Off-the-shelf coil and toroid with very high number of turns of thin wire - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: April 18, 2022, 07:58:24 pm
content: Hi,I am looking for off-the-shelf coil just for some random experiments with the (very lose!) specs listed below. In a nutshell it has to work as sensor of ultra low magnetic fields and/or ultra week induced currents.The most important thing is to have as high number of turns as possible and relatively small size. Because it is just to test some ideas I am just looking for a 1 or 2 off. So I was wondering if someone has some suggestions as to where I could find something that could meet those specs. I have been thinking that there might be some applications where coils with the specs below might already be built for.If I could find one like in the old radios ferrite antennas types but with thousands of turns that would work, I would just remove the ferrite rod).- practically no current flows through it so the wire can be extremely thin- highest number of turns possible with the rough dimensions ranges below. Ideally, if there are some made for specific applications, in the thousands of turns.- inside diameter is what is important. Anything from 10mm to 25mm or even 30mm ID- length anything from 20mm to 50mmAny idea if anything along those lines is made for some applications already? Or where I could find such coil?Also the same for a toroid with the same ID of 10mm to 25mm but height of 10mm to 30mm. Again those are just loose ranges/specs.Thank you

author: cdev
author_rank: Super Contributor
timestamp: April 18, 2022, 08:09:29 pm
content: Can you count? Start winding a coil.. Any 3 year old could understand thatThank youCould you be more specific about what you are trying to sense the magnetic field of? And where, and approximately the frequency of the signal. There are clip on magnetic probes that are designed for use in measuring DC voltage carried signals flowing along wire loops, sometimes they contain hall sensors and they can measure currents in a wire non-invasively. Thats how a clamp meter works. Keep in mind that if your signal is DC, any other coil is not going to do well trying to sense itas all you will get is AC, and only when the magnetic field is moving, so the signal you will pick up is quite different, and quite possibly impossible to use for the same use as it was being used for originally..

author: eugene
author_rank: Frequent Contributor
timestamp: April 18, 2022, 10:46:46 pm
content: cdev is correct that toroids are not good as sensors. In fact, everything about the design is meant to isolate the coil from the outside world. Moreover, coils of wire in any form are not good for measuring DC magnetic fields; they are good only for detecting changing magnetic fields (AC.)You haven't described the characteristics of the magnetic field that you want to measure. If it is AC and you think a coil of wire will work, then my suggestion is to buy a small coil of fine magnet wire and roll your own. I don't know what sources are available where you are located, but you can probably find 42AWG easily because it is popular for guitar pickups.But, be aware that it is so fine that it can easily be broken. Further, the insulation will be just a coating an "enamel" like substance. I put enamel in quotes because it was once the popular choice but is very difficult to remove, especially from something as fine as 42AWG. In more recent decades other insulations have become available that can be removed with a drop of solder on the tip of a soldering iron. However, enamel is still available either as old stock or because some people like to stay with the old recipes.If you want to measure a small DC field then you can still use a coil but you need to record the output over time with something like a storage oscilloscope or some other device with an ADC. The idea is that you put the coil in the field than flip it over (or remove it from the field entirely) recording the voltage from the coil the entire time. You then need to analyze the recorded data to extract the difference between the field with the coil in both orientations. This is called a "flip coil." It is a very old technique and the analysis can be challenging to get right.DC fields, and slowly changing AC fields can be measured with a Hall sensor. Because of high-volume commercial products like cell phones, Hall sensors meant to be used as a digital compass are truly inexpensive; Probably less money than a roll of magnet wire. These can fairly precisely measure the earth's magnetic field, which is on the order of 50 microtesla. If you need to measure smaller fields than that, then you have a difficult problem that requires expensive equipment.The downsides of inexpensive (or even expensive) Hall sensors is that they often have a DC offset and they typically require I2C or SPI to communicate with them. The DC offset is easy to calibrate out, but you need to be aware of it. As for digital communication, you can buy raw Hall sensors that are completely analog, but they require a low noise power supply and a very good instrumentation amplifier. Or spend lots and lots of money on laboratory grade instruments, but I don't think that's what you're looking for.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 18, 2022, 11:23:30 pm
content: The one thing that a toroid does get you, is if you have windings arranged in sectors rather than uniformly, four sectors say -- you can sense saturation along both axes in the plane of the component. This, plus a number of other improvements that I forget, is the basic idea behind the fluxgate magnetometer. Although the structure is magnetically open (with respect to ambient fields -- somewhat obviously, I guess, since that's what it's there to sense!), with the right choice of materials and geometry, quite some gain can be had to begin with (i.e. ambient fields of ~nT become µ~mT inside the material), and thus easily sensed by voltage or pulse width measurements.Tim

author: Bud
author_rank: Super Contributor
timestamp: April 18, 2022, 11:29:00 pm
content: Current sense transformers would be probably the closest to your desired specification, you can find them in early computer power supplies and CRT TVs, or can buy new from suppliers like Digikey. They would not have thousands of turns though, just hundreds at most.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 18, 2022, 11:42:03 pm
content: Mains CTs come in the thousands -- usually on stripwound steel cores rather than powder or ferrite.Tim

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 19, 2022, 03:51:32 am
content: I keep thinking "ignition coil" but I don't know what they look like on modern vehicles. The old ones might have been potted, but I know some high performance ones were in oil. Not sure if it would do what you want.

author: jonpaul
author_rank: Super Contributor
timestamp: April 19, 2022, 12:05:23 pm
content: Beware of self capacitance limiting the test frequency on coils, toroids or,transformer100s..1000s turns.Off,the shelves xenon lamp trigger coils are under a $ redialy availableJon

author: cdev
author_rank: Super Contributor
timestamp: April 19, 2022, 02:13:43 pm
content: The discussion on the Uni-T UT-210e meter might give you some info you could use?This is an inexpensive hand held clamp meter that includes the microprocessor and hall sensor and is a very handy test instrument that might be possible in the futureto use to extract the DC measurement from at an adequate bandwidthto use for reading something like a DC loop signal. Thats what I want. To be able to take that signal, as if it was a reading of the loop itself.. The alternative means of measuring the signal are inadequate for various reasons. A differential probe on afloatingscope could measure it with a shunt inline. (if its not floating there is way too much noise for me) But frankly, thats a major PITA. Clamp meter with a broadband data output would be a much more elegant solution.By way of trying to show why I would find this useful, I have a piece of what seems realistically to be a fairly esoteric piece of foreign made HVAC equipment whose protocol is not public that I would like to talk to but they don't publish it. I have a fantasy of reverse engineering the protocol and interfacing to it to help me save money on energy. Its my own hardware, I own it.I think we could do significantly better. The controls they sell for it are just insanely overpriced. I already own several of them. So I am not trying to get something for nothing. Ive already spent that money. Lots of it.I've never needed to make coils with more than 100 turns. Yes, sometimes very thin enameled wire is needed to have. I always save it when I find some.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 19, 2022, 02:41:23 pm
content: It looks like you're trying to build a flux gate magnetometerhttps://www.sensorland.com/HowPage071.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2022, 05:37:01 pm
content: Oh, well if you can cut into a cable to separate the wires, that makes things 100x easier, and 10x easier still if you can simply lift one lead and wire in an isolated receiver, or check if it's already isolated and doesn't care if you just shove an oscilloscope in there, etc. Or the receiver is grounded but has significant voltage drop (the easiest 4-20mA receiver (if that's what this is?) is just a dumb resistor hanging off an ADC) so you can read that verbatim.Tim

author: guenthert
author_rank: Frequent Contributor
timestamp: April 19, 2022, 11:13:02 pm
content: How about Tesla coils? There are cheap ones on the secondary market which could be repurposed. Those certainly have a high number of windings, given that they are core-less (air-core), I'm not sure how well they work as sensor for low magnetic fields (should do alright at radio-frequency though).Extremely small magnetic fields are detected using SQUIDS, aren't they?

author: ricko_uk
author_rank: Super Contributor
timestamp: April 23, 2022, 03:32:49 pm
content: Thank you all!Several interesting suggestions and few things to try!Thank you

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 23, 2022, 06:04:06 pm
content: Removing the high voltage winding from microwave ovens and replacing it with a few windings of thick wire is a popular method to make spot welders.You could do the opposite.Split the metal core (usually thin welds on the edges) and then take out the high voltage winding. It has quite a lot of turns of thin wire and it has a fairly big overall circumference and therefore it picks up external magnetic fields better then smaller coils.

author: ricko_uk
author_rank: Super Contributor
timestamp: April 23, 2022, 06:29:35 pm
content: Thank you Doctorandus_P

author: eugene
author_rank: Frequent Contributor
timestamp: April 23, 2022, 09:13:38 pm
content: I still don't understand how you expect to use a transformer to measure a magnetic field. If you give us more details about what you want to accomplish then we might be able to help you move in the right direction.

author: cdev
author_rank: Super Contributor
timestamp: April 24, 2022, 12:01:53 am
content: A toroid can couple your radio to a magnetic loop and do a really good job of it. Fairly broadband, too. The toroid goes right around the conductor. The loop conductor goes through the coupling transformers major axis.. Would Litz wire be useful? It might be helpful, but.not as much as one might hope. .WHY NOT?Quote from: T3sl4co1l on April 18, 2022, 11:23:30 pm

author: Uunoctium
author_rank: Frequent Contributor
timestamp: April 24, 2022, 09:13:35 am
content: Salvaged this controlcoil from an over 40yrs old power contactor, which was constructed similar as an reed-relay.The company Guenther (1) was so kind, printing the coil data on it (img2). Very useful in combination with an signal-tracer tracking hum-sources...Solution II: Humbucker(1)https://shop.griederbauteile.ch/info/g/Guenther.pdfhttp://www.wguenther.de/guentherindex.htm

author: eugene
author_rank: Frequent Contributor
timestamp: April 24, 2022, 01:32:48 pm
content: Quote from: cdev on April 24, 2022, 12:01:53 amOK. Current in a conductor through the axis of the toriod core will couple to the coils. I would call that an exercise in current measurement not magnetic field measurement, but I'll accept it as my mistake for not understandingQuote from: T3sl4co1l on April 18, 2022, 11:23:30 pmThat sounds like something different again, used to detect an external field in the plane of the toroid. though I admit that I don't really understand it completely.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 24, 2022, 04:30:13 pm
content: A relay coil? Slavage one from an old car or mc. If you dont want a core use a reed reelay coil.https://en.wikipedia.org/wiki/File:Delta_Electronics_DPS-350FB_A_-_board_1_-_OEG_SDT-SS-112M_-_case_removed-3045.jpgFor low fields GMR and maybe AMR sensors.AA/AB-Series Analog Magnetic SensorsThere's probaly a magnetometer in your cell phone. Rip it out! Exquitely sensitive to mag fields.An LSM6DSLTR? They'll even chuck in a gyro for free!