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Old Sep 24th 2017, 04:56 AM   #11
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Originally Posted by yafimski View Post
Hi,

Thanks for the answers.
I would like to keep the discussion in the realms of informative responses. I think we've cleared up explosions enough

So far I've increased from 65 turns to 130 turns around the nail. I'm using the 9v battery. How much current should I use? (since there is still no effect of the electromagnet on the neomagnet).


Thanks!
You haven't said how big this battery is ??? if it's the small 5x3x2 cm type , you're wasting your time....
connect your coil across a car battery , that should do something ...pay no attention to fear mongers , they have no hands on experience in the real world......

It would be more sensible to choose a cylinder magnet of about the same diameter as the nail ...

And the diameter (AWG) of the coil wire is important , I assume you're using enameled copper wire .
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Old Sep 24th 2017, 05:52 AM   #12
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Yeah i'm using the small 9v battery.
As a caution according to what the others said here, I'd like to ask a second opinion.

Does anyone have an alternative suggestion to a car battery? Seems exaggerated to me... it's probably possible in a safer way / less voltage.

Thanks
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Old Sep 24th 2017, 01:09 PM   #13
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Originally Posted by yafimski View Post
Yeah i'm using the small 9v battery.
As a caution according to what the others said here, I'd like to ask a second opinion.

Does anyone have an alternative suggestion to a car battery? Seems exaggerated to me... it's probably possible in a safer way / less voltage.

Thanks
There are DC sources which you get from an AC outlet by a transformer/rectifier circuit. Why not use a car battery? Just limit the current with a high power resister. The amount of resistance is determined by how much current you want. All I said was simply not to short out the car battery. I didn't suggest not using one.
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Old Sep 24th 2017, 05:25 PM   #14
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Originally Posted by oz93666 View Post
To get a decent magnetic field you have to match the coil resistance , number of turns , and battery, to get the best output ...
What are you talking about regarding "match coil resistance"?

Originally Posted by oz93666 View Post
.. thicker the wire , lower resistance , so more current ...but thicker wire means less turns in a coil ...
Who cares what the resistance of the wire is since it plays a very very small role in the amount of current going through the wire. Current has to be limited in order not to short out a battery by placing a resistor in line. That can only be avoided if the wire is so long that there is sufficient resistance due only to the wire. If so then one adjusts the voltage to get the desired current and thus the desired magnetic field.

And there is no reason whatsoever that a thicker wire means less number of turns since the wire can be wound around itself thus increasing the number of turns per unit length, n.
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Old Sep 24th 2017, 07:27 PM   #15
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Originally Posted by Pmb View Post
What are you talking about regarding "match coil resistance"?.
We want to maximise the magnetic field created ....it's proportional to I x n .... current times number of terns in the coil ....

You can use thin copper wire , so then we have many terns , high n ... but then this coil has a higher resistance , which will limit the current ....

thicker wire allows more current , but you can't get many turns in the same size of coil ....

So the starting point is the power source what is the voltage? and what is it's internal resistance? ....

A small 9v battery has too high an internal resistance to get a reasonable current , you won't get over about 1A !!! ... if that's the only battery available then you must use very thin (enameled) copper wire ...figure out how much of this wire has the same resistance as the internal resistance of the battery , and use that length to make the coil ....

But as I said , my experience tells me the 9v battery won't do the job ...

It would save yafimski a lot of wasted time if he explained what exactly he's trying to achieve...

An electric lock for a door ? that opens when energised ? if so what power do you have ??? it all comes down to power source ...

You can use mains supply ( through a simple diode) to energise the coil (very big coil, fine wire , can be bought on eBay)
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Old Sep 24th 2017, 07:57 PM   #16
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Originally Posted by oz93666 View Post
We want to maximise the magnetic field created ....it's proportional to I x n .... current times number of terns in the coil ....
Then you used the term "match" incorrectly. By the way, there is no maximum magnetic field since there is maximum number of turns or maximum current. The field can theoretically be as large as one wants, within physical limits.

Originally Posted by oz93666 View Post
You can use thin copper wire , so then we have many terns , high n ... but then this coil has a higher resistance , which will limit the current ....
He can also use gold wire since its conductivity is greater than copper. Then again its clear that he wants to be realistic and the gauge of the wire isn't of great importance.

To imply the current is limited implies that there is a minimum voltage. In any case such a limit is not realistic for the reason I explained above.


Originally Posted by oz93666 View Post
thicker wire allows more current , but you can't get many turns in the same size of coil ....
So what? Who says that the could has to be a particular size?

Originally Posted by oz93666 View Post
So the starting point is the power source what is the voltage? and what is it's internal resistance? ....
There are many such sources of power as I already explained.

Originally Posted by oz93666 View Post
A small 9v battery has too high an internal resistance to get a reasonable current , you won't get over about 1A !!!
So? Who said otherwise? It's silly to think that one would use a 9V battery to generate a very strong magnetic field and the whole purpose of a Neodymium magnet is that it be a strong magnet.


Originally Posted by oz93666 View Post
... if that's the only battery available ..
irrelevant. The OP never mentioned that he wanted to limit what he uses as a source of power. Please stick to the subject at hand.

Originally Posted by oz93666 View Post
It would save yafimski a lot of wasted time if he explained what exactly he's trying to achieve...
He already did. Read the OP. Other than that its none of our business.

Last edited by Pmb; Sep 24th 2017 at 09:51 PM.
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Old Sep 27th 2017, 12:13 PM   #17
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Originally Posted by oz93666 View Post
You can use mains supply ( through a simple diode) to energise the coil (very big coil, fine wire , can be bought on eBay)

Can you suggest a particular name or product to buy on ebay? What is a 'mains supply'?

Thanks for all the comments so far!
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Old Sep 27th 2017, 12:20 PM   #18
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Originally Posted by yafimski View Post
Can you suggest a particular name or product to buy on ebay? What is a 'mains supply'?

Thanks for all the comments so far!
I assume by "mains supply" he's referring to the electric outlet one finds in homes, offices and factories.

Go to Amazon and search for "power supply" and choose what you'd like to use. While I understand that you'd want the greatest magnetic field that you can generate, its not very realistic since there is no upper limit to it. What you should do is set a limit for yourself and then build to that limit.

You'll also need a power resistors to put inline with the coil to limit the current.
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Old Sep 27th 2017, 02:29 PM   #19
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From what I understand, the 9v battery is roughly 500mAh, so that means it's wattage is around 4w or 5w.

Since I am trying to influence the magnetic field of the neomagnet, and it's just one magnet, I'm thinking of buying this power supply:
65W AC Adapter for Lenovo IdeaPad Yoga 11 13 2pro Power Supply Battery Charger | eBay

Can I just get this kind of computer charger, split the end and take out the wires and connect them to the electromagnet? 65W is 10x more powerful than I have now.

I'm also not sure how to know the magnetic force of the neomagnet. The product page doesn't say anything about that...


Thank
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Old Sep 27th 2017, 03:36 PM   #20
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Originally Posted by yafimski View Post
From what I understand, the 9v battery is roughly 500mAh, so that means it's wattage is around 4w or 5w.
Where did you get that figure from? It's incorrect to say "the 9v battery is roughly 500mAh' since that depends on the brand. Some batteries are better than others. Hence the different costs. And "mhA" refers ti the amount of energy stored in a battery. The ever ready battery is supposed to keep that bunny going "forever" meaning longer than other brands.

See http://data.energizer.com/pdfs/l91.pdf

You can get more amperage from putting 9V batteries in parallel.

Originally Posted by yafimski View Post
Since I am trying to influence the magnetic field of the neomagnet, and it's just one magnet, I'm thinking of buying this power supply:
That's a battery charger. Not a power supply. What you want is something that will hold a current steady and that means a good DC power supply. A battery charger may or may not do that. All its supposed to do is charge batteries. Do you know for certain that it will hold a constant current under a particular load?

Originally Posted by yafimski View Post
Can I just get this kind of computer charger, split the end and take out the wires and connect them to the electromagnet? 65W is 10x more powerful than I have now.
What do you mean by "computer charger"? Split what ends?

You appear to be going to great lengths to avoid getting a solid power supply. Is that due to limited funds?

Originally Posted by yafimski View Post
I'm also not sure how to know the magnetic force of the neomagnet. The product page doesn't say anything about that...
A magnet doesn't have a force. All it does is create a magnetic field. The force you're thinking of is the force that it exerts on other objects. That's difficult to calculate. It's not like there's a well-know formula for the force on an object due to a magnet. It's more complicated than that. There are several parameters which need to be given in order for it to be a well-stated problem. Even then I know of no formula for it. The field generated by a magnet is complicated. There are approximations used when the field is generated by an object. E.g. a distances which are large compared to the size of a bar magnet one can use the dipole approximation. Then you need to specify what the object is acting on, etc. Not easy stuff with ready formulas.


I suggest that you find a solid reliable power supply which is affordable. The experiment from there.
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