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Calculating a magnet size needed from an Amp-Turn reading

Calculating a magnet size needed from an Amp-Turn reading

Calculating a magnet size needed from an Amp-Turn reading

This might be complete retardness, but I am struggling with being able to calculate a needed magnet strength to actuate a certain reed switch that I am thinking of using for a design.  The specification sheet for the switch claims it requires 140-170 amp-turns to actuate it, I would like to convert this to a measurement that is more conducive how magnets (NdFeB, SmCo etc.) are measured.

I have found the needed test coil information to make a resonable calculation of what the needed magnetic flux density is in order to operate the switch (using the simplified formula).  But transferring this over to a magnet strength needed does not seem to make sense (numbers obtained are much to small for any type of magnet).  Any help would be much appreciated in pointing me in the right direction.

Also does anyone know a source for calculating off-axis magnetic flux density of a disc magnet? seem to keep stumbling across the same few equations that are only reliable for on-axis calculations.  

Fairly new to the world of magnetic design and I might just be making assumptions that are not completly correct that are causing my qualms



RE: Calculating a magnet size needed from an Amp-Turn reading

Hello Nick,

I run into the same situation every once in awhile.  A few years ago I created a chart that to estimate the conversion between Amp-turns to Gauss.  It is based on data from Moskowitz's book.  I've found it good for a first approximation.  You can see it here:

For your switch (140-170 amp-turns), it looks like you'll need somewhere between 150 to 250 Gauss.

Regarding your 2nd question about calculating off-axis magnetic flux density: It can't be done analytically.  One has to use FEA/BEA to calculate off-axis components.  

You may want to look into a freeware/shareware FEA/BEA solver (see the FAQ section of this forum).  As an alternative, most permanent magnet suppliers will be able to calculate the numbers for you easily.

RE: Calculating a magnet size needed from an Amp-Turn reading


Thank you for your Reply! I'm having troubles viewing the file that you directed me too... dango. Anyway i was just curious how you came about that  realization of 150-250 Gauss, is that from experience? Or was there a certain calculation that you ran through? I was doing some prelim calculations based on the coil that I found the company had used to test the switches, but was getting extremely small answers.... And alos do you know of a good FEA (free preferably) that would be advantageous?


RE: Calculating a magnet size needed from an Amp-Turn reading

To view the file in the link I provided, just click where it says "Click here to download" (the words are the link).  The file is a PDF of a graph.

I came up with the 150-200 Gauss value from the chart.

As suggested by the replies in the other group you posted to, this'll just be a start.  The next best step is to get some magnets and experiment with them.  

Regarding the FEA package: Did you review the FAQ section of this forum like I suggested?

I recommend experimenting with actual magnets before getting an FEA package, especially if you are not familar with magnetic modeling.  It'll take less time to experiment than it would to learn an FEA package.

RE: Calculating a magnet size needed from an Amp-Turn reading

Thanks again mike!! Much appreciated!

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