trying to make an electromagnet

Discussion in 'Electronic Basics' started by andy, Jul 11, 2004.

1. andyGuest

I'm trying to make an electromagnet for a project i'm working on. The
thing i'm trying to build is an automatic plant watering system for a
greenhouse. What I want is to have a tank of water empty into a drip feed
hose once or twice a day.

The way i'm thinking of doing it is to have the bottom end of the tank
permanently connected to the hose, with no valve, then fill the tank
through a standard ballcock valve. The ball will have a short metal rod
glued on it, with a loudspeaker magnet or similar placed above it so that
the magnet normally holds the valve closed, with the ballcock up. The idea
is to have an electromagnet winding round the metal bit on the ballcock,
which i can send a pulse of current through to cancel out the attraction
from the permanent magnet, and make the valve drop down. Then the tank
will fill up until the ballcock sticks closed again, and slowly drain into
the hose.

All of which seemed like a good idea until i started trying to make an
electromagnet - i can't make it anywhere near strong enough.

What i've done so far is to get a 250g reel of enamelled copper wire
(26SWG/0.45mm) which luckily has both ends exposed without having to take
it off the reel, and build this circuit:

+6V-------------
|
wire
reel
|
|----------
| |
10ohm LED
resistor |
| |
| 200 ohm
| resistor
| |
0V-------------+

I chose the 10ohm resistor to give a current of about 600mA - from what i
can find out, the rated current for this wire is around 800mA.

The led lights up, which shows that there is a connection through the wire
reel.

When i put a galvanised steel bolt inside the reel with the current on,
it attracts metal objects _very_ weakly (only just noticeable), but
nowhere near enough to do what i want.

car central locking solenoids etc. run off 12V and produce a fair amount
of force.

any suggestions on what i'm doing wrong and whether this idea can be made
to work would be appreciated.

2. JR NorthGuest

It might be easier to use a small DC gearmotor to operate
the valve. A reversing circuit with limit switches would
mirror the action you want.
JR

3. andyGuest

The trouble with doing it that way is if the power goes (it will be
running off a solar panel), then the valve could get stuck open. I'm
trying to make it so the cycle of filling and emptying the tank and then
stopping happens purely mechanically, with the electrical bit only used to
kick off the cycle. then if the power dies, the worst that can happen is
it sits there doing nothing.

5. andyGuest

Thanks for the suggestion. i think i'll stick with the magnet idea for a
while, because doing it that way gives a bit of extra pull to hold the
valve closed beyond the end of the normal travel - if i use a latch
there's going to be some slack taken up when the water drains again, and
the valve may end up leaking slightly.

The other way i thought of was to just use a permanent magnet, and have a
solenoid pushing the ballcock away from it to start the watering cycle.

6. JamieGuest

i have been reading the thread here and i think what you
need is a simple Toilet flush value system that can be use to
refill the primary tank after it has emptied into the lower
run off tank.
the lever can be yanked by a door solenoid via a timer
circuit running off a solar charged battery system.
the pulse is only long enough to drive a large transistor to
pull a 6 volt coil.
the battery he uses can hold enough charge for aprox 4 days of use
with no solar charging since the initial pull is only a short brief
moment to get the water flushing which prevents the plug from dropping
in the hole until it's empty.

i know because i helped a friend set up a system like this
that uses no commercial power at all.

7. Rich GriseGuest

I don't know why you seem stuck on a permanent magnet and homemade
solenoid, especially since your first solenoid yielded such dismal
results. Personally, I'd rely on gravity to hold the valve closed;
I can't think of a circumstance where a permanent magnet would be
better than something that's just lying there. And for a solenoid,
especially if you're planning on putting the thing into actual use,
I'd definitely buy something - shop around surplus places, repair
shops, and car junkyards - something that just sprang to mind is
a car trunk or gas lid solenoid. When I was a kid, we took apart
a washing machine. What a parts gold mine! I doubt if they're as
cool these days.

Anyway, continue to learn about solenoids and magnetism and
permeability and reluctance and all those various black arts, but
for production, KISS. ;-)

8. andyGuest

i guess partly because that's the idea i thought of after considering
various other things (e.g. buying a proper solenoid valve - expensive and
needs current to hold it open; windscreen wiper motor on a normal valve -
might get stuck open if the power goes; etc.) So having decided to try
that i want to see if i can make it work. Also if i can make this work,
the total power consumption will be very low - it should just need a
single very short pulse of high current to start the watering cycle, with
barely any current needed between cycles, which is good for a solar
powered system.
maybe, except then to open it you need something that can push against
gravity for as long as the valve is open. which means more amp-hours used
per day.
i thought of central locking or starter motor solenoids. maybe i'll give
something like that a try if this doesn't work.

9. andyGuest

I found out last night i hadn't built the circuit the way i thought - when
it's done right and a thick steel rod used for the core it is starting to
give a reasonable amount of force. Also the coil has more resistance than
i thought - about 20 ohms, which makes the current a fair bit less than
the wire's rated value. so if i rewire it as two separate coils, i should
be able to get more oomph.
P.S. sorry if it seems like i'm just giving loads of reasons not to take
your advice - i have been taking in what you've all said, it's just i want
to try it the way i thought of first and see if it works. The thing i
could do with some idea about atm is whether / how much i could go over
the rated current for the wire if i keep it in a short pulse. Also what
kind of metal rod would make a good core for the solenoid - i'm planning
on getting a thick steel bolt for this.

a standard solenoid won't be any good for the idea i thought of i think,
because the magnet and the solenoid are separate parts in my design.

10. Terry PinnellGuest

As you're sticking with the electromagnet approach, I'd focus on the
core you're using. A 6" nail is a pretty good choice. Knock it about a
bit (like bang it with a hammer on a vice), as that apparently
improves its magnetisation potential. Experiment with winding lots of
wire on it, and periodically checking to see how it performs. Once you
have it capable of supporting the ballcock, you can move on to the
next stage of your idea, namely cancelling a permanent magnet's
attraction. I'm not at all sure that stands up - have you seen it
done?

I appreciate your curiosity is now probably driving you, but I reckon
I would either adopt one of the other suggestions, or at least look
hard for a surplus electromagnet, rather than making your own.

11. andyGuest

i've got it working with a bit of steel rod as the core and the back of a
small speaker as the magnet. but this isn't going to be strong enough to
hold up the ballcock reliably, so i need to get a stronger magnet and
increase the current in the coil a bit. I'm already using a whole 250g
reel of 26SWG wire, so i don't want to add any more turns to the coil.

my idea at the moment is to use one of those small neodymium magnets and
put that on the ballcock, with the coil mounted in position, rather than
the way i thought of first.
maybe. part of the thing is i want to publish the design when it's done,
so using standard parts would help with this.

12. John FieldsGuest

---
Check this out:

http://www.floatvalve.com/dimensions.html

These are cheap (<\$10) float valves designed to be used to shut off
the flow of liquid when a tank fills, but by mounting them
upside-down, capturing the float when the tank empties, and not
letting it rise when the tank fills (until you release it) they'll
work the way you want. What I'd do would be to affix a small
permanent magnet to the float in such a way that it would be attracted
to the core of an electromagnet mounted to the bottom of the tank so
that when the tank emptied and the magnet and the core of the
electromagnet got gloser and closer, the mutual attraction between the
magnet and the ferrous core of the unenergized solenoid would be
enough to overcome the opposing force generated by the buoyancy of the
float as the tank later filled. Then, later, when the tank filled and
you wanted to empty it, what you'd do would be to apply a pulse of
current to the electromagnet which would cause its core to magnetize
so that the polarity of the end of the magnet and the end of the core
closest to it would be the same. with a strong enough pulse, the
attraction would be nullified (or made repulsive) and the float would
rise, allowing the tank to start emptying. Later, when it emptied
enough, the magnet would again be strongly attracted to the solenoid
core and the cycle would start over.

If you wanted to get slick, you could even use a level detector to
determine when the tank got full and use its output to fire the
solenoid... Voila!, fully automatic, unattended watering with nature
supplying all of the electricity and water!

If you didn't want to wind your own electromagnet you'd probably be
able to use the coil from a relay, and I'd suggest that no matter
which you did you pot the whole thing to make sure you won't have any
problems because of the thing's being immersed in water for years.(?)

If you _did_ want to wind your own electromagnet what's going to
determine the force it can develop will be the number of turns of wire
you wind around the core, the current through the wire and the
permeability of the material you use for the core.

The fusing current of #26 is about 20 amps and it has a resistance of
about 41 ohms per thousand feet at 20°C, so assuming that it can
survive a short 10 amp pulse out of your 12V supply every once in a
long while means that the shortest piece of wire you can wind on the
core will be about 30 feet.

13. andyGuest

that's like what i'd thought of in reverse - maybe i'll try that too.
that's the key bit i needed to know - i thought it was much lower, like

15. andyGuest

does that mean encase it in resin? is it best to do this while you're
winding it, or afterwards?

16. Rich GriseGuest

An ordinary toilet doesn't need the handle held to finish the flush (at
least in the US, if it's in good repair).

http://home.howstuffworks.com/toilet3.htm

When the flush valve opens, it floats, because it's hollow, and water
can get up underneath it and buoy it up. When the water runs out, the
valve falls closed.

You need one tug, which can be done with that pulse that you've
described. Charge a cap all day, and click-kerflush!

So take a whole toilet mech - you can use the input valve to stop the
filling - I think they're sold separately anyway - and put it in the

17. Rich GriseGuest

How Dare You! You Inconsiderate Lout! I slave and sweat and toil and work
and think and create and cogitate and theorize, and This is the Thanks I
Get? Humph! [0]
Well, like I said, go ahead and do what you're thinking of - I'm curious
to see how it comes out.

As far as iron, I've read that "soft iron" makes good cores/pole pieces
for this stuff, because it's got good permeability, but doesn't stay
magnetized when you turn it off. As far as getting some, well, your
google is as good as mine.

18. John FieldsGuest

---
Fusing current is the current which causes the wire to get hot enough
to melt, and maximum rated current is the current which causes some
specified limit to be reached, usually the temperature or the
resistance of the wire. They're interrelated, and since copper has a
positive temperature coefficient of resistance, the hotter the wire
gets, the more its resistance will increase.
---
---
You have to know what the specification is for and how it's being
defined before you can draw any meaningful inferences in what you
think the spec is saying. For instance, a 300mA spec may be relevant
to a piece of #26 stranded PVC insulated wire in a 20°C ambient
temperature environment and may have more to do with the insulation
melting than anything else, while the 600 mA spec for a piece of solid
#26 Heavy Formvar insulated magnet wire in a 20°C environment may have
more to do with the change in resistance of the wire.

20. andyGuest

what's the difference between fusing current and rated max current? I
found a web page that puts the rated current at about 300mA for 26SWG. I
calculated about 800mA based on rated current for another wire that had
the value given in the maplins catalogue. And then i found a page that
says fusing current for 26SWG is 15A, which is a lot higher - i would have
thought the difference would have been more like 2-3 times higher.