PLEASE NOTE
If you've come here by way of my
High-voltage projects sub-menu
, you will have seen my warnings on electrical safety. If you haven't read
this yet, please click on that link and do so before proceeding.
Induction coils, in one form or another, have been around for a long time -
well over a century and a half. Rather than giving a history myself, I've
decided to refer you to
this link
,
which does it well and provides some excellent photographs of induction
coils from various eras. (If you follow the links from this page, there is
some absolutely fabulous historical stuff - take the time to check it
out. Seriously!)
Alfred P. Morgan, in "The Boy Electrician", in Chapter XII:
"INDUCTION COILS", gives instructions for making a "medical coil" and a
"spark coil". The first of these is not too hard to make; the second looks
like a lot of very tedious work which can all go to waste if it's not
done very carefully and with meticulous attention to detail. Quite honestly,
I can't see myself having the patience to undertake such a challenge, so you
won't find instructions for doing so here (not in the short term, anyway).
It's possible to create something which serves as a very reasonable spark
coil from a car ignition coil. This obviates the need for the huge amount of
very careful winding needed to make such a beast from scratch; and it's
possible to pick up a second-hand ignition coil quite cheaply from a car
wrecker. One day, after he became aware of my growing interest in such
things, my Dad brought one home and assembled it - with a few other bits and
pieces - into a very respectable spark coil; see the first menu item below.
A.D.Bulman, in "Model Making for Young Physicists", gives instructions for
"An induction or shocking coil", which corresponds pretty much exactly to
Morgan's "medical coil". My Dad and I did build a shocking coil based to
some extent on Bulman's instructions; this is the second menu item below.
So, what is an induction coil, and how does it work?
This link
provides a good introduction, along with some historical background.
It's a kind of step-up transformer, but it's not intended to take a
normal sinusoidal AC input and output another sine-wave at a higher voltage.
No: the idea is to make a really high voltage just for an instant,
and then repeat the process over and over. In a spark coil, you get a series
of quite noisy and rather threatening-looking sparks; in a "medical coil",
the basic idea is the same, but the coil itself (usually) and its output
voltage are both considerably smaller.
If you're not sure what a transformer is, have a look at
this link
which gives a reasonably simple introduction. If you really know very
little about electromagnetism, consider reading my
Electromagnetism - an introduction
page (which also contains the transformer link just mentioned).
Like an ordinary step-up transformer, an induction coil has a primary
winding with relatively few turns, and a secondary winding with lots
of turns (thousands of turns in a spark coil; turns-ratios of about
100:1 are common in auto ignition coils).
In an ordinary step-up transformer, an AC (alternating current) input
produces an AC output. As a direct result of the turns-ratio, the output
voltage will be greater than the input voltage.
An induction coil, however, is quite different in its operation. In order to
produce a series of short high-voltage "spikes", rather than a smooth
sinusoid, we arrange for a DC input to the primary to be continually
switched on and off. It's the switching off that performs the "magic".
Note, however, that one of my two induction coil projects - the shocking
coil - actually uses an AC, rather than DC, power supply. It still generates
high-voltage spikes; and it is these which provide the "shocking"
experience, rather than the very small AC component which gets through to
the secondary. See the relevant page - a menu item at the end of this page -
for more details.
The core and primary winding may be considered to be an electromagnet. As
mentioned in my
Electromagnetism - an introduction
page, transients occur when a DC voltage is switched on or off:
In an induction coil, when the switch-on transient occurs, the magnetic
field builds fairly quickly, but not generally quickly enough to produce a
really high voltage across the secondary. However, at switch-off, the
damped-sinusoid transient associated with the collapse of the magnetic field
is quite rapid. Dramatically high voltages are induced for that very short
time in the secondary. Just what we want!
The method of switching the power on and off varies from one induction coil
to another. Again, I recommend
this link
which illustrates the point very clearly
[note: this no longer works - please see update above (except that it
now does work again - yippie!)].
When I was in high school in the mid-sixties, there was a big black spark
coil which I seem to recall having seen operating, very briefly, just
once. (I think the teachers were probably frightened of it, and
reluctant to use it!) It was almost certainly able to produce nice big, fat,
noisy sparks at least two inches (five centimetres) long.
It was of the "traditional" type, with its own interrupter mechanism - so
that, essentially, the primary / interrupter combination formed a big
buzzer. With current flowing in the primary of such a beast, the core
becomes a magnet and attracts the interrupter so that the current is
switched off. Being spring-loaded, the interrupter flies back and switches
it on again. Then the cycle repeats. At each such switch-on or switch-off
event, a transient occurs, with the switch-off transients producing the high
voltage output from the secondary, and thus causing the sparks.
The voltages produced across even the primary by the switch-off transients
can present a problem. Of course, they're nowhere near as large as those
across the secondary; but they can be big enough to cause arcing between the
switching contacts. This is undesirable for two reasons: firstly, it
represents wasted energy which is not getting to the secondary, where it's
wanted; and secondly, the sparks can cause damage to the switching contacts
themselves, basically from oxidation.
It's possible to do something about this. Most induction coils have a
capacitor connected across the switching contacts. This solves, or at
least significantly reduces, the problem there.
A capacitor is basically two metal plates with a small gap between them. The
gap is filled with an insulating substance called a dielectric. These
days, capacitors come in all shapes and sizes. In the early days of
electrical research and technology, they were rather big and clunky, and
known as "condensers". The concept was that electric charge could be
condensed into them. (Actually, the term was in use until
comparatively recently; I can remember my Dad using it, and not being
completely comfortable with the term "capacitor", even while he was
explaining to me how they worked, in the 1960's!)
So, how does a capacitor help in reducing primary-generated sparking?
Initially, before the power is switched on, the capacitor is shorted by the
contacts. At switch-on, the first transient occurs, and the current in the
primary produces a magnetic field. This opens the contacts, so that the
capacitor is no longer shorted by them but becomes a part of the circuit, in
series with the primary. Thus the current going through the primary starts
to charge the capacitor. While this current is flowing, the magnetic field
is maintained, and the contacts have time to open wide enough so that a
spark won't form easily between them.
As the capacitor's charge approaches its maximum possible value, the current
rapidly drops off and the second transient occurs - the magnetic field
collapses and a large voltage is generated across the primary. Remember: the
contacts are still open at this stage - and fairly widely so, because of the
action of the capacitor as just described. Now, without the magnetic field
to keep them open, they begin to spring shut again - but by the time they
actually approach each other closely enough so that a spark might form
between them, the high-voltage transient has already happened and most of
the energy has been dumped into the secondary, to appear as a spark across
the spark-gap.
When the contacts actually do close, the capacitor is discharged and the
process begins again.
Of course, the first transient does have an effect on the result. While it's
happening, it does induce a voltage across the secondary - in the opposite
direction to the initial voltage caused by the second transient. However,
that voltage is so small in comparison that it scarcely matters -
overwhelmingly, it's the second transient that has by far the greater
effect.
It's not exactly simple to understand, is it? There's a lot happening within
a short period, and timing is obviously crucial. I still find it surprising
that such an inherently simple circuit can cause so much puzzlement. But
that's electromagnetism for you!
Choosing the right capacitor is a balancing act. If the capacitance
(measured in farads, after Michael Faraday) is too small, so that the
charge it can hold is small, it will charge up too quickly - and the
contacts will not have had enough time to move very far apart before the
magnetic field collapses. So a wasteful, destructive spark will occur
between them after all.
On the other hand, if the capacitance is too large, it takes more energy to
charge it - energy that therefore doesn't go into the magnetic field. When
the field eventually does collapse, there's less of it to do so, with the
result that the second transient is less intense than we'd like, and the
voltage produced is correspondingly not so great. [A somewhat non-technical
way to describe the overall effect is "soggy", as opposed to "snappy" (but
not too "snappy") when things work as they should.]
To be honest, even that is an over-simplification. The capacitor itself has
transients of its own when charging and discharging; and of course these
have their own effect on the overall operation of the induction coil. Just
adding the capacitor complicates things more than you'd probably think.
Circuits involving both inductors (coils) and capacitors - both in series
and in parallel - have their own "resonant frequency"; indeed, that is the
basis of radio and other electronic devices.
I've yet to see a really satisfactory explanation of everything that
goes on in an induction coil. I make no claim to do any better than anyone
else in this regard!
A rule-of-thumb figure that is often given for a good capacitance value is
0.1 microfarad (i.e. one ten-millionth of a farad; one farad is a
huge capacitance and values are often given in microfarads - or the
still smaller units of nanofarads or picofarads). Car ignition capacitors
are normally around 0.1 microfarad. (Note: somewhat quaintly, the
auto industry is one area where the old term "condenser" is still quite
often used!)
These days, of course, with modern electronic techniques, it's possible to
switch the current off very quickly without contact-points and the
associated sparking - so that big voltages appear where they're wanted,
across the secondary, with no messing about. This is the principle of
electronic ignition in cars. The higher the voltage of the spark at the
spark plug (and thus the more power delivered there), the better the fuel
burn, and the better the efficiency of the engine as a result.
On one hand, ignition coils - and other inducton coils - are quite simple
devices. On the other hand, as we've seen, it turns out that - in common
with many electromagnetic phenomena - explaining how they work can be quite
tricky.
This link
contains comments by a number of contributors who each have their own
"take" on ignition coil theory - among other things! Well worth a read, and
quite amusing in parts.
Are you interested in building your own induction / spark coil? (It can be a
big job!)
In "The Boy Electrician", Alfred P. Morgan gives instructions for making a
spark coil from scratch. He says that the core should be made from a tight
bundle of "soft" iron wires (soft in the magnetic sense, meaning that they
won't hold onto any magnetism they may experience). To make them soft, he
recommends bringing them to red-heat in a wood fire and allowing them to
cool very slowly, covered with ashes.
Most things I've read about induction coil cores give pretty much the same
general idea. The core should be made from "soft" iron and laminated in some
way, to reduce eddy currents as far as possible.
The primary is normally wound on first. It consists of perhaps a couple of
hundred turns of thick copper wire.
The secondary, consisting of thousands of turns, is made from very
thin wire. Great care is needed; because of the very high voltages which (we
hope!) will exist across it when it's working, each layer is carefully
insulated from the previous one, using old-fashioned-sounding things like
card soaked in melted beeswax! (It staggers me that people in Victorian
times were able to make decent spark coils at all - but they did!)
The work can be simplified, to an extent, by making the secondary up from a
series of short coils called "pies", which are slid onto the primary when
complete and soldered together in series. Presumably, if one develops a
problem, it's less work to rewind just that one than to replace the whole
monstrosity if it's in a single piece! (The one described in "The Boy
Electrician" is made from just two "pies" - as is the one described in the
final off-site link at the end of this page. Coincidence...?)
Building the interrupter presents plenty of challenges of its own...
Seriously: do you really want to get involved in something this
long, tedious, and messy?
Some do! Here are links to the work of three such enthusiasts:
Induction coil builder #1
Induction coil builder #2
Induction coil builder #3
A few comments about these "Induction coil builder" links:
The first describes a modest project using a ferrite flyback transformer
core. This might be a good way to start, if you're planning on trying to
build induction coils. (Interestingly, in this version, the secondary is
wound first, with the primary wound over it, like in an
ignition coil - see
this site
for a helpful diagram.)
The second shows a rather neat setup with a home-made induction coil used to
power a smallish Tesla coil (which is even equipped with a "magnifier"!).
Good explanation, with photographs, of how to make the induction coil
secondary from "pies".
The last of these three links leads to quite a big website which features an
in-depth analysis of induction coil phenomena. Several photographs of an
oscilloscope screen showing transients, somewhat as in my graphic above -
but with greater attention to detail (showing quite a bit of evidence that
there's more to be said about how these beasts work, if we're honest.) This
webmaster's big spooky induction coil, also made from "pies", is a must-see!
The site also features his Tesla coils. Some deliciously scary stuff here.
I wish you the very best of luck if you want to join these intrepid souls,
and build your own induction coil. I'd like to myself; but do I have the
patience...? (Well, if I ever do, you can bet I'll post the details here,
presumably as a third menu item.)
A long time ago, as you'll see from the two menu items following, my Dad and
I took a much easier road, and still managed to get some satisfying results.
To close these comments:
One last link
on induction coil construction. This one appeals to me. It's a set of
detailed instructions for building your own monster, originally published in
1959. It has that same lovely "olde-worlde" feel to it as Alfred P. Morgan's
"The Boy Electrician". Yummy! Even if you never build the thing, just
reading the article will probably bring a smile. (The site also describes
lots of other science projects.)
UPDATE, Wednesday, 8th December 2010
I've just revisted this page and noticed that there are some links which
don't work any more. When I get a bit of time to spare, I'll attempt to
effect some repairs...
In the meantime:
By idly typing "shocking coil" into Google, I've found
this
page which relates to something from that wonderful old bygone era which
still holds a fascination for me. The article, entitled "How To Make An
Induction Coil", apparently first appeared in something called "Boy
Mechanic Vol1" which, it seems, was once a part of the "Popular Mechanics"
phenomenon.
By clicking on the
Table of Contents
link further down the page, I found myself in an absolutely wonderful
environment in which there are further links to a vast number of
other "things to make and do" pages of the type that appeals to "boys"
like me. For example, check this out:
Have a look around the site, and lose yourself for a while in that lovely
old world when life was simple, and kids took the trouble to find sometimes
quite hair-raising things to keep them (more or less) out of mischief long
before such things as violent video games became all the rage!
INDUCTION COIL PROJECTS
"Old Sparky": my ignition coil circuit
My home page
Preliminaries (Copyright, Safety)
Induction coil projects
Driving A Washing Machine With Motorcycle Power
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