# What is the physical quantity which makes a Reed switch open or close?

I would like to generate the activation map of a Reed switch, similar to the following:

Using a magnetic finite element modeling software such as FEMM.

Assuming the Reed switch closes for X Ampere-Turn, what quantity do I have to measure in the software to determine whether the position and orientation of the magnet considered make the switch close?

I can calculate plots of the H field, linear integrals along arbitrary contours, integrals over arbitrary areas... But I don't really know which one to consider, especially considering that the field varies inside the Reed switch between the strips.

Edit: I understand these types of XY closure graphs, I would like to generate one using FEMM. For this I need to know what to measure in the software (and where/how) that I could convert to Ampere Turn to know whether the switch should be closed or not.

• Ampere-turn is a unit of magnetomotive force $F$, which is related to magnetic flux $\Phi$ by $$F=\Phi R,$$ where $R$ is magnetic reluctance. I don't know this topic well, but it seems you have to find/measure magnetic reluctance and sizes of your device in order to convert magnetic flux density (AKA magnetic field $\vec B$), provided by a magnet, to magnetomotive force. – Ruslan Mar 24 '17 at 15:07

Whether the reed switch opens or closes depends primarily on two things:

• The orientation of the bar magnet with respect to the reed switch: is it parallel or perpendicular? A parallel magnet will create 3 lobes of activity like so

and a perpendicular one will produce 4 lobes:

• Whether the magnet is inside or outside the lobe

If the reed switch need a minimum of $X$Ampere-turns to close, then any bar magnet with a strength equal to or grater than this that moves into one of the lobes, will make the switch open or close. Of course if your magnet is much stronger than $X$Ampere turns the size of the lobes will increase.

You might find the diagrams and animations here useful.

*Images from standexelectronics.com

• Thank you for your answer. I am well aware of that, I already understand those types of graphs. There is no such thing as a magnet of X Ampere Turns, those graphs are made for a specific Reed of a specific AT rating, and for a specific magnet. I would like to know what to measure in the software (and where/how) that I could convert to Ampere Turn to know whether the switch should be closed or not. Do you know? – Mister Mystère Mar 24 '17 at 14:22