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Some quick questions regarding solenoids

  1. May 1, 2009 #1
    Hey guys,

    I'm still working away on some projects I've been building/studying. I'm not actually building any practical circuitry but just experimenting with some results...

    Anyway - I've discovered that domain realignment within iron cores in high frequency solenoids can be too slow to keep up with the high frequencies.

    I am wondering what kind of conventional core material you can suggest that can realign its domains in the 500khz ballpark

    Using straight iron cores gives me great results for magnetic flux being held within the core, but Iron domains are too slow to re-align at 500khz

    I've been reading around and I've read a bit about steel mixed with silicon to allow for higher frequency domain realignment... Am I on the right track? I wasnt able to determine if this stuff would have the same performance with magnetic flux though.

    Someone pointed me at making my own custom cores using fine grain iron filings and 2 component foam... but again, Not sure what kind of results that might yield.

    Yet another suggestion I've received was to use copper rods or copper tubes. And a final suggestion was to use copper tubing with solid aluminum cores (Insert aluminum dowel into copper refridgeration tubing). I am skeptical of these last suggestions as I know that copper and alulminum are not ferromagnetic but I was not able to discover much information about how copper or a copper/aluminum mix would behave as a core material in a solenoid.

    I suppose I can try all of these ideas, but in the interest of saving $$ I would greatly appreaciate any suggestions for a core material that can derive a strong & clean magnetic field while undergoing high frequency repolarization

    Thanks very much in advance for your consideration!

  2. jcsd
  3. May 1, 2009 #2


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    Above about 500 KHz, powdered Iron and ferrite cores are used but not generally for making electromagnets.
    Above about 4 MHz ferrites are about all that work.

    They are mostly used for making inductors of large inductance without having to wind hundreds of turns of wire and for making wideband transformers with efficient coupling between windings.

    Such transformers are useful for matching some antennas to their feedlines.

    Portable AM radios often use an antenna wound on a piece of ferrite rod. These would be readily available if you needed a source of ferrite. There are many grades of ferrite and makers like Amidon have lists on their web sites.
    They also come as toroids which are a donut shaped piece of ferrite.

    This is a quote from Wikipedia:
    Ferrite has a cubic crystalline structure with the chemical formula MO.Fe2O3 where Fe2O3 is iron oxide and MO refers to a combination of two or more divalent metal (i.e: zinc, nickel, manganese and copper) oxides. The addition of such metal oxides in various amounts allows the creation of many different materials whose properties can be tailored for a variety of uses.
  4. May 1, 2009 #3
    Thanks very much! Yes actually a toroid would be best so I will take a look into ferrite toroids - thanks again

    I'm also noticing that these things are VERY sensitive to outside influences. I find that these things will wonder off of the desired frequency quite easily... As it stands I'm still using a singnal generator that I have access to to drive the frequencies in these coils - but I would like to build an LC circuit for this role - Since I'm still learning about LC circuits, I havnt chosen one yet but since I would like to have a system that readjusts the output frequency to the solenoid in the face of frequency drift I wonder if there is a standard LC circuit already in existance for this purpose.

    I was looking at how a Theremin opperates using this principal - there is a high frequency oscillator in the system - the system is sensitive to outside interference - outside interference drives frequency drift in the oscillator - A second oscillator is set to oscillate freely - The free oscillator's output is matched to the system's oscillator - as the system's oscillator begins to drift, the free oscillator compares its output to the oscillator in the system and the difference between these two frequencies is sent to a sound output module. I was thinking of using that difference in frequency to modulate the system oscillator to keep its oscillations fixed for my purposes....

    That said, this quite a complicated circuit - is there a more elegant design for this purpose?
    Last edited: May 1, 2009
  5. May 1, 2009 #4
    There is not a conventional tank circuit for this application? I'll continue to look on my own but any tips you guys can provide would be more than appreciated :)

  6. May 1, 2009 #5


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    I'm not sure what you are asking. "Tank Circuit" has no meaning unless you say what it is connected to.

    You haven't tried ferrite yet, so you don't know if it will drift or not

    To make an oscillator with ferrite cored coils, you would need to be careful with temperature variations as the permeability of ferrite varies with temperature. But it isn't too bad and at 500 KHz shouldn't be much of a problem.
    AM transistor radios use such oscillators all the time and they don't seem to drift much.
    If fact, the components from defunct radios would be a suitable source of coils for your project. You may even like to look around for new replacement coils for transistor radios.

    It is possible to phase lock a free-running oscillator to a stable source such as a crystal oscillator, but you want to use it in a Theremin, which I think changes frequency according to proximity to your hand. So, you need an oscillator that is inherently unstable and one that is as stable as possible and then you mix the two together to get an audio resultant.

    There are various steps in getting any required level of stability and this can get expensive or it may be simple.
    Perhaps you need to read up on oscillators, though, to get some background.

    You could do a search for "VFO" on Google to get an idea of the circuitry for stable oscillators.

  7. May 1, 2009 #6
    Sorry, as I am still learning, my terminology isnt up to snuff - I guess what mean is LC circuit
    Youre right - I am likely getting ahead of myself, I just thought there might be a clean cut solution to a frequency drift issue which i do anticipate I will experience. There will be heat buildup and there will be EM interference from outside sources that I anticipate will throw the desired frequency off...
    Good call - though I'm hoping for at least 8" diameter toroids so, i doubt i'll find them in radios :\ I'll have to see if I can find them to order that large otherwise I'll just try the iron powder custom built core solution
    Well, i was just suggesting the theremin to give an idea of the type of control system i was thinking about.... Phaselocking with a crystal oscillator is an option
    Cant go wrong with reading up more.. I did discover VFO's - that sounds like a better choice though they look like quite a bit is involved with them - I wasnt sure if there was a more straightforward approach.
    Thanks, I'll read up some more and let you know how it goes - I'm very thankful for your input so far - I know I must seem pretty clueless but I really find this stuff fascinating and I'm finding myself really absorbed by the material, its really rewarding to learn about. There's just so much though! But I'm getting there :)

    Thanks again
  8. May 2, 2009 #7


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  9. May 2, 2009 #8
    Its a long story lol - Suffice it to say, it gives me a good amount of working area in the middle for circuitry. I'm working with triodes so they'll take up some extra room.

    I understand triodes have high fidelity with VHF frequencies so they should be able to handle the high frequences and keep the signal clean.

    Ill check out the video when I get off work but by the sounds of that post, its not really related to what I'm studying. I'm just interested in studying resonance in a coil and looking at what kinds of things will cause the desired frequency to drift and how various interferences affect resonance frequencies - basically just interested in coil resonance - nothing too special really..

    Lol I can already see you whipping out some valid criticisms about torroid size and triodes lol but hey, what can I say - I cant help but be compelled to study this stuff! That said, I really appreciate your input so far, criticisms included!

    Keep it comin!
  10. May 2, 2009 #9


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    I have some toroidal power transformers that have very low leakage currents so that is a good use for them. These would be about 6 inches in diameter.

    No, this is the second time someone has mentioned triodes in the last week. Someone else was talking about cartridges for record players. Must be nostalgia week.

    They work at VHF if you get suitable valves but they need to be in grounded grid mode or else you have to neutralize them.
    Before you use one read up on "Miller Effect". Nasty.
  11. May 2, 2009 #10
    Interesting - sounds like its another consideration in determining the frequency of resonance? At least, that appears to be the case with consideration for how I intend to be using them being that the resonance frequency is determined in part by the capacitance in the circuit...

    Anyway, a stable output frequency is something I'd prefer, in consideration of core heat buildup induced frequency drift, I'm going to spend this weekend looking for a nice LC Circuit to keep the coil at the resonance frequency.

    Here is a video that has an experimental setup that clued me in that these kinds of coils are highly sensitive to outside interference.
    http://video.google.com/videoplay?docid=-721789270445596549&hl=en [Broken]
    Skip to 11:20 in the video and at 15:35 he demonstrates that the resonance in the coil is thrown off just by moving his hand near it. I dont really attribute any credibility to the rest of the video (not that i could anyway lol) - This video conveniently had a good demonstration of the effect that I'm trying to counter balance...

    Since the frequency at which resonance occurs within the coil changes with respect to outside interference as well, then it is necessary to modulate the 'stable' frequency that is being fed through the coil. Ideally I would like the circuit to react and adjust the output frequency so as to keep the coil on resonance

    The output signal voltage is less critical (within spec for the system though obviously) so long as the frequency is on, then I'm happy - is stability within a few Hz a reasonable degree of stability to ask for at those frequencies?

    6" toroids would work too I think.. Just gotta find me some now.. I'll have a look at that site you mentioned earlier - I'm not in a rush though, I still need to sort out the proper LC Circuit which is quite an undertaking from my perspective - So many interesting options!

    Anyway, thanks again!
    Last edited by a moderator: May 4, 2017
  12. May 2, 2009 #11


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    Still not sure what you are doing.

    If you are worried about "outside influences" you just put your tuned circuit in a metal box and try to limit how much the temperature changes in the box.

    In extreme cases, it is possible to put a heating element in the box and regulate the temperature to some temperature like 110 deg F which you hope will never occur in your house. If you do that your tuned circuit should stay at the same temperature all the time.

    The other thing that can change temperature is the supply voltage. It is not unusual for the supply voltage to vary and cause most of the frequency drift of an otherwise good oscillator.

    Drift of a few Hz is quite normal at those frequencies. If that was a problem, you would have to look at using a quartz crystal oscillator.
  13. May 3, 2009 #12
    The concepts involved with this test setup are interesting to me. I've been spending hours reading.. its very engrossing! There's so much to learn, I am finding most of it very fascinating

    Crystal oscillators would work... but I am also very interested in what you think about how effective the following technique might be:
    It looks like this technique yields stable oscillations but it does not appear that the frequency is variable. Sounds like it can be used as a timer circuit (is that the right term?) for amplification/output via triode to drive resonance in my test coil. I will still need a way to modulate that stable frequency...

    What do you think??
    Last edited: May 3, 2009
  14. May 3, 2009 #13


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    Those magnetostriction devices look interesting but they depend on the dimensions of the device. So, they are not variable.

    I'm not sure why you don't get a solenoid (or make one ) and make an oscillator.
    Or test one with a signal generator, with and without capacitors, in series and parallel.

    There are all sorts of techniques to get oscillators doing what you want, but you really need to try some basic testing on one.
  15. May 3, 2009 #14
    I recently used some VCXOs (voltage controlled crystal oscillators) in a circuit at about 1 MHz. I think their max tunable range was about +/- 200 ppm.

    I not so recently (about 1955) used some 6AM4's in a grounded grid circuit at 420 MHz. It has 4 grid leads which makes (made?) it ideal for this application.

    I once had an AM radio that had ferrite-slug-tuned coils for both the RF amplifier (550-1600 kHz) and the local oscillator.
  16. May 4, 2009 #15
    Yea, theyre really cool hey? I've been reading though and theyre essentially the same operation principal as a crystal oscillator, in which case, its just as easy and more compact to use one of those instead. Plus, not as much of a concern for overheating. So, it looks like you were right from the start!

    Yea, I cant wait to start building! I was waiting for some better core material - I didnt really see the need to simulate the timing circuit before I got the proper core material and I'm waiting for some trimmer caps before I build an oscillator... So in the meantime, I'm just trying to get as much of the circuitry down as I can.. But youre totally right, I need to start playing with these things so I dont need to burden you with so many questions... (Sorry about that)

    Yea, I'll totally build a couple of these things and see how they perform - I see on wikipedia that there is a list of common ones - Since I expect that wikipedia often has the 'lite' version of whatever information I'm looking for, can you suggest a resource that is a more complete collection of oscillator circuits so I can see whats available out there?

    Thanks again!!
  17. May 4, 2009 #16
    Man! I wish I was around in those days - It was the golden age of EE as far as I'm concerned!

    A VCXO sounds like just the job for me, thanks for the tip! Should yield a stable output frequency and variable. I'll have to find some that operate at a lower frequency though. Minute variations in input voltage will be a concern at that point but I think I should be able to find a way around that...
    Yes, I had considered trying something like this but I didnt see very much information about it being common practice but this is another setup that would work great to achieve the results I'm looking for! Thanks very much! I will try this as well. Can you recommend any additional reading on this type of oscillator/amplifier circuit? Is there a particular name for it?

    Thanks again!
  18. May 4, 2009 #17


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    Getting busy with a soldering iron is the best way to learn about tuned circuits and oscillators.

    As a first oscillator that is easy to get going, I'd suggest a Hartley oscillator.
    This has many variations but always includes a tapping on the main coil to provide feedback.
    This means they are almost certain to work first time.

    Please skip the valves and go to the following site:


    Don't worry about all the capacitors across the coil. Just one variable one will work fine.

    Or you could look at this one:
    Ignore the stages to the right of R6 (and including R6) for now.

    You do not need a printed circuit board to make simple circuits like this one. As long as the components are mounted securely, that is all you need.

    I have attached a simple calculator for working out reactances and resonances for tuned circuits.

    Attached Files:

    • XLC.zip
      File size:
      1.2 KB
  19. May 4, 2009 #18
    Wow that first link is looks Great!

    I've been reading about crystal oscillators, VCXO's and TCXO's as well as frequency multipliers and frequency dividers - this stuff is fascinating!

    I am thinking about just using the quartz crystal from a wristwatch - it is in just the right frequency ballpark but those are not VCXO's. I'll have to do some more hunting for what kind of electronics use 1-100+ kHz crystal oscillators so I can salvage some or just find out where I can buy some. That frequency ballpark would be ideal for me though.

    Anyway, after all that reading, I see you've brought me some more! I'm going to go absorb that now so I can do as you suggest and build that Hartley Oscillator. Ill need to get some trimmer caps still though... Maybe I can salvage one...

    I'll let you know how it all works out. I guess my next step is researching some good appliances to salvage components from... That knowledge would be useful in general but it'll have to do until next payday lol GF put her foot down after the oscilloscope and signal generator

    That calculator is perfect, I'm sure it will come in handy! Nice find, thank you :)
    Last edited: May 4, 2009
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