Ferrites: L changes with current change.
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From: Bill Sloman on 18 Dec 2009 20:28 On Dec 19, 1:46 am, "amdx" <a...(a)knology.net> wrote: > "Bill Sloman" <bill.slo...(a)ieee.org> wrote in message > > news:a9f61b7d-4521-4bc1-8657-7ace03993f14(a)j19g2000yqk.googlegroups.com... > On Dec 18, 10:24 pm, "amdx" <a...(a)knology.net> wrote: > > > "Eeyore" <rabbitsfriendsandrelati...(a)notcoldmail.com> wrote in message > > >news:4B2BEDEE.6F9EE29A(a)notcoldmail.com... > > > > amdx wrote: > > > >> I have ask before about changes of inductance vs. AC excitation in > > >> ferrites > > >> before and didn't get a satisfactory answer. > > >> We can use a particular potcore at 5ua or 10amps. but how much does the > > >> inductance change over this range? > > > > Depends on the material. It'll change with temperature too. > > > >> What current does the manufacturer use to make his curves. > > > > They sweep the current. > > > >> I'm thinking about a transformer at the input of a radio receiver with > > >> a > > >> signal measured in microvolts. > > >> Say we built a 50 ohm to 200 ohm transformer using the manufacturers > > >> specs, > > >> and made the primary > > >> inductance 4 times 50 ohms at our design freq.. It would work fine at > > >> 100 > > >> ma > > >> to many amps. > > >> But at 5ua is there enough inductance in the primary to make it > > >> function > > >> as > > >> a proper impedance transformer? > > > > Losses may be too high. Try winding one for the job. > > > > And get a copy of Epcos's Ferrite Magnetic Designer tool. > > >http://www.epcos.com/web/generator/Web/Sections/DesignTools/Ferrites/.... > > > > Most manufacturers make 'copies' of the popular ferrites. There are > > > often > > > direct > > > cross-references beteen say Epcos, Ferroxcube/Philips/Yageo and TDK for > > > example. > > > >http://www.pe-coils.com/MnZn_Feirrite/MATERIAL%20COMPARES.pdf > > >http://www.dianyuan.com/blog/u/2008-12/6123_1230623550.pdf > > >http://www.google.co.uk/search?hl=en&q=ferrite+cross-reference&meta=&... > > > > The cheap Chinese manufacturers tend not to have the latest low-loss, > > > high > > > temp parts though but that's a PSU issue. > > > > Epcos's data also advises different ferrites for different applications. > > > > Graham > > > I should add this a general question, I don't have a specific job in mind. > > Just wondering what happens to AsubL at 0.1 microamps. > > Get hold of a data book on ferrite cores. Most of them include flux > density versus field strength curves for the various ferrites used in > their cores. > > EPCOS at least makes the data availlable on the web - here's the link > for their N27 material > > http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/Ferrit... > > -- > Bill Sloman, Nijmegen > > Ok Bill, I'm over my head on this because I don't understand all the terms. > If we look at page 3 of the link you gave, first graph, you will see the B > vs u(o) curve. > At 25mT it is already down 34% from the peak and I would bet a dollar that > the curve > drops faster the lower you go with the B field. Look at page 4, which shows the field versus the magnetising current - it looks more or less like a straight line at zero field, which suggests that permeability doesn't drop all that fast below 25mT. Presumably what is happening is that is that the "sticky" component of the alignment of the magnetic dipoles responsible for the hysterisis is falling out of the permeability at low fields - as you can see from the curves on page 4 it goes away at high fields. > My guess (only a guess) is that if I wound my theoretical transformer > (described above) > and had my 0.1 microamp driving it, it would be no where near 25mT. Somehow > this > B vs u(o) curve will relate to permeability which is then related to (A sub > L). I would then > use (A sub L) to calculate the turns on the theoretical 50 ohm primary (200 > ohms inductance). Every specific core pair has magnetic path length, specified in the data sheet, and the exciting field (in Amperes per metre) is just the current looped around that core divided by the magneitc path length in metres. In your case this is just your 10uA times the number of turns divided by the magnetic path length of your core. > It just seems that at very low currents the permeability will drop so low I > will not have my > 200 ohms inductance and the transformer will not work as designed. Wrong. > Since I have never seen this discussed and people build working radios > everyday, > I'm probably all wrong, but I'm not sure where. Your error lies in assuming that the permeability drops to zero at zero magnetising current. It doesn't, and in fact in this case probably continues to fall off roughly linearly to about 60% of the peak permeability. If you really want a more stable inductance, gap your core until the inductance is around 10% of the ungapped figure - which usually takes a layer or two of 60 micron transformer tape between the core halves - and wind a new core with three times the number of turns (the square root of ten times more turns, to be precise but it is difficult to gap the core this precisely, which is why the manufacturers do it for you by grinding down the centre leg of a gapped core set). Since 90% of the magnetic path is now in the gap, current dependence goes down by a factor of ten. Bigger gaps provide even more stable inductances, if you can afford the extra series resistance and inter- winding capacitance. -- Bill Sloman, Nijmegen
From: amdx on 19 Dec 2009 07:11 "Bill Sloman" <bill.sloman(a)ieee.org> wrote in message news:9deeb9f3-7bb4-4543-b82b-10a51a9a43f1(a)j19g2000yqk.googlegroups.com... On Dec 19, 1:46 am, "amdx" <a...(a)knology.net> wrote: > "Bill Sloman" <bill.slo...(a)ieee.org> wrote in message > > news:a9f61b7d-4521-4bc1-8657-7ace03993f14(a)j19g2000yqk.googlegroups.com... > On Dec 18, 10:24 pm, "amdx" <a...(a)knology.net> wrote: > > > "Eeyore" <rabbitsfriendsandrelati...(a)notcoldmail.com> wrote in message > > >news:4B2BEDEE.6F9EE29A(a)notcoldmail.com... > > > > amdx wrote: > > > >> I have ask before about changes of inductance vs. AC excitation in > > >> ferrites > > >> before and didn't get a satisfactory answer. > > >> We can use a particular potcore at 5ua or 10amps. but how much does > > >> the > > >> inductance change over this range? > > > > Depends on the material. It'll change with temperature too. > > > >> What current does the manufacturer use to make his curves. > > > > They sweep the current. > > > >> I'm thinking about a transformer at the input of a radio receiver > > >> with > > >> a > > >> signal measured in microvolts. > > >> Say we built a 50 ohm to 200 ohm transformer using the manufacturers > > >> specs, > > >> and made the primary > > >> inductance 4 times 50 ohms at our design freq.. It would work fine at > > >> 100 > > >> ma > > >> to many amps. > > >> But at 5ua is there enough inductance in the primary to make it > > >> function > > >> as > > >> a proper impedance transformer? > > > > Losses may be too high. Try winding one for the job. > > > > And get a copy of Epcos's Ferrite Magnetic Designer tool. > > >http://www.epcos.com/web/generator/Web/Sections/DesignTools/Ferrites/... > > > > Most manufacturers make 'copies' of the popular ferrites. There are > > > often > > > direct > > > cross-references beteen say Epcos, Ferroxcube/Philips/Yageo and TDK > > > for > > > example. > > > >http://www.pe-coils.com/MnZn_Feirrite/MATERIAL%20COMPARES.pdf > > >http://www.dianyuan.com/blog/u/2008-12/6123_1230623550.pdf > > >http://www.google.co.uk/search?hl=en&q=ferrite+cross-reference&meta=&... > > > > The cheap Chinese manufacturers tend not to have the latest low-loss, > > > high > > > temp parts though but that's a PSU issue. > > > > Epcos's data also advises different ferrites for different > > > applications. > > > > Graham > > > I should add this a general question, I don't have a specific job in > > mind. > > Just wondering what happens to AsubL at 0.1 microamps. > > Get hold of a data book on ferrite cores. Most of them include flux > density versus field strength curves for the various ferrites used in > their cores. > > EPCOS at least makes the data availlable on the web - here's the link > for their N27 material > > http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/Ferrit... > > -- > Bill Sloman, Nijmegen > > Ok Bill, I'm over my head on this because I don't understand all the > terms. > If we look at page 3 of the link you gave, first graph, you will see the B > vs u(o) curve. > At 25mT it is already down 34% from the peak and I would bet a dollar that > the curve > drops faster the lower you go with the B field. Look at page 4, which shows the field versus the magnetising current - it looks more or less like a straight line at zero field, which suggests that permeability doesn't drop all that fast below 25mT. Presumably what is happening is that is that the "sticky" component of the alignment of the magnetic dipoles responsible for the hysterisis is falling out of the permeability at low fields - as you can see from the curves on page 4 it goes away at high fields. > My guess (only a guess) is that if I wound my theoretical transformer > (described above) > and had my 0.1 microamp driving it, it would be no where near 25mT. > Somehow > this > B vs u(o) curve will relate to permeability which is then related to (A > sub > L). I would then > use (A sub L) to calculate the turns on the theoretical 50 ohm primary > (200 > ohms inductance). Every specific core pair has magnetic path length, specified in the data sheet, and the exciting field (in Amperes per metre) is just the current looped around that core divided by the magneitc path length in metres. In your case this is just your 10uA times the number of turns divided by the magnetic path length of your core. > It just seems that at very low currents the permeability will drop so low > I > will not have my > 200 ohms inductance and the transformer will not work as designed. Wrong. > Since I have never seen this discussed and people build working radios > everyday, > I'm probably all wrong, but I'm not sure where. Your error lies in assuming that the permeability drops to zero at zero magnetising current. It doesn't, and in fact in this case probably continues to fall off roughly linearly to about 60% of the peak permeability. If you really want a more stable inductance, gap your core until the inductance is around 10% of the ungapped figure - which usually takes a layer or two of 60 micron transformer tape between the core halves - and wind a new core with three times the number of turns (the square root of ten times more turns, to be precise but it is difficult to gap the core this precisely, which is why the manufacturers do it for you by grinding down the centre leg of a gapped core set). Since 90% of the magnetic path is now in the gap, current dependence goes down by a factor of ten. Bigger gaps provide even more stable inductances, if you can afford the extra series resistance and inter- winding capacitance. -- Bill Sloman, Nijmegen Thanks, Bill I did some quick calculations of B/H on graph number 4 to find u(o)u(r). It varies from 1.7, peaks at 4 and then drops to 0.4, this is a 10 to 1 range but the variance is not at the low end where I would have expected. Again I don't know quite how B/H relates to A sub L but I think it figures into it. If I get some time I'll get numbers together for a transformer I made for a Flag antenna. I'm curious about the B in my transformer compared to the values shown on the graph. Mike
From: Eeyore on 20 Dec 2009 21:56 amdx wrote: > "Eeyore" <rabbitsfriendsandrelations(a)notcoldmail.com> wrote > > amdx wrote: > > > >> I have ask before about changes of inductance vs. AC excitation in > >> ferrites before and didn't get a satisfactory answer. > >> We can use a particular potcore at 5ua or 10amps. but how much does the > >> inductance change over this range? > > > > Depends on the material. It'll change with temperature too. > > > >> What current does the manufacturer use to make his curves. > > > > They sweep the current. > > > > > >> I'm thinking about a transformer at the input of a radio receiver with a > >> signal measured in microvolts. > >> Say we built a 50 ohm to 200 ohm transformer using the manufacturers > >> specs, > >> and made the primary > >> inductance 4 times 50 ohms at our design freq.. It would work fine at 100 > >> ma to many amps. > >> But at 5ua is there enough inductance in the primary to make it function > >> as a proper impedance transformer? > > > > Losses may be too high. Try winding one for the job. > > > > And get a copy of Epcos's Ferrite Magnetic Designer tool. > > http://www.epcos.com/web/generator/Web/Sections/DesignTools/Ferrites/Page,templateId=render,locale=en.html > > > > Most manufacturers make 'copies' of the popular ferrites. There are often > > direct cross-references between say Epcos, Ferroxcube/Philips/Yageo and TDK for > > example. > > > > http://www.pe-coils.com/MnZn_Feirrite/MATERIAL%20COMPARES.pdf > > http://www.dianyuan.com/blog/u/2008-12/6123_1230623550.pdf > > http://www.google.co.uk/search?hl=en&q=ferrite+cross-reference&meta=&aq=f&oq= > > > > The cheap Chinese manufacturers tend not to have the latest low-loss, high > > temp parts though but that's a PSU issue. > > > > Epcos's data also advises different ferrites for different applications. > > > > Graham > > I should add this a general question, I don't have a specific job in mind. > Just wondering what happens to AsubL at 0.1 microamps. You must have too much spare time on your hands ! Graham due to the hugely increased level of spam please make the obvious adjustment to my email address
From: Eeyore on 20 Dec 2009 22:00 amdx wrote: > Ok Bill, I'm over my head on this because I don't understand all the terms. So study the subject ! I had to. > If we look at page 3 of the link you gave, first graph, you will see the B > vs u(o) curve. > At 25mT it is already down 34% from the peak and I would bet a dollar that > the curve drops faster the lower you go with the B field. > My guess (only a guess) is that if I wound my theoretical transformer > (described above) and had my 0.1 microamp driving it, it would be no where near 25mT. Guesses are a waste of time. Either you KNOW or DON'T KNOW ! > Somehow this B vs u(o) curve will relate to permeability which is then related to (A sub > L). I would then use (A sub L) to calculate the turns on the theoretical 50 ohm primary (200 > ohms inductance). > It just seems that at very low currents the permeability will drop so low I > will not have my 200 ohms inductance At what frequency ? And that's the vector sum of Xj ohms + Rdc ohms by the way. > and the transformer will not work as designed. > Since I have never seen this discussed and people build working radios > everyday, I'm probably all wrong, but I'm not sure where. Take the advice and get the Epcos program. It tells you all about saturation and change in inductance with amps. Graham -- due to the hugely increased level of spam please make the obvious adjustment to my email address
From: Eeyore on 20 Dec 2009 22:02
amdx wrote: > Again I don't know quite how B/H relates to A sub L Just call it Al. Everyone else does. Graham -- due to the hugely increased level of spam please make the obvious adjustment to my email address |