homemade solenoid coil

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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

Re: homemade solenoid coil

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I's say the power supply's oscillating/pulsing.  How do you know it

John Fields
Professional Circuit Designer

Re: homemade solenoid coil

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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.

Re: homemade solenoid coil

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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 ?


Re: homemade solenoid coil

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The variation in votage is because it's jumping around, for whatever

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Re: homemade solenoid coil

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

Real Programmers Do things like this.

Re: homemade solenoid coil

On Sat, 03 Jun 2006 18:10:38 -0700, Jamie

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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.


Re: homemade solenoid coil

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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


Re: homemade solenoid coil

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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

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.


Re: homemade solenoid coil

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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!

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