RE: [Elecraft] OT: Effect of Compression and ExpansionontheInductanceof Toroids?
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:[EMAIL PROTECTED] Sent: Monday, August 29, 2005 9:22 PM To: Ron D'Eau Claire; elecraft@mailman.qth.net 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
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 -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
RE: [Elecraft] OT: Effect of Compression and ExpansionontheInductanceof Toroids?
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:[EMAIL PROTECTED] Sent: Monday, August 29, 2005 9:22 PM To: Ron D'Eau Claire; elecraft@mailman.qth.net 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: Elecraft@mailman.qth.net 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
