You are welcome, David! You asked about a range of values for Q. As you say, Q is frequency-dependent and each type of inductor will have an optimal frequency range where its Q is highest (assuming that high-Q is our goal, here). Q can be expressed several ways. One is the ratio of the inductive reactance to the effective series resistance. If you make an LC tuned circuit with a perfect capacitor and a real inductor (not far from the truth because capacitors typically have much higher Q than inductors), then the width of the resonant peak will be the center frequency divided by the Q. Also, if you induce ringing in the tuned circuit (by applying a step or an impulse input), then Q will be the number of cycles of ringing which pass between points where the amplitude goes down by a factor of "e" (the base of natural log). If the Q is 100, then, and the initial amplitude is 1, then after 100 cycles, the amplitude will be 1/e, after 200 cycles it will be (1/e)^2, etc. If the frequency is too low, then the DC resistance of the wire dominates because the reactance is low and the Q is low. If the frequency is too high, skin effect greatly increases the wire resistance and core losses (if there is a core) will be much higher and the Q once again is low. Also, for many air-core inductors, the limit on Q is actually self-resonance. Distributed capacitance of the winding causes the inductor to self-resonate at a frequency below what would otherwise be its optimal Q frequency. Anyway, all that aside, an excellent inductor Q is about 300. 100 is good. 30 to 50 is OK, and below that is poor. If you really, really try hard and have no additional constraints (like size or wire geometry), then I think it is barely possible to achieve a Q of 1000 with an inductor, but that would be the upper limit for wound inductors. You can also create inductors using conductive cavities at microwave frequencies and these could have an even higher Q than this, but of course, the inductance value is tiny (in the nanoHenries). Regarding saturation: core material boosts inductance by "amplifying" the magnetic field of the coil. If you have a coil which would make a magnetic field of 1 Gauss by itself with 1 Amp flowing in it, and then put in a core material with a (relative) permeability of 100, the magnetic field will be 100 Gauss. However, if you change the current in the coil up and down, you will see that as you go to higher currents, eventually there will be "diminishing returns" and finally the magnetic field will almost level off - increasing only by the same rate as it would if there were no core material. This particular magnetic field strength depends only on the type of material. However, the more turns of wire you apply, the less current that will be needed to saturate the core. So, you are correct that, for a given core size, lower inductance results in a higher current needed to saturate. The core size comes into play because the relationship between the inductance and the magnetic field strength for a given current depends on the core geometry. I can't explain much further here because it gets really confusing and I would need to go look up some things myself. The reason it becomes so confusing is because most of the theory of inductors and transformers was developed around 1880 to 1900 when there was not one standard way of measuring or expressing magnetic field strength. So, different authors explain saturation a little differently from other authors. Also, to make magnetic design easier, a "magnetic circuit" analogy was developed which treated magnetic flux (the integral of magnetic field over some area) as if it were electric current flowing in a closed loop. The resulting calculations can be easier than doing it the "physics" way, but they obscure some of what is going on in my opinion. I do agree that the ARRL stuff is a good place to start. Another good place is the old TI/Unitrode seminars on switching power supply design (although, obviously, less focused on RF). These can be downloaded free from TI but it takes a while to poke around and find all the sections that are available. Here is a link to start: http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=3DSEM40101= 4 I will caution, though, that the author of most of these (Lloyd Dixon) uses terminology loosely and also uses the magnetic circuit analogy without explanation. One thing that drives me crazy about his writing is that he seems to think that the law of conservation of energy means that systems try to use the least energy that they can (as opposed to simply meaning that energy in must equal energy out). He tries to explain the distribution of current in the windings of transformers by saying that the law of conservation of energy causes the current flow to distribute itself in such a way that it minimizes the "energy transfer". First of all, that's not the law of conservation of energy. Secondly, what does he mean by "energy transfer"? It seems that he means energy transfer into and out of the magnetic field during each switching cycle. If so, then he should say that the "principle of least action" causes the current flow to arrange itself to minimize the transfer between electrical and magnetic energy during each cycle. This error alone cost me a good day of scratching my head until I figured out what he was trying to say! OK, enough ranting. Sean On Tue, Jan 1, 2013 at 3:13 PM, David wrote: > On 28/12/2012 07:13, Sean Breheny wrote: >> Hi David, >> >> Here are a few of the most common applications of inductors (you >> probably know this but I want to refer back to this so I will list >> them): > > Sean, > > Thanks for your excellent reply. It has taken me a few days to digest > and there are questions in line below, but this helps immensely. I can > now go and read datasheets for the different types of inductor with a > slightly better understanding. > > I will have to spend some time reading the ARRL Handbook for inductors > again to understand better how Q relates to the frequency (I find it a > very well explained book, even though I'm not into ham radio). > >> For RF tuned circuits/impedance matching/filters, you want high-Q >> inductors (i.e., low loss) > > What region of values would be considered "high-Q" for these RF circuits? > >> RF circuits like these will not usually employ molded inductors >> because they are not the lowest loss type of construction. Toroidal >> inductors are common. For VHF and up, SMD inductors are common - often >> with visible turns of wire as you mentioned. > > This explains why a number of the amateur radio circuits (for HF bands) > I have been studying use custom wound toroidal inductors. Previously I > had assumed this was only for power levels involved. > > I could wind my own with toroids, but for prototyping I'd much rather > use something with a known inductance value, hence the original question > about what type to use. > >> Switching power supply energy storage inductors generally must be >> large enough to avoid saturation and to minimize core losses. > > I assume we avoid saturation as the inductance of the device decreases. > As I understand when saturation occurs the inductor acts less like an > ideal inductor and more like a short (or small DC resistance). > > Thanks once again for taking the time with your reply, it was incredibly > useful. > > David > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .