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AC Magnetic field measurment

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riccardo

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Hi,

I would like to measure the magnetic field strength within a solenoid which has a current oscillating at around 100kHz.

In theory I could use a smaller coil with a resistor between the terminals and then measure the waveform voltage with a scope.

Does anyone know the calculations necessary to convert that information into a magnetic field measurement?
I'm assuming it will not be totally straightforward as I would need to consider the number of turns, area of pickup coil, induced voltage/current and the frequency.

Any pointers would be appreciated!
 
Hi,

I would like to measure the magnetic field strength within a solenoid which has a current oscillating at around 100kHz.

In theory I could use a smaller coil with a resistor between the terminals and then measure the waveform voltage with a scope.

Does anyone know the calculations necessary to convert that information into a magnetic field measurement?
I'm assuming it will not be totally straightforward as I would need to consider the number of turns, area of pickup coil, induced voltage/current and the frequency.

Any pointers would be appreciated!
https://www.pa.msu.edu/courses/2000spring/phy232/lectures/ampereslaw/solenoid.html
 
Are you looking for precise measurments or just a visualization?
You could just get a standard off the shelf drum core inductance, put a 50v or so zener across it and use that, I do something like this to check vehicle igntion coils and injectors.
 
That link is for DC. I believe something a little more complex is required for high frequency AC measurement due to the effects of the measuring coil's reactance.

I'm looking for something fairly precise so that I can compare the field strengths in induction heater coils.
 
You can get hall effect sensors that will measure constant fields, not sure if you can get one that will measure an extreme field like that present in an induction heater though.
Wouldnt a field in a induction heater be run on ac?, or are you wanting to measure at dc.
 
Hi,

You might be better off just calculating the field. You should be able to measure the current and come up with some values.
It would not be easy to place a small coil inside another one and get the position just right. As you can see from the graph in the attachment, the field varies as you move along the axis, but it will also vary along the cross section so this makes it difficult. If you do place a coil inside, it would probably be best to find a way to calibrate it anyway so you know you are getting true reading values.

The attachment shows how to calculate the field inside the coil along the axis which we call 'x' here. If you want the field at the very center, set x=0.
x1 is the left side end position, x2 is the right side end position, as measure from the center of the coil. This means the length of the coil is L=x2-x1, and so x1=-L/2 and x2=L/2.
 

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  • Solenoid-01.gif
    Solenoid-01.gif
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When I needed to do magnetic field measurements at work, I ended up buying a meter for about 300 dollars. It is Hall sensor based, can measure AC and DC, can make peak and average measurements and is easy to use. The range is impressive from a few tenths of a Gauss up to a 2 Telsa. I recommend something like this because the time you will take to design and build something as good and as versatile as this meter is considerable.

I dont think they make the model I bought any more because the one I bought looks like a hobbiest built it ( I mean that in a good way), but now that company's products look like the cases are much more professional looking.
 
I dont think they make the model I bought any more because the one I bought looks like a hobbiest built it ( I mean that in a good way), but now that company's products look like the cases are much more professional looking.

Sure you heard of Yaesu Musen :) . The same process.

I still recall the first Yaesu equipment I saw, maybe some 35 years ago. If I compare that with current ones, there is a difference...! Until I bought an FT901DM. What a nice xcvr!

BTW, if a coil is used to measure, wouldn't affect somehow something in the one under test? Hall sensor is not a coil, I know.
 
The coils to be measured are large and open so it is no problem getting a small coil probe inside to measure it. Measuring the current in the active coil is not easy either as it is possibly hundreds of amps at 100s of kHz
I prefer to make a probe coil if possible then I could see how the field strength is distributed in and around the active coils under various conditions.

I have a hall based detector that says it can do AC, but I assume that it means 50Hz type AC. If I put it in such a high frequency coil I think it will probably just get melted.
 
Hi,

Hall sensors that sense 100kHz are probably expensive, unless they improved the technology recently.

Putting a coil inside is tricky too, because it's not a spherical detector. To get the actual field readings you'd have to be able to sense the direction of the field also. Otherwise you have to assume it is oriented parallel to the coil length directly in the center. As you move toward one end, the direction will change, first slightly then more until it reaches 90 degrees from axis, then more until it points back the other way on the outside of the coil.
We could do some calculations to show the field and direction of an ideal coil, but there are probably pics on the web. Also, this will be for an ideal coil and we have no idea if your coil is near ideal or not (ideal with respect to the geometry, not with respect to the electrical characteristics). It is probably close though.
It would probably be simpler just to measure the center of the coil, along the axis, but if you move outside the coil rotate the coil in a plane parallel to say a vertical slice along the length. Rotate the coil back and forth until you get the highest reading, keeping the very center of the coil at a constant distance from the center of the large coil.

There should be no reason why you cant get relative readings this way. There wont be any base calibration to go by, but if you see a reading of 2 and then somewhere else a reading of 1, you know the field decreased probably by a factor of 2.

Inside and outside you should be able to keep the coil parallel to the big coil, as long as you dont get too close to the ends. The field should be symmetrical about a circular path around the coil diameter.
 
Hi,

Hall sensors that sense 100kHz are probably expensive, unless they improved the technology recently.
That's a good point. The one I bought is good to 20 kHz. Perhaps one can buy an inexpensive low bandwidth meter and use it to calibrate a higher speed sensor IC. The ICs are much cheaper than the complete meter. I've used current measurement hall sensors that operate to 120 kHz, so I expect that a sensor IC for magnetic field measurement, with high bandwidth, is feasible. The problem is that often the magnetic field sensors are used for position/angle measurements that don't require high bandwidth. So, it might not be easy to find them.
 
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