Calculating a magnet size needed from an Amp-Turn reading

[nlocken]

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

Thanks

Nick

 

[MJR2]

140-170 NI is a rather small amount. But this is what is being used to close a small airgap in this reed switch. We need some geometry to better explain what this means.

How do you measure your RE magnets and how would you need to understand the electromagnet in those terms.

The off axis solutions are typically proprietary since they require a bit more effort. They were very useful twenty years ago. Most of the time we are using FEA today since rarely is anything a simple disc magnet.

 

[logbook]

Small signal reed relays typically only take 5 to 50 amp-turns. 170 amp-turns sounds like a high voltage reed.

Now a reed coil is coaxial with the reed. A permanent magnet will not be and this would confuse the issue anyway.

Unless you are an academic I would buy a few magnets and see how close they have to get to make the switch close. Use the distance of approach as the metric to set the margin on the strength of the magnet. Remember that rare earth magnets typically don't work well when thick. An iron carrier will also enhance the field, allowing a smaller magnet to be used.

 

[nlocken]

Thank you both for your replys!

MUR2-
What type of geomytry is needed? Switch geometry? coil used to obtain the 140-170 NI? And I was mainly trying to understand the weakest possible magnet when been placed in a certain arangment so as to not actuate other reed switches.

Log-
You are absolutly correct! it is a high voltage reed that Im interested in actuating. I suppose that does make the problem a bit more interesting considering the alignment.... Is there anyway to get a round about guess of what size magnet i would need if I plan on using the magnet about a half an inch away from the center of the switch and probibly a quarter of an inch away from the glass?

Thanks again!
Nick

 

[itsmoked]

nlocken; logbook is right on!

Even if we could tell you exactly what magnet to buy; part number, etc. you are short circuiting a subject you need a better "feel" for. You NEED to get some sample magnets of known strength and size and START MESSING WITH THEM! You have a subject here that is perfect for messing with. So many others are very expensive or impossible to mess with take the opportunity here to do it. You will absolutely be launched forward in your hunt for subject knowledge. You will have a dozen more questions immediately show up that you will also be able to answer and all the while you will be filling in other questions like:

How to mount the reeds?
How to mount the magnets?
What's a better diameter?
What's the best shape?
Who's a nice magnet company to work with?
What are turn around times like?
What is the best system geometry?
How close can two units operate?
Which reeds work better?
How do multiple ones work next to each other?
All questions we cannot answer for you and may be more important than you original question.

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[waross]

Are you planning on actuating your reed switch with magnetic coil or by physically moving a magnet towards and away from the reed switch?
Is this a bi-stable reed switch?
A point of confusion with your small numbers for magnetic flux density;
This depends on amp turns per inch of magnetic circuit, not just amp turns. You must also factor in the permeability of the magnetic parts in the reed switch. Permeability is not a fixed constant for magnetic materials; you must consult the permeability table for the particular alloy involved.
My old textbook states that the permeability of air is 3.19. In magnetic materials the permeability ranges from 143 to 11,000. Try assuming a permeability of 5000 in your calculations. This will probably make your numbers more believable.
Remember that when designing a magnet coil to fit in fixed physical dimensions, the wire gage and the applied voltage are the factors that determine the DC amp turns. Adding turns does not appreciably increase the amp turns because as turns are added, the resistance increases and the amps decrease. The number of turns does affect the heat produced and the inductance of the coil. A high inductance may cause an operating delay that may or may not be desirable, depending on the application.
yours

 

[logbook]

waross, a relative permeability for air of anything other than about 1 is just plain wrong. The only thing I can think is that there is a problem with units and your book is using units such that you are actually quoting the absolute permeability of air/free space.

 

[waross]

Hello logbook
I agree with you 100%, except I don't see a problem.
However, whether the permeability of air is absolutely 3.19 or relatively 1, is not particularly important in comparison to the permeability of the magnetic components of the switch wich can be in the range of 5000.

yours

 

[nlocken]

Thank you all for all your insightfull inputs. I am greatfull for them all. I have begun experimenting with the limited time that I have availible to me. Hopefully the answers that I need will come to me soon.

Nick