What characteristic on the material datasheet defines Flux in cores vs current  - Page 1
Forum section: electronics / rf-microwave

author: Qmavam
author_rank: Regular Contributor
timestamp: August 30, 2019, 04:04:22 pm
content: I want to make the maximum Flux in a core with the minimum DC current in my coil.What characteristic do I look at on the material data sheet.It may be just iron, but what is the name of the characteristic.A drawing may help.Thanks, Mikek

author: Qmavam
author_rank: Regular Contributor
timestamp: August 30, 2019, 04:24:45 pm
content: OK, found my own answer, Reluctance.Since reluctance is not stated on datasheets, I found,Thus, it is related to mu and area.So I want maximum area and maximum permeability in the material.Feedback appreciated.Thanks, Mikek

author: duak
author_rank: Super Contributor
timestamp: August 30, 2019, 05:39:09 pm
content: Mikek,Magnetic Reluctance (Rel) is analogous to resistance in electrical circuits. Because of the way things are you can't get almost pure parts like resistors and capacitors that just snap together to get the job done.I don't recall any manufacturer putting Rel on a core's data sheet. I remember having to calculate the Rel for the various cores I thought might work for a design - good place for a spreadsheet. The first time I designed a ferrite core inductor I tried to use a toroid but because it had a varying DC current running through it, it saturated at 1/10 of the desired current. I ended up with a gapped E-E core to handle the DC. It just took a little time to re-learn the stuff from college. wrap my mind around the problem and then grind out an answer.In your case, I'd consider an ungapped core to make it easy to vary the mu with DC. I expect a smaller core to be easier to saturate than a larger one simply because there are less iron atoms.

author: Yansi
author_rank: Super Contributor
timestamp: August 30, 2019, 05:45:13 pm
content: Flux will be proportional to ampers times turns, magnetic conductivity (the shorter the average length of the magnetic flux line) and the effective area of the magnetic circuit.

author: Qmavam
author_rank: Regular Contributor
timestamp: August 30, 2019, 06:10:04 pm
content: Quote from: duak on August 30, 2019, 05:39:09 pmYes, that's what I found, (relearned)QuoteAs you see, I want to saturate the core and so far have only tested one core that saturates easy enough with my limitations. And it didn't the proper frequency characteristics.QuoteI thought that was what Ididn'twant, as I understand it, a gap make it harder to saturate, (hard to saturate air)QuoteYes, there has been a thought to grind part if the toroid off to reduce the cross section just for that reason.

author: duak
author_rank: Super Contributor
timestamp: August 30, 2019, 10:36:40 pm
content: Mikek, that's correct, I think you want an ungapped core. Find a material that has low enough losses for your frequency band, pick a core size and calculate how many turns (N) you need to get the maximum inductance from AsubL. Calculate the Reluctance of the core based on its dimensions and mu. Then based on the saturation flux (Bmax) determine the current needed for saturation: I = Bmax * Reluctance / N (If my memory and math is correct.) Note that I'm assuming that the same winding is used for both the RF signal and the DC current. If you're using a second winding for the DC on one core, then use the appropriate N. If you're trying to saturate one core by using a second core and winding, I don't know enough about the pieces to say how to determine the results.I'm not sure that grinding away part of the core helps. It reduces Bmax but it also reduces the inductance, perhaps even more so. If I were a math whiz, I'd be able to figure it out quickly. But I'm not, so I won't.It may be that you won't be able to find a core material with a high enough mu and low enough losses to be easily saturated without burning up the winding from the DC current needed. Maybe you'll need tapped inductors with some sort of switching mechanism with stepper motors. If this were a long wire antenna, maybe inductors made from springs with slug cores attached to one end of each spring. Pulling on the antenna would stretch the springs that would reduce their inductances - only one moving part!Long weekend coming up - have a good one, eh?

author: radiolistener
author_rank: Super Contributor
timestamp: August 30, 2019, 11:42:07 pm
content: Qmavam, may be this is what you're looking for...http://g3ynh.info/comps/Vari_L.html

author: Qmavam
author_rank: Regular Contributor
timestamp: August 31, 2019, 01:47:31 am
content: Quote from: duak on August 30, 2019, 10:36:40 pmI made an edit here after checking a toroid which I had cut a couple of slots part way through, they dropped the inductance by 75%. Ouch!QuoteYes, that is why I moved away from the turns on the same core, and isolating the ac winding from the dc winding, (otherwise it's a transformer)Quote

author: Qmavam
author_rank: Regular Contributor
timestamp: August 31, 2019, 01:50:44 am
content: Ya, those do not fit into my working idea.You might read a little of my thread with subject,Help understanding ferrites, I want saturation .Thanks, Mikek

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2019, 03:08:32 am
content: Yeah, tons of turns, and as little airgap as possible in the path around. So, for the U and toroid shown, make sure the facing surfaces are smooth, planar and parallel, and use high permeability materials if possible.You probably want the cross section of the toroid thinner than of the U, so the toroid section saturates first, reducing its winding inductance (and reducing its losses by effectively turning some of the core to airgap, as I noted in the other thread).Tim

author: Qmavam
author_rank: Regular Contributor
timestamp: August 31, 2019, 05:32:52 pm
content: My latest thought is to use iron as the electromagnet. I'm know it has a high permeability, that is highly dependent on purity.But it looks like poor iron has a permeability higher than almost any ferrite available. and with iron I can make almost any shape I want.I won't splurge for high purity iron and will use what I can find in the shape I want.I'm hoping with this particular geometry I"ll get very little transformer action.Mikek

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2019, 06:49:37 pm
content: The across-the-toroid configuration is better; as drawn, you can only saturate the part under the iron core which isn't a huge amount of effective air gap. Also the resulting fringing field couples into the iron, lowering Q considerably.Tim

author: duak
author_rank: Super Contributor
timestamp: August 31, 2019, 07:32:47 pm
content: Apparently these gizmos are called cross field transductors. There was a link in one of your other chains to some info on them (CRVCO??). Here's link to some more info:https://archive.org/details/TheForgottenUseOfSaturable-CoreInductorsTransductorsI made a quick test with a 3 mH common mode choke with various DC bias electromagnets. A coil from a contactor big enough to fit the choke into allowed me to reduce the choke's L to about 0.6 mH. Adding a bit of steel around the coil to increase the flux reduced it further. A supermagnet about 20 * 10 * 6 mm touching the choke's core reduced its L to about 0.5 mH.Esoteric materials aren't needed. Using steel for the DC core should be just fine. A soft or mild steel like 1008 or 1010 is easy to find, work with and has quite high permeability and Bmax. There should be little or no coupling into the DC core because it is at right angles to the RF core. To make the DC magnetic circuit as efficient as possible, reduce its reluctance by maximizing the contact areas between the two cores and minimizing the air gap. For easy toroidal DC cores how about cutting notches in some stacked steel washers just big enough for the ferrite core?Tim's comment about how much of the RF core is saturated is true. To meet your L variation target, you'll probably need to experiment with various DC bias electromagnet sizes.

author: Qmavam
author_rank: Regular Contributor
timestamp: September 01, 2019, 01:05:03 am
content: Quote from: duak on August 31, 2019, 07:32:47 pmThanks for posting the article, I was getting a little discouraged. I tried a few things today, I used some transformer laminations that fit my toroid perfect but altered the inductance only a very little, then realized the orientation wasn't proper. I tried the solenoid from a contactor. I worked and altered the inductance about 50%.I tried a square nut to see if I could measure the AsubL, it didn't work well, very small inductance. I tried an iron rod, still not much.Per the article, I have both 4C6 and 3D3 pot cores in the 2616 size. I had the 4C6 wound with the crossfield method. but was disappointed with the change. I don't have enough turns, I'll try again.If my project works, I would need 9 of the inductors in series, so I want to keep the voltage at saturation low. Even at 5Vdc, I would need a 45 Volt power source. So I was hoping for even less voltage at each inductor..QuoteI'll get some toroids on order tomorrow. I have the one 26/16 4C6 potcore that will work over my frequency range and Q requirement. So the others are just to test how to get max inductance change. I'll see if I can get the 18/11 4c6 potcore mentioned a 1/2" toroid and a 7/8" toroid.If you have any other thoughts about cores, let me know I'll order from Digikey tomorrow.Thanks, Mikek

author: Qmavam
author_rank: Regular Contributor
timestamp: September 01, 2019, 03:05:50 pm
content: Quote from: T3sl4co1l on August 31, 2019, 06:49:37 pmOnly saturating the part of the gap and not being an effective air gap make sense to me, although when I do that same experiment with magnets, I'm pretty sure I eliminate most of the core. I'm using a 125uH inductor and can reduce it to 2.2uH with magnets. The caveat is I'm using 1/4" x 1/16" neodymium magnets stacked 7 deep.OH, thinking it is because the magnets are so strong the flux finds it's way around the whole core, I used just one magnet on each side. Ya, only goes from 125uH to 85uH.There is a sweet spot that goes to 60uH if I offset (around the circumference) the magnets about 1 diameter.QuoteNot sure I get the "resulting fringing field" ie, because the DC core is only in on spot? does that mean if I use the geometryof the OP shown here the fringing would be less?Could use an iron horseshoe shape.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2019, 05:52:48 pm
content: Quote from: Qmavam on September 01, 2019, 03:05:50 pmNot really meaningful. If mu_r = 2000, 125uH is tiny, and it's only modestly saturated (mu_r < 20 say) at less than 1.25uH. If mu_r = 15k (typical of solid core ferrites for CMCs), a reduction of 50x is still a quite modest mu_r = 300!The figure of merit is, getting mu_r closest to 1, for the least bias current.Again, mu_eff seems to be best in the 20-60 range for power inductors, maybe on the lower side for RF inductors. I'm guessing this applies as well for saturated ferrite as it does for air gapped or powder cores.QuoteThe ferrite under the magnet will be saturated, turning effectively into air; the AC field from the inductor windings then fringes out into the space around that air gap. But that space is filled with solid metal, so it all goes into eddy currents. Bad Q.The OP arrangement has the most possible fringing space, which means the lowest saturated mu_r and the least eddy currents (and the most stray fields when saturated, but that's true of any solenoidal sort of winding).Tim

author: Qmavam
author_rank: Regular Contributor
timestamp: September 01, 2019, 07:59:03 pm
content: Quote from: T3sl4co1l on September 01, 2019, 05:52:48 pmOk, moving on, I have searched and can't find a U core, they are unavailable or $57.So, snap on suppression cores came up. They are basically a U. I have one on hand no idea what material.I tried a quick rest with out any change in my inductance. Ya only 20 turns, 3 amps and weird overhang, But I thought, something?Any thoughts about suppression cores? They're available in many shapes and sizes. Lots of room for wire. Available in 75 material with 5000 permeability.btw, I'm aware when I get serious, I need to remove the epoxy coating to get good contact.Don't laugh-- just looking for solutions :-)Is it wishful think there would be enough flux inside the snap on core?See drawing.

author: duak
author_rank: Super Contributor
timestamp: September 02, 2019, 07:08:04 pm
content: Could we have a picture of the setup turned to the right by 90 degrees?Are the snap on cores you have high permeability? The ones I worked with all had relatively low mu. If you take the RF toroids out of the gaps and apply current, will the snap on cores stick together strongly? If not, you've got a permeability or winding problem. Measure the L of the DC coils with the cores separated, together and finally with the RF cores interposed. The L is a measure of how efficient the magnetic circuit is.I think you're going to want a DC core material that has a higher mu and Bmax than the RF core so I suggest going back to soft iron, at least to get a handle on the geometry. Transformer iron is difficult to work with because it's silicon steel and harder than the back of Superman's head. It's true that the soft iron will introduce some losses but I think that they won't have a big effect of the RF characteristics. My LCR meter goes up only to 1 KHz, so I can't easily test this. I do see that external iron has an effect on the L and DF but don't think it'll be a big deal in the end.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: September 02, 2019, 07:19:15 pm
content: Quote from: Qmavam on August 30, 2019, 06:10:04 pmA gap also changes the reluctance. two "resistances" in series.

author: Qmavam
author_rank: Regular Contributor
timestamp: September 02, 2019, 09:43:02 pm
content: Ok, I have something that works, with a caveat.I used 3 washers and cut a slot and a flat on them, I wound 630 turns on it. (Hoping I over did it.)My coil is 105uH and when I pass 360ma through the dc coil it goes down to .7uh.That was first pass no attempt to optimize.I do have a problem with residual magnetism in the 3 washers. After power is removed the washersact like a weak magnet. They have enough magnetism to hold the 105uH coil at about 80uH.I don't know yet if a reversing switch and a current pulse would demagnetize it, but I'll give it a try.I still need a proper material, one that doesn't keep any residual magnetism.

author: Qmavam
author_rank: Regular Contributor
timestamp: September 02, 2019, 09:57:50 pm
content: Quote from: duak on September 02, 2019, 07:08:04 pmI already disassembled it and use the core elsewhere, But I put nylon nuts in place of the toroids and took a picture.QuoteI don't have any data on the snap on cores."will the snap on cores stick together strongly?"Are you asking if they are magnetic? No. But the edges do have edges that fit together flat.I can buy a snap on in material 61 which is a high mu, but know I need to knowif it holds residual magnetism when I remove current.I think you're going to want a DC core material that has a higher mu and Bmax than the RF core so I suggest going back to soft iron, at least to get a handle on the geometry. Transformer iron is difficult to work with because it's silicon steel and harder than the back of Superman's head. It's true that the soft iron will introduce some losses but I think that they won't have a big effect of the RF characteristics. My LCR meter goes up only to 1 KHz, so I can't easily test this. I do see that external iron has an effect on the L and DF but don't think it'll be a big deal in the end.You will see I added another post about using washers and had good response, except for the residual magnetism.Thanks for the reminder to check Q.Mikek

author: Qmavam
author_rank: Regular Contributor
timestamp: September 03, 2019, 04:47:32 am
content: I tried cutting a toroid in half and gluing it to another toroid at a right angle, I wound 200 turns on the dc coil.It was a 122uH coil, I had to apply 1.6 amps to the dc coil to get it down to 60uH. that's not exceptable, and it got hot!I could put a half core on the opposite side, get another 200 turns maybe....Mikek

author: Qmavam
author_rank: Regular Contributor
timestamp: September 04, 2019, 02:54:23 pm
content: I have a working dc controlled variable inductor.I used the E core out of my junque box pictured above.I don't know whatmaterial it is, But, it does not have residual magnetism like the washer cores I used previously.It's not final because the inductor core I have is nota high frequency core, it's only good to1MHz, I need 10MHz.The inductor is 123uh when I apply current to the dc coil.The dc core has 700+ turns on each leg + 1400+ of #28 wire. I'm a glutton for punishment!Test is at 100kHz.67ma=101uH Q=28133ma=69uH Q=49200ma=20uh Q=69Q= gets wild in here, jumps around I saw high of Q=1000 and low 368.267ma=5uH Q=139333uH=3.2uH Q=103Q seems strange to me, I'm using a DE5000 LCR meter.I've been told as I saturate Q will go down it seems to go up.Not important until I get the proper toroid core. At this point I see my options asFairrite #61 or Feroxcube #4C65, I was warned about #61 and magnetizing,I suspect the same with 4C65, but I won't know until I try.Anyone know of another material that will have good Q up to 4MHz?Also how can I find the AsubL of the E core since it has a 1/4" air gap?Here's a picture of the E core with toroid in center leg. I ground off mostof what little material was in the center lag gap. See previous picture.Mikek