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 Author Topic: Solenoid performance  (Read 589 times) 0 Members and 1 Guest are viewing this topic.
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 « on: November 05, 2012, 10:11:02 am » Bigger Smaller Reset

My son is trying to make a solenoid to 'launch' a 1cm ball bearing as part of a physics project.  We got some magnet wire and wound it on a spool, planning to use a large spike to provide the movement.

My questions:  generally, are you looking for high current, high voltage or (yikes) both in order to generate the maximum magnetic field?  Is DC or AC better (his idea was just to use household current but that is a bit scary to me)?  We tried a 6V SLA battery last night and didn't get anything.

Also, what kind of resistace would you expect from the winding (we used the full 1/2 lb spool of magnet wire...don't know how much length that is)?  I put an ohmmeter on it and unless I am reading it wrong, it only seemed to be about 150 ohms.  In my mind that would create a pretty good short.

The other option would be a store-bought solenoid, but we are trying to move the bearing quite a distance vertically and it looks like most of the smaller ones have very little throw (<1 cm)...which wouldn't be a problem if they can provide decent enough force.
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SW Scotland
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 « Reply #1 on: November 05, 2012, 11:56:10 am » Bigger Smaller Reset

Firstly, magnetic force is a linear function of Ampere-Turns  ie amps time the number of turns of wire.  So the answer to your question is AMPS.  You either need more amps, more turns or both.   Of course adding turns means increased resistance so to keep amps up you also need to increase voltage.   You say your coil is 150 ohms.   That is way too high to get a decent force from a 6 volt system.   You should be looking at a pulse current in the order of several amps, which at 6 volts implies a resistance of around 1 to 3 ohms.   So, a lot thicker wire required.  You also need a hefty battery, something like a small car starter one.

Secondly, you need DC

Third, don't even think about using mains power - you clearly do not have the knowledge (and I'm not being disrespectful)

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 « Reply #2 on: November 05, 2012, 12:04:55 pm » Bigger Smaller Reset

To get more current flow you can cut your magnet wire into smaller pieces and wind them in parallel. If you cut your wire into 3 pieces you would have a 50 ohm coil. 5 pieces a 30 ohm coil. 10 pieces a 15 ohm coil. I think you can see where the math is going here. as the resistance lowers the current will go up and will be divided btween the multiple wires. You can also experiment with winding the coil around a piece of steel or iron pipe. And you may have to play with timing your pulse (be careful, you will also produce some pretty good sparks from that coils, and they can shock!!!
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 « Reply #3 on: November 05, 2012, 01:38:05 pm » Bigger Smaller Reset

Thanks for the replies.  I also found an excellent article that covers many of the same principles.

http://www.ledex.com/basics/basics.html

Also a calculator:

http://www.daycounter.com/Calculators/Magnets/Solenoid-Force-Calculator.phtml

To get more current flow you can cut your magnet wire into smaller pieces and wind them in parallel. If you cut your wire into 3 pieces you would have a 50 ohm coil. 5 pieces a 30 ohm coil. 10 pieces a 15 ohm coil.

Would these parallel sections be physically adjacent to each other (i.e. side by side along the length of the coil)?

Third, don't even think about using mains power - you clearly do not have the knowledge (and I'm not being disrespectful)

No disrespect taken.  I am actually quite comfortable with mains power in 'regular' application.  I know enough to see that my 150 Ohm coil would only draw around 1A (which I now know is not enough).  So cutting the resistance in half or a third might do the trick, but then the question is about heat dissipation.
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 « Reply #4 on: November 05, 2012, 04:42:28 pm » Bigger Smaller Reset

No disrespect taken.  I am actually quite comfortable with mains power in 'regular' application.  I know enough to see that my 150 Ohm coil would only draw around 1A (which I now know is not enough).

If you are comfortable with mains power, then you could use a SCR to connect rectified mains power across your 150 ohm solenoid for a short period.
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