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Electromagnet for reed switch (battery powered)

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kael

New Member
I'm looking for a small electromagnet that can trigger a reed contact.
This video shows a similar example:
The magnet should be as small as possible: shortest side max. 15mm the other side about 30mm, less is better.
The reed I need to trigger is encapsuled in a plastic waterproof cover, where the magnet can be attached to.
Also it should be possible to work for about 100h or so on a battery (about 4x 20ms/min), preferable 9V or 2xAA.
I looked extensivly on several location but without any luck.
Maybe a coil used in some kind of device is useable or it is ready available?
Any ideas anyone?
 
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Sceadwian

Banned
The video you posted is for a camera trigger, and it used a LOT of a current briefly. Is this meant to be held open continuously as it sounds like you're describing, or a certain number of pulses like the video describes?

I don't know what you have looked for extensively but you have not yet describe on a basic level your intentions! You're NOT going to find a perfect web site part match for the solenoid that will do what you want, you need to determine the force required to trigger the reed switch you want, and create a custom coil to meet your needs, you'll find virtually no 'off the shelf' parts for magnetic solenoid trigger coils.
 

ccurtis

Well-Known Member
Why not just secure a reed relay? It has the coil and reed switch built into a single unit.
 

cachehiker

New Member
What's the sensitivity of the reed switch in amp turns?

What are its dimensions? Are you planning on placing the reed switch inside the coil as shown?

You can calculate how many turns of a certain gauge you can wrap on a bobbin from a wire chart.

You can then calculate the DC resistance, the DC current, and the amp turns it will likely produce.

Aim for 20% headroom, make sure the coil and the battery can handle the resulting power and DC current, and then start winding.

At 4*25ms out of every second (10% duty), getting 100 hours out of pair of AA's will require the circuit to stay under 20-30mA.
 

kael

New Member
The sensitivitu of the excisting reed switch is not known. It can be triggered with a fridge magnet from about 12mm distance, if that is a help?
Where to start with the wire chart and calculation of the turns?
 

cachehiker

New Member
No. I have fridge magnets that will hold a veritable textbook and fridge magnets that are overburdened with a post-it note.

Pick up a piece of 3/8" OD copper tubing an inch long and flare the ends well.

One layer of 26 gauge magnet wire (16 mils) will be about 60 turns. Three layers will be about 180 turns.

180 turns * pi * (3/8" + (3 * 16mils)) ≈ 240 inches.
240 inches * 40.8 ohms/1000 feet ≈ 0.8 ohms.
3 volts / 0.8 ohms ≈ 3.75 Amps.
3.75 Amps * 180 ≈ 675 amp turns.
but 26 gauge can only handle a few hundred mA so you power it with a 200mA constant current source.
200mA * 180 turns = 36 Amp Turns.

That may or may not be enough to close the reed switch and will suck the battery dry in 10-15 hours.
I've worked with reed switches requiring 30 amp turns up to 90 amp turns. Most are somewhere around 50.

Anyhow, do you get it? You probably need 60 amp turns at about 25mA. That's about 2400 turns of 30 gauge. 2400 turns of 30 gauge would be around 280 feet.

BTW, don't count, find a 250 foot roll, concentrate on the winding process, and use it all. If you need another 5mA in the end, that's not a big deal.
 
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cachehiker

New Member
You can also scavenge the coil out of some reed relays. Most reed relays have pretty sensitive reed switches though, so you may not get 50 amp turns without overheating it.
 

kael

New Member
Okay your extended example gave me much better look on what to look at. I didn't had any idea about the Amp Turns but know I do and know what to look for when working with the reed.
Making my own coil is not so easy of course, it needs to be waterproof also.
I was thinking of using this one:
http://www.conrad.com/Kuhse-GTo18-0.5000-12VDC,-49-N-electromagnet,-12-V%2fDC-1.4-W-M3.htm?websale7=conrad-int&pi=503656&ci=SHOP_AREA_83990_0214811
It only gives an indication of its fors in N, no Amp Turns here.
Otherwise is seems the right size, IP65, low power, 12VDC... I hope it works on 9V too.
 
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cachehiker

New Member
Is it waterproof? If so it will probably work but you don't need a ferrous core for this application. The Magneto Motive Force (MMF) generated by the coil with an air core is sufficient since the field will concentrate itself in the reeds of the switch. If you're measuring sensitivity, it's actually undesirable since it will divert some of the field away from the switch and leave you with an unknown amount of MMF concentrated in the switch.

We use an older Hamlin reed switch test coil with 5000 turns. They're a bit pricey though. Prior to that we used a hand wound 1000 turn coil on a plastic bobbin for sewing thread. I suspect a good dip or three in conformal coating could've been used to make it waterproof.

BTW, I was distracted at work and didn't fully account for the 10% duty cycle. If the quiescent current of the remainder of the circuit is low enough, you could easily get 100 hours from a pair of AA's with 200mA of coil current.
 
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Sceadwian

Banned
If properly designed though a magnet with a good core will use far less power than an air core, especially for solenoid applications. I don't think I've ever seen a physical solenoid designed with an air core before unless the magnet has to pass through the coil itself.
 

kael

New Member
so the copper 3/8 tubing (or even plastic?) is just used fore the actual winding of the coil? Afterwards it can be removed or rest in place?

How needs the coil then orientated to the reed? Parallel along each other? Or at 90° angle, the reed at 1 of the ends of the wounded coil?
Where will I get best magnetic field?
What is the effect of the AmpTurns sensitivity when the reed in encapsuled in plastic IP65 housing? Do I need double as much then or what?

I did some previous testing with the coil of a small 12V relais (about 15x25mm) but I didn't managed to trigger a glas-reed even holding it against the coil.
And how can I regulate the current through the coil? If I put 9 or 3V over it through a BC 547.
 

ccurtis

Well-Known Member
@ccurtis: a reed relais will not work since I need to use an existing reed switch.
You could possibly connect the contacts of the reed relay in parallel with the existing reed switch without disconnecting the existing reed switch.

If you must use an external coil, you will need a significantly larger amount of energy from the battery when powering a coil placed next to a reed switch than a coil wrapped around a reed switch since the magnetic field inside the coil is considerably more dense than outside the coil (if the length of the coil was infinite, ALL of the flux would be inside the coil). And if you must use a coil placed next to the reed switch, it should have a soft iron core and be placed parallel to the reed switch. Although it concentrates the magnetic field inside the coil, the total magnetic field strength is increased by the iron core and the increased field radiates from the ends of the core around the outside of the coil where you want it to be.

Edit: You could reduce the amount of energy required by using a magnet for the core, passing the current through the coil with a polarity that aids the field of the magnet enough to close the reed switch. Of course, you will have to make sure the magnet/coil is far enough away from the switch so that the switch does not stick closed when the current is removed.
 
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cachehiker

New Member
Amp turns may be thought of as voltage. Reluctance as resistance. Permeability as inverse resistivity.

The magnetic flux will follow the path of least reluctance. If the only ferrous material near a coil is a reed switch in the middle of it, the flux lines will be concentrated there even if the coil has a somewhat large inside diameter (within reason) relative to the switch. That's why I mentioned copper (non-ferrous) tubing. It's easy to find for purposes of experimentation, easy to cut to length, and easy to flare the ends. A plastic spool or bobbin is better but will be harder to get. The big drawbacks are copper is conductive and flared ends will only allow for so many layers of winding. OTOH, we're dealing with low voltage so enameled magnet wire should provide enough insulation. The aforementioned 2400 turns of 30 gauge is 24 layers though and it might be too much for a simple flared end. If the IP65 enclosure is non-ferrous, it shouldn't affect things much at all.

I would've personally expected most 12V relay coils to activate the reed switch but I've never tried it. OTOH, it's designed for a different application where it may have a considerable amount of leverage to pull in the contacts. Was it perchance a small low power relay coil or a monster contactor coil?

The most efficient way to limit current is to the wind the coil with the right DC resistance for the available voltage. Stepping down the voltage with some kind of regulator will always waste some of the energy available from the battery.
 

alec_t

Well-Known Member
Most Helpful Member
but you don't need a ferrous core for this application
True if the reed is inside the coil; but in this app it will be alongside the coil, so I think a ferrous core will be necessary.
 

kael

New Member
Yes, that's correct the reed contact is fixed to a alloy frame and also encapseled in a plastic waterproof housing. So I also was wondering if it it possible to use coil without core in this kind of setup.
 

kael

New Member
@cachehiker, I looked at again at you calculation example from 13th july, I understand your explanation but I can't figure out how you came to 2400 turns of 30 gauge for getting 60AT at 30mA, 3V? Can explain this figure please?
 

colin55

Well-Known Member
How many cycles do you want? The reed switch has vey limited life.

you could easily get 100 hours from a pair of AA's with 200mA of coil current.
No you can't. 100hrs x 10% duty cycle (four 25ms pulse in a second) x 200mA = 2AHr The cells have less than 1AHr when drawing 200mA.

You can buy a reed switch for $3.49 fromRadio Shack
Model: 275-233 | Catalog #: 275-233

0.5-Amp SPST Reed Relay at 12VDC

Price:
$3.49


Fast response, small size and low cost make this 0.5 amp relay a good choice for projects that requi...
See the Full Product Details
 

cachehiker

New Member
I made an error somewhere. I should quit posting from work where I constantly have too many windows open.

2400 turns * pi * (3/8" + (24 * 10mils)) ≈ 4640 inches = 386 feet (oops! transcription error.)

3V / (386 feet * (103 ohms / 1000 feet)) ≈ 75 mA.

2400 turns * 77mA ≈ 180 Amp Turns.

Is that about what you were getting?


Let me try this again with 36 gauge:

1" long / 5 mils ≈ 200 turns per layer

2400 turns * pi * (3/8" + (12 layers * 5 mils per layer) ≈ 3279 inches = 273 feet

3V / (273 feet * (414 ohms / 1000 feet)) ≈ 27 mA

2400 turns * 27mA ≈ 65 Amp Turns.


If I get the time I'll sketch something out illustrating the ferrous core effect. It's really not much though. If the coil can activate the reed switch in a given position with the core, it (in theory) will activate it in the same position without the core and vice versa. The real difference is in the switching speed, the stored energy, and how big of a flywheel diode you need to put in parallel with the coil.

We use about a million reed switches and solenoids a year where I work. Nobody else could be bothered with working out some of these effects so I did.
 
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ccurtis

Well-Known Member
If I get the time I'll sketch something out illustrating the ferrous core effect. It's really not much though. If the coil can activate the reed switch in a given position with the core, it (in theory) will activate it in the same position without the core and vice versa.
I'm interested to see that. According to the classic equation for magnetic field strength, B, in the center of the solenoid: B=kμ[SUB]o[/SUB]nl where k is the relative permeability of the core material; equal to 1 for air, and 5000 for iron. n is the number of turns. l is the current in amperes. μ[SUB]o[/SUB] is the permeability of space.
 
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