Effect of Compression and ExpansionontheInductanceof Toroids?
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Yes, I think you may be absolutely right, Don. I measured a maximum effect
of 9% in the one example I tested - a random core lying on my bench that had about a dozen turns on it. That agrees with your experience of finding up to a 10% change available. I saw your suggestion before, and it certainly sounds plausible. I wonder if something else might be at work here too. Or perhaps I'm looking the same effect you are in a different way. Here's my thought. The increased inductance/turn of the toroid compared to ordinary air-wound coils is caused by the very high permeability of the core compared to air. That makes the inductance fairly independent of the spacing between turns since spacing the turns hardly reduces the magnetic flux in the core that is available to each turn. But I wonder if crowding the turns together doesn't slightly increase the inductance by providing a lower permeability path for the magnetic flux, since the distance the flux must travel along the toroid before it passes through all the turns of the coil is shorter. In other words, the same effect one sees by using closer spacing in an air wound coil, only much, much less due to the efficient magnetic path provided by the core. Does the formula you are using account for a reduction of flux density around the distance of the torus due to the losses in the core, or does it assume a constant flux at all points? It's always interesting when experiment fails to support predictions. True, it most often turns out to be an invalid experiment that causes that result, but I can't see the problem here, especially considering that the effect of added distributed capacitance has an inverse effect on the reading on this type of 'meter'. Ron AC7AC -----Original Message----- From: W3FPR - Don Wilhelm [mailto:[hidden email]] Sent: Monday, August 29, 2005 9:22 PM To: Ron D'Eau Claire; [hidden email] Subject: RE: [Elecraft] OT: Effect of Compression and ExpansionontheInductanceof Toroids? Ron, I have the same inductance meter, and it does a good job because it does measure using the LR time constant rather than frequency, but 'physics is physics', and with a true toroidal inductor, there is still no dependency on the turns spacing (the math says so). Now, for the practical side of things, I do believe that your results (and others with similar findings) deviate from the 'classic physics' treatment of the ideal (ignoring practical behavior) - and when the toroid turns are irregular (not evenly spaced around the circumference of the core), there is some part of that coil behaving as a solenoidal inductor where turns spacing IS a factor. Remember that a toroid is simply a solenoidal coil formed into a circle with the ends of the coil meeting. This ideal toroid has equal turns spacing all the way around. So my current conclusion is -- IF the toroid is equally spaced around the core, the inductance is not dependent on the spacing of the turns, BUT, if the turns are compressed anywhere around the core, additional factors rear their ugly heads because the inductor is a combination of a solenoidal coil and a toroid coil, and the math becomes quite complex - just how much of each effect depends on just how much deviation from an ideal toroid shape exists in the configuration at hand. In a frequency dependent tuned circuit, how much of the frequency shift is due to the inter-turn capacitance or a change in inductance is (to me) only a matter of curiosity - the plain fact is that some change in the resonant frequency (or inductance) can be achieved by changing the turns spacing, but we all know that the adjustment range is small - the really big determining factor is simply the number of turns. As an example, I often improve the 10/12 meter Low Pass Filter characteristics of a KPA100 by squeezing the toroid turns "just the right way", but I determine what is the 'right way' by monitoring the impedance with my MFJ259B as I do it. Sometimes it is 'this way', while other times it is 'that way' - the batch to batch change in the permeability of the cores likely accounts for more variation than the turns spacing itself. BTW, this change does not really help the KPA100 output, but it improves the base K2 10 meter efficiency at 10 watts or lower when the KPA100 is installed. The overall inductance change that I have experienced is about 10%, so that is within the normal design tolerances using 10% resistors and capacitors - so except for satisfaction of the curiosity factor, I would say just to wind the toroids with the proper number of turns, and 'tweak' them in-circuit as required and as close as is permitted by your measurement capability. 73, Don W3FPR _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
RE: OT: Effect of Compression and ExpansionontheInductanceof Toroids?
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You are quite correct Ron, the variability will be small.
Flux is dependent only on Ampere-Turns, so the total flux will not change unless the number of turns is changed. There will be some flux leakage, and I figure that is the parameter that changes more than anything else because that would modify the effective perneability of the core. The inductance of an ideal toroidal core depends only on the radius of the toroidal core, the number of turns and the radius of each turn - there are no other terms in the equations (other than constants - core permeability is one of those 'constants'), so any variability must depend on just how far from the ideal a particular toroidal coil is - and the only thing that can change once the toroid is wound (with tight turns) is the effective permeability (due to flux leakage). One very nice thing about toroids is that they do have very little flux leakage (that is why toroids are 'self-shielding'), but we do know that some flux leakage does exist since one can de-tune a resonant circuit by placing a finger next to the toroid - if it were an ideal toroid with no flux leakage, adjacent objects would have no effect. Even an air-wound toroid will display this self-shielding effect if it is wound with perfectly even spacing between the turns, but if the spacing is altered, more flux leakage will exist and the inductance will change because the effective permeability will be modified. 73, Don W3FPR > -----Original Message----- > > Yes, I think you may be absolutely right, Don. I measured a maximum effect > of 9% in the one example I tested - a random core lying on my > bench that had > about a dozen turns on it. That agrees with your experience of > finding up to > a 10% change available. > > I saw your suggestion before, and it certainly sounds plausible. > I wonder if > something else might be at work here too. Or perhaps I'm looking the same > effect you are in a different way. Here's my thought. The increased > inductance/turn of the toroid compared to ordinary air-wound > coils is caused > by the very high permeability of the core compared to air. That makes the > inductance fairly independent of the spacing between turns since > spacing the > turns hardly reduces the magnetic flux in the core that is > available to each > turn. But I wonder if crowding the turns together doesn't > slightly increase > the inductance by providing a lower permeability path for the > magnetic flux, > since the distance the flux must travel along the toroid before it passes > through all the turns of the coil is shorter. In other words, the same > effect one sees by using closer spacing in an air wound coil, only much, > much less due to the efficient magnetic path provided by the core. > > Does the formula you are using account for a reduction of flux density > around the distance of the torus due to the losses in the core, or does it > assume a constant flux at all points? > > It's always interesting when experiment fails to support > predictions. True, > it most often turns out to be an invalid experiment that causes > that result, > but I can't see the problem here, especially considering that the > effect of > added distributed capacitance has an inverse effect on the reading on this > type of 'meter'. > > Ron AC7AC > > > -----Original Message----- > From: W3FPR - Don Wilhelm [mailto:[hidden email]] > Sent: Monday, August 29, 2005 9:22 PM > To: Ron D'Eau Claire; [hidden email] > Subject: RE: [Elecraft] OT: Effect of Compression and > ExpansionontheInductanceof Toroids? > > > Ron, > > I have the same inductance meter, and it does a good job because it does > measure using the LR time constant rather than frequency, but 'physics is > physics', and with a true toroidal inductor, there is still no > dependency on > the turns spacing (the math says so). > > Now, for the practical side of things, I do believe that your results (and > others with similar findings) deviate from the 'classic physics' treatment > of the ideal (ignoring practical behavior) - and when the toroid turns are > irregular (not evenly spaced around the circumference of the > core), there is > some part of that coil behaving as a solenoidal inductor where > turns spacing > IS a factor. > > Remember that a toroid is simply a solenoidal coil formed into a > circle with > the ends of the coil meeting. This ideal toroid has equal turns > spacing all > the way around. > > So my current conclusion is -- IF the toroid is equally spaced around the > core, the inductance is not dependent on the spacing of the turns, BUT, if > the turns are compressed anywhere around the core, additional factors rear > their ugly heads because the inductor is a combination of a > solenoidal coil > and a toroid coil, and the math becomes quite complex - just how much of > each effect depends on just how much deviation from an ideal toroid shape > exists in the configuration at hand. > > In a frequency dependent tuned circuit, how much of the frequency shift is > due to the inter-turn capacitance or a change in inductance is > (to me) only > a matter of curiosity - the plain fact is that some change in the resonant > frequency (or inductance) can be achieved by changing the turns > spacing, but > we all know that the adjustment range is small - the really big > determining > factor is simply the number of turns. > > As an example, I often improve the 10/12 meter Low Pass Filter > characteristics of a KPA100 by squeezing the toroid turns "just the right > way", but I determine what is the 'right way' by monitoring the impedance > with my MFJ259B as I do it. Sometimes it is 'this way', while other times > it is 'that way' - the batch to batch change in the permeability of the > cores likely accounts for more variation than the turns spacing > itself. BTW, > this change does not really help the KPA100 output, but it > improves the base > K2 10 meter efficiency at 10 watts or lower when the KPA100 is installed. > > The overall inductance change that I have experienced is about > 10%, so that > is within the normal design tolerances using 10% resistors and > capacitors - > so except for satisfaction of the curiosity factor, I would say > just to wind > the toroids with the proper number of turns, and 'tweak' them > in-circuit as > required and as close as is permitted by your measurement capability. > > 73, > Don W3FPR > > -- > No virus found in this incoming message. > Checked by AVG Anti-Virus. > Version: 7.0.344 / Virus Database: 267.10.16/83 - Release Date: 8/26/2005 > No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.344 / Virus Database: 267.10.17/84 - Release Date: 8/29/2005 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Leigh L. Klotz Jr WA5ZNU
Well now, that certainly clears that subject up, no?
Fred K6DGW Auburn CA CM98lw Leigh L Klotz, Jr. wrote: > The toroid formulas are approximations, but good ones. Here is a > physics-based explanation that gives some of the complexities, but there > are literally books on inductors. > > http://tinyurl.com/9brxb > > Leigh / WA5ZNU _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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