Mad Teddy's induction coil projects

Mad Teddy's web-pages

Induction coil projects


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:

  Driving A Washing Machine With Motorcycle Power  

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!



"Old Sparky": my ignition coil circuit

My shocking ("medical") coil

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