using inductor instead of coil to produce electromagnetic wave in small range

[miskol]

Hi,

i want to produce electromagnetic wave in a range around 20 cm where a hall sensor will be used to detect the signal and produce voltage readings.

my current plan is to used a microcontroller to provide the pulse to a transistor circuit that will provide the high-amp pulse (up to 8 amp as supported by the inductor) to the inductor. please advise on my progress.

i am also still finding the mathematical equation that can be used to estimate the coverage area of the electromagnetic wave. if anyone could help show me the way i would be delightful.

Thank you.

 

[Darkcrusher]

Hi!

I dont understand what you are trying to do, I guess you mean you want to estimate the field ? Coils can be hard to simply calculate. Even if you numerically simulate them, there are still some practically concerns.


Since you are talking about coverage, I guess you will have some sort of receiver. You can map the coverage by placing the receiver around the coil and increase distance. Next you should repeat in different directions.

Hope this helps!

 

[miskol]

yes, in practical i should do that by placing receivers at different places to map the coverage. but the reason why i need the mathematical equations is because if possible i want to theoretically estimate the distance of the magnetic field based on the circuit's parameters such as the value of ampere introduced to the inductor.

 

[Darkcrusher]

I understand..

Well in that case use a very simple round loop with round cross section. It's very easy to calculate the field in the center. For the off axis field you need more advanced integrals and stuff but then you might consider using graphs or sim software

 

[miskol]

but firstly, is it possible to use inductor to produce magnetic field that can reach a distance up to 20cm? i understand that a coil is normally used for increased distance, but it should be hard for an inductor because the magnetic field is focused on the magnetic core instead of unfocused like in the case of using a coil.

thus the reason why i'm searching for mathematical equations on this.

however, i'm also concern whether my circuit is appropriate. please also help on this.

Thank you.

 

[Darkcrusher]

Well you came to the right place. If you make your coil diameter very large you can do it.

If you want small coil and big distance , you should establish a resonance between your transmitter and reciver coil. This will focus your fields.

Cheers

 

[jiripolivka]

I think you are confusing a "pulsed magnetic field" with a high-frequency electromagnetic wave.
Your DC magnetic pulse will create a "static" magnetic field according to the Biot-Savart law, its intensity decreasing by the square of distance.
If you create, however, a short pulse of current and introduce it into a loop or wire coil, then you create a series of harmonic responses of high- frequency electromagnetic field. Each of such waves will behave according to the laws of radiation while the difference between their individual behavior will be proportional to wavelength of each wave.
With longer waves, within the "near zone", Fraunhofer description yields three types of decreasing-intensity fields, one decreasing by the distance, another by distance squared, and the last one by distance cubed. According to loop geometry you will see the effect of wave polarization.
You should read good textbooks on electricity and magnetism to have some background, and even better, you should experiment with your simple equipment. Maybe you would not discover new unknown things but you will get a good hands-on experience about how different fields behave. Good luck!

 

[miskol]

Darkcrusher: thank you for the information. however as i have stated, my focus is to use inductor and not a coil.

jiripolivka: i am still finding good textbooks to help my understanding on these matter since i'm fairly new in this. my simple understanding now is that if current is introduced to the inductor, it would produce magnetic wave. my concern now is to apply pulsed-current to the inductor because most transistors can provide very high current if operated in pulsed transmission instead of continuous. then at the end of the distance, i will use a hall effect sensor to detect the magnetic wave. i've not yet practically test the simple circuit construction, as this operation involve high-current that i've never yet experienced before. luckily i've not yet tested earlier, if not i would have not know anything about the importance of flyback diode protection. so because of that, i'm still searching of anything that is important that i might miss before proceeding with the experiment.

Please help, thank you.

 

[FvM]

finding the mathematical equation that can be used to estimate the coverage area of the electromagnetic wave

For the switching speed respectively the frequency range achieved by your circuit, you can simply treat the problem as magnetostatic one. No electromagnetic properties (coupling of electrical and magnetical field) have to be considered. As said, Biot-Savarts law allows a simple field calculation at the axis of round coils. For complex shaped coils or off-axis positions, it's most likely more simple to use a numerical simulator/solver, e.g. the free student version of Quickfield.

my focus is to use inductor and not a coil

A magnetic core will reduce the distant field strength in most cases, because it concentrates the field to the proximity. A numerical simulator would be very useful for evaluating the core effect. I don't understand, what you want to achieve with a core. Of course, an air coil is an inductor as well.

As you intend to generate variable magnetic fields, a sense coil (with an amplifier) is possibly a more sensitive detector than a hall sensor.

 

[miskol]

when i embark in this project, i was given these two components, inductor and hall effect sensor. with the objective of just to get a reading from the hall effect sensor at a certain distance when the inductor is introduced with current.

at the beginning stage, i studied a bit on electromagnetic and understands that the core will increase the capability of an inductor because of the focused magnetic field but it also will decrease the magnetic field, which is important in my application. however, even though usually others will use coil, but it is difficult to wind the same coil in many numbers. research papers have showed that the coil winding need to be precise with one another, thus usually will be sent out to companies to do it which will cost much. thus this is the only reason why we are using inductor, it is easier and cheaper to get off-the-shelf inductor, so we are testing whether it is usable or not.

 

[FvM]

Can you please clarify about the involved type of "inductor". It can mean a lot, from effectively closed core to halfside open pot core, bobbin and rod core. Preferably tell the manufacturer and type.

research papers have showed that the coil winding need to be precise with one another

Not generally, I think.

 

[jiripolivka]

Dear Darkcrusher:

I am not sure how fast response your Hall sensor has. Those I know are slow, so it would sense a "DC" magnetic field, decreasing intensity from your loop/coil/solenoid with the distance squared.
To introduce current pulses into your coil, you can use a transistor switch but I would try a relay contact- lossless and fast.
Experiment will tell you what next to improve.
Then again- introducing short current pulses into a loop or coil means the current harmonics, instead of a "DC" magnetic field, will rather generate electromagnetic field pulses. This is (with very short pulses) a base of UWB technology. Very short pulses generate a wide (ultrawideband) spectrum, with quite peculiar features.

 

[miskol]

Can you please clarify about the involved type of "inductor". It can mean a lot, from effectively closed core to halfside open pot core, bobbin and rod core. Preferably tell the manufacturer and type.

it is a hash choke. Manufacturer - Bourns JW Miller (5200 Series), Part Number - 5219-RC.

---------- Post added at 13:00 ---------- Previous post was at 12:55 ----------

Dear jiripolivka:

- yes, i noticed that too, their sensitivity is low. using refrigerator-magnet, the readings is small.
- i will try using relay after i test with the transistor.
- i'll try to get more literature.

Thank you.

 

[FvM]

Yes, thanks for clarifying. It's a iron powder rod core choke, and should generate some external field. The current source would need a current limit to protect itself respectively the choke. The field should be at least detectable in a 20 cm distance. According to the datsheet, the 5219-RC is rated for 15A DC and can surely carry more for short duration. You didn't say much about the intention of your experiment, so I won't guess further about a reasonable procedure.

 

[jiripolivka]

In the very beginning you stated that you intended to use a Hall detector to pick up magnetic field intensity over ~20 cm, generated by a "coil".
I am still not sure what you really want to do.
Transmitting a pulsed signal over ~20 cm , or up to 10 meters, can be done with a loop (like 2x2 meters) laid on the ground while a smaller loop or a pickup coil can be connected to an earphone or an AC mV meter, and you can move it around the larger loop and check the output signal at many situations.
Then you can try your Hall sensor and compare the results.
For similar experiments you do not need to buy a coil, simply make the larger loop from a thick wire or cable, and wind some 50-100 turns of the same wire on a paper tube, e.g. one inch diameter.
As a signal you can start to take the audio output from your radio, normally input to a 4 or 8 Ohm loudspeaker. Or you can use a 3 V buzzer and connect your loop in series.
Otherwise it is too much talk before you DO something. Try it and see!

 

[miskol]

The current source would need a current limit to protect itself respectively the choke. The field should be at least detectable in a 20 cm distance.

- please advise if my current circuitry is usable. i plan on testing at max 8 A first.
- how did you estimate that it is detectable. this is why i'm finding the equations, so that i could estimate if the field is detectable.

---------- Post added at 12:00 ---------- Previous post was at 11:49 ----------

Dear FvM and jiripolivka,

basically what i want to achieve is that, when a magnet is located near the hall sensor, it can be detected by the sensor and the adc value (from microcontroller) can be used to determine the intensity of the magnet field. the adc value should then can be used to determine the distance between the sensor and the magnet, the higher the measured intensity the higher the ADC value is.

as for my case, the magnet is replaced by the inductor which is initially fixed at the distance of 20cm (this should be the maximum distance). the current is fixed too, lets say at 8 amp (if such current is already detectable at distance = 20cm). then if successfull, i will try several test by varying the distance with the same inductor and fixed current rating.

Please advise, thank you.

 

[FvM]

- please advise if my current circuitry is usable. i plan on testing at max 8 A first.

You need a current limiting resistor or another method to limit the current

- how did you estimate that it is detectable. this is why i'm finding the equations, so that i could estimate if the field is detectable.

Due to the core, it's no longer a simple magnetostatic problem as an air coil would be. As already suggested, it's typically a case for a numerical sover.

See below a Quickfield simulation, assuming 16 turns 8 A excitation. A flux density simiar to the earth magnetic field (30 µT) is achieved in about 100 mm axial distance, at 200 mm it's only 4 µT. This rather low flux is mainly a effect of the unsufficient low coil diameter. Diameter equal to distance would be e reasonable dimension.

 

[miskol]

Dear FvM,

thank you very much for the Quickfield simulation image. I'm very delighted to see it, makes me motivated. I've downloaded the Quickfield software for student and have been trying to reproduce the model. Unfortunately there's so few tutorials provided. their forum also lacks information. i'm still working on it though.

however from the simulation results, it is troublesome to find that with 8 Ampere excitation the highest flux density is 0.05 mT. the hall effect sensor i have now is from Honeywell (SS94A2) and have a minimum magnetic range of +-50 mT.

does this means that SS94A2 cannot be used to detect 0.05 mT?


Thank you.

 

[jiripolivka]

I do not know if your Hall sensor can offer the sensitivity needed. Typical Hall sensors can detect > 50 mT "DC" field. You can possibly increase the sensitivity by keying the current in your coil/loop by ~ 1 kHz, and use a selective amplifier after your Hall sensor.

A better technique is used in magnetometers. You should read a good textbook on magnetometry and design a suitable detector.
The simplest way I described above- feed an audio signal to your loop and use a pickup coil with an audio amplifier as a detector. At least you can use this simple setup for a comparison with your Hall sensor.

 

[FvM]

I wasn't aware of the rather low sensitivity of usual hall sensors, although I suggested a sensor coil before. A magnetic compass would be better sensor in this case.