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homemade solenoid coil

hi folks,

i'm having some really bizarre behavior out of a homemade solenoid, please read and help!

i've got some really thin "magnetic wire" from radio shack, i think it's 30 gauge, that i'm using to create a solenoid coil. what i did was took a drinking straw from one of those kid's box drinks, the really thin straws, and i wrapped two layers of the wire around the straw and solder the ends to a breadboard and attached leads to it. then i stuck two different sized nails inside the straw and applied current from a 12 volt radio shack power supply (actually 14 volts). i am touching the coil so i can disconnect it when/if it gets hot because it isn't protected by any kind of resistor or anything.

the behavior i get is unusual, or at least i didn't expect it. i apply power and i get this rhymic kind of "jump" out of the nail. it will get pulled to the center of the coil and kind of align itself so that equal amounts of the nail are on either side of the coil. if i push the nail out of the coil it will jump back into the coil, which is great. but the unusual thing is that the behavior is in pulses, it's not like there is a constant kind of "push" or "pull", but instead it sort of sits there and has sort of a "push, push, push, push ..." kind of heartbeat to it every second or so, or 1/2 second maybe. it's cycle is very predictable. if i tilt the coil so that the nail kind of slides out i can get it in equalibrium where the nail will slide down, get jerked back up, slide down, jerked back up, etc, and move like a cylinder in a car engine, up and down.

so my question is, i suppose, why does it pulse like this instead of just having a constant force ? i was thinking it was something to do with overcoming friction, that the nail would just sit still until some kind of potential was high enough to make it overcome the friction it has against the straw, but that's not it. intuitively, just watching it, i can tell this is not what's going on. another thing i thought is maybe the power supply was pulsing, but that doesn't seem to be the case either.

what's going on ?

a basic electronics group seems like the right place to ask this question.

Reply to
purple_stars
Loading thread data ...

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I\'s say the power supply\'s oscillating/pulsing.  How do you know it
isn\'t?
Reply to
John Fields

John Fields wrote: [snip]

hi john,

i can't say about the power supply one way or the other. i just now tried to verify it and i don't know. i started with the coil disconnected from the supply and measured the supply and it says 14 volts. then i hooked up the coil and i can't get a reading anymore, the digital meter just jumps around to different numbers and won't settle out anywhere, i can't even tell what it is approximating.

Reply to
purple_stars

john, you had it, it was the power supply i think. i changed over to a

12 volt "wall wart" and i got the expected behavior, the nail would move to the center of the coil and then would just sit there. if i pushed against it, it offered resistance, not pulsed but a constant steady resistance though i did feel a "hum" against my finger.

i'd like to make this work better. here are a few issues i'd like to work on if anyone has any kind of suggestions to make this better.

1) it gets warm, the coil does. i think a resister in series would cut down on this, maybe ? or should i reduce the voltage with a voltage regulator or how should i do it ? it gets warm pretty fast when attached to the 12 volt wall wart. i'm pretty sure it would melt the insulation on the wire if i left it connected up for a few minutes. 2) how do i get more power out of it ? i know i can add turns. somewhere i read that using pure iron instead of steel is better. does the diameter of the nail matter ? also does the proximity of the nail to the sides of the straw matter, there is a fair amount of room in there at the moment because the straw was bigger than the nail is. should i get a bigger or smaller nail and/or straw to add more power ? 3) also, what about voltage .. is higher voltage better, worse, indifferent ?

thanks.

Reply to
purple_stars

Or it's periodically current-limiting, shutting down, and restarting, either an active current limit in a switcher or a thermal shutdown in a linear regulator.

John

Reply to
John Larkin 

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The variation in votage is because it\'s jumping around, for whatever
reason.
Reply to
John Fields

sounds like your over loading the filtering on your power supply and getting a 60 hz pulse..

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Reply to
Jamie

hook a small 12V lamp, (or a LED and a Resistor) in parallel with the coil. it seems to me that the powersupply you have is interracting with the coil in some way. (I expect you'll see the light going on and off as the powersupply tries to recover from your mistreatment of it)

you're heavily overloading that powersupply I once built a coil similar to that (except I used surplus 24ga telco wire in three layers on a 100mm (long) nail) and my dad hooked it to a "solid" 12V supply (it was a car battery). a fraction of a second later the coil was a cloud of vaporised PVC.

build a coil with more layers of windings and/or put a resistor or a 12V lamp in series with it, this will reduce the load, and hopefully you shpould get closer to "textbook" behavior

Bye. Jasen

Reply to
Jasen Betts

Hello purple stars,

I haven't been on this group in awhile, but all the advice given you above makes sense. According to physics, B=unI, where u is the permeability, iron has about 200 times more then air. n=number of turns per meter (ie take the number of turns you have and divide by the length in meters or cm and convert to meters) I=Current, that's the biggie. Oh, and B is the magnetic field.

If you want to increase the field, you increase any of those 3 quantities, but you can also use larger wire (say, 22 ga) with what you are doing now, you will increase the field because the wire can carry more current.

I believe your first problem was what John Fields said, either that or your power supply was shorted and was bouncing back and forth. I just built a couple of solenoids yesterday. A steel core is much, much better than air if you cannot get iron.

Rather than putting a resistor across the input leads, you may use a capacitor ( a big one, say 1000uFd, but be very careful). Rig it so the supply charges the cap, then disconnect (with a switch), the supply. Then, using a switch, discharge the cap thru the solenoid coil. To test it, put a compass next to it (say within 10 cm) and watch that needle flip flop. You could also just switch the power from the cap to the coil and keep the power supply connected. It's not so much the voltage, but the current that gives the field. I was also using a couple of 6V lantern batteries from wally world, and they were dead in no time. Then I connected an 8 pack of NiMhs and they got really hot really fast. I was trying to correlate the movement of a compass needle in degrees to the field I was generating with the solenoid. Magnetic fields follow the inverse square law just like gravitational fields Also, you may want to google for solenoid, electromagnets, etc. Have fun.

Joe

Reply to
Joe

That's nowhere near thin enough, unless you want to run several amps through your coil. This is why it gets hot and loads down the supply. Look around for some #36, or old relays, dishwasher solenoids, pinball machines, whatever. The amount of magnetic force, so to speak, is proportional to ampere-turns, which is #amps * #turns. Use more turns of thinner wire, and you won't waste so much energy as heat, which is I^2*R. :-)

Good Luck! Rich

Reply to
Rich Grise

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