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# Inspecting magnetic force necessary to activate a reed switch

## Inspecting magnetic force necessary to activate a reed switch

(OP)
hi,

I am working on a project where i have to activate a 6AT reed switch from a distance of about 10 mm or a little more, I am going to use 10 mm X13 mm X 140 mm magnet will it have enough force to activate this switch I need to know before I try to get this large magnet volume because it will cost me as it is in customized shape and have relatively high volum as you can see from the size?
Replies continue below

### RE: Inspecting magnetic force necessary to activate a reed switch

(OP)
I forgot to mention that the magnet will be split vertically in terms of magnitude so if you look at it standing vertically with 140mm length the right side would be the S pole while the left side would be the N pole or visa versa, I would appreciate the help

### RE: Inspecting magnetic force necessary to activate a reed switch

this is a pretty simple simulation by modeling with a 2D or 3D soteware. the forcedepends on the type and size of magnet and the relative position to the Reed switch. I donot have the licence to use any magnetic software, but there are lots of people over there who can help

### RE: Inspecting magnetic force necessary to activate a reed switch

You can put a pole piece on the magnet to increase the local field and get better actuation.
Modeling is important.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

### RE: Inspecting magnetic force necessary to activate a reed switch

Where will the reed switch be, relative to the magnet?

### RE: Inspecting magnetic force necessary to activate a reed switch

I think the pole piece would need to be on the reed switch to make it more sensitive to a remote magnet.

### RE: Inspecting magnetic force necessary to activate a reed switch

(OP)
thank you all for your feed back, I have reviewed a data sheet concerning the use of reed switch by a manufacturer and as i understood it would be best if the N pole of the magnet is opposite the S of the reed switch and of course the S of the magnet should be opposite the N and so I considered the right part of the tall magnet is N and the left is the S pole and this magnet will be sliding exactly against the Reed switch with an estimated 13 mm space I know that seems a lot of space but please do not forget that the magnet is 10 mm thick and 13 mm width so it should have a strong magnetic field? the question is will it be enough to activate a 12 AT reed switch 13 mm away with PVC isolation

### RE: Inspecting magnetic force necessary to activate a reed switch

What type of magnet is it? Alternatively, what are it's Br and Hc? Is the magnet thru-magnetized along the length, or are there two poles per face?

I would ask the manufacturer, since they already know all these details.

### RE: Inspecting magnetic force necessary to activate a reed switch

I'm really puzzled by what you think you're going to get when you don't provide any information about the magnet or the switch. It's like asking whether my car can do 100 mph on some unknown material without describing the car or the the material.

### RE: Inspecting magnetic force necessary to activate a reed switch

Normally the permanent magnet lies parallel to the length of switch, it doensot matter which direction is N or S, since the magnetic elements in the switch are magnetically soft, whose polarities are determined by the permanent magnet.
In order to activate the reed switch, one will have to know how much flux density is needed from the magnet. there is no simple relationship betrween AT (amp turn) and flux density (Guass). a rule-of-thumb may be used: 1 Gs per 1AT will activate teh switch when both magnet poles are near the two ends of the switch. so about 12 Gs is needed to activate a 12 AT reed switch. i feel, if the magnet is AlNiCo5 with the given size (magnetized along the length), at a distance of 13mm, you should have plenty of margin to make the switch close. if it is a RE magnet, you will get much more than enough field.
curious why you use 140 mm that long magnet? is your reed swtith super long?

### RE: Inspecting magnetic force necessary to activate a reed switch

(OP)
Thank you RyreInc for your feed back, I just need to let you know that yes it is a thru-magnetized along the length so the right side of the magnet length is one pole and the other is the other pole, about the type I am not that expert but i found that magnets are numbered N35 through N52 the more the number the more strength , but cost is crucial for me special that the magnet is relatively big.

IRstuff, thank you very much & I would like to tell you information that I am sure of which is the pick of my reed switch it is this model
http://www.digikey.com/product-detail/en/MITI-3V1%... and i need to activate two ate the same time with a 130 mm separation space between each other this is why i need the magnet that long, I am afraid I am restricted in the mechanical feature of the magnet because it is going to slide into 12*12 mm plastic trunk,

everyone, I think I have made a huge step through solving the issue permanently- if you agree with me-, i have redesigned my application so that the space between the switch and the magnet is only few millimeters of plastic trunk material,l will use two plastic trunks, inside the first is the Reed switch and inside the other is the sliding magnet and the two is stuck to each other back how does this sound everyone? I believe this would allow me to use even N35 magnet to activate the switch with this kind of magnet size do you think this will work?

### RE: Inspecting magnetic force necessary to activate a reed switch

Seems to me that the concept is somewhat inefficient. Don't you only need two magnets 130mm apart? The switch is only 7mm long, so one might think that two 25mm magnets mounted on a 140mm long bar would be a cheaper alternative than making a 140 mm long magnet.

### RE: Inspecting magnetic force necessary to activate a reed switch

(OP)
thank you IRstuff for your remark, to make things clearer for you and for all I need to mention that there shouldn't be any cut in signal in any switch at anytime so as you know the two switches are vertically above each other once the magnet starts to slide against the first one the first is activated and once the magnet reaches the second to activate the first would be still on , so this is basically the idea just to have a continuous operation.

### RE: Inspecting magnetic force necessary to activate a reed switch

I guess, for me, I would be looking for some flip-flops to do that function, rather than a gigantic magnet.

### RE: Inspecting magnetic force necessary to activate a reed switch

(OP)
Guys, I really need an answer to this question , I know that y-30 is a low force magnet but if i use it in 10 mm thickness wouldn't be enough to activate this reed switch from two layers of trunk PVC? because i found a cheap source for it and i need to know the answer has anyone tried to activate a reed switch with a Y-30 thick magnet?

### RE: Inspecting magnetic force necessary to activate a reed switch

I don't know much about prices, I just extrapolated from K&J's website, and this magnet specifically: http://www.kjmagnetics.com/proddetail.asp?prod=BZZ... I simply took the ratio of the volumes and multiplied that by the price. Also, a custom magnet will cost more than an off-the-shelf one, depending on quantity.

I also don't know about manufacturing feasibility--3mm is pretty thin, and sintered magnets are pretty brittle.

If space isn't an issue than go with the ferrite over the neo.

I would go ahead and just buy the parts and test them.

### RE: Inspecting magnetic force necessary to activate a reed switch

(OP)
thank you
it seems that going for 3-4 mm is unpractical because of weakness I am forced to go for the y-30 i found because i received a .38 USD price for this size without any holes or tooling , and this is certainly a good price for me, it goes up to 0.8 USD with express which is also an acceptable cost and i will stick to 10 mm thickness i will only have to stick a longer iron thin plate with a hole to do the hang the lift thread in this is the only solution i may stick it with double face in addition to the magnetic force, i hope this would prevent it from going right or left , what do you think ? if that wont work i would use a thin plate with three leads one vertical and two horizontal to be bent around magnetic but not from reed switch side but the other side so the magnet will still face reed switches do you think this will work because 9-14 USD is a very huge cost for my aim , if that works the only problem would be if that magnet vendor is nothing but a scam !

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