This is probably one of the easiest high voltage projects I have ever stumbled across. For the cost of an ignition coil and a trip to RadioShack, I was able to intermittently interfere with antenna reception in my household for years.
The cool thing about car ignition coils is that they are essentially miniature solid-state Tesla coils—just like their older cousin, they essentially convert a low voltage, high current power source (such as a 12V, 10 amp car battery) to a high voltage, low-amperage current (such as that irritating 1000V, .003 amp shock you get from a doorknob). Purists will stop me here: technically speaking, Tesla’s original patent called for an open-air circuit, with energy transfer through a “resonant coupling” between two coils with no electrical connection (like a radio antenna and receiver). But the legacy of the Tesla coil lies in the general idea of transformers (the electrical component, not the Hasbro automatons), which are vital to almost every modern electronic device.
Transformers (ie, increasing or decreasing voltage while doing the opposite to current) rely on one of Nikola Tesla’s most important inventions: alternating current. We’ve all heard the description of the electrons wiggling back and forth in the wire instead of flowing though, but that description obscures the true beauty of AC. A better way to visualize AC is as waves in the lattice of metal atoms. When I took high school chemistry, transition metals were typically described as a “sea of delocalized electrons.” This epithet explains many meaningful properties of metals: they’re literally just a fixed atomic lattice containing a bunch of electrons that constantly float around, uncommitted to a single parent atom . This means that something like an incident electric field can very easily cause the electrons to move: in DC current, a constant electric field makes all the electrons move in one direction. While this carries kinetic energy very well, a common misconception is how it does so: electrons move incredibly slowly, and so the current is not due to a torrent of electrons suddenly cascading through the circuit. Rather, the observed current comes from a large number of electrons getting a tiny boost in velocity– their power is in their sheer numbers, rather than by their individual speeds. In fact, it actually takes about 4 hours for a given electron to make the round trip from your light switch to your ceiling fan– most of the power that travels in the circuit is due to large numbers of slow electrons arriving at the light bulb at the same time.
In alternating current, it’s not the particles themselves carrying the energy—it’s a disturbance in them. In fact, when you flip a light switch, an energetic electromagnetic field travels through the wires in your wall at the nearly the speed of light (because, after all, light is an electromagnetic field). It might slow down some due to other interactions with the metal in the wire (light also slows down when it travels through non-conducting materials, like distilled water), but for all practical purposes the change in current travels super fast. Tesla’s AC was brilliant because it exploited the fact that the particles don’t even have to move at all to carry a lot of energy. Going back to the sea metaphor, imagine how long it takes a single water molecule to travel from Russia to the US, even if currents are steadily eastward. Nonetheless, things like tsunamis can still do a lot of damage; not because of individual particles carrying energy over the entire ocean, but rather because every particle between Russia and the US jumps up and down a little bit to carry the wave. Likewise, when you hit an AC switch, you basically trigger a series of well-timed kicks of energy to the metal lattice (the wire), each of which sends a nice wave traveling through the sea of electrons. Because the light bulb converts the energy of the sloshing electrons into heat and light, you need to keep making waves (using power) in order to keep it going.
The cool thing about kicking the electrons is that the resulting current changes in time– and so it can cause other things to change in time as well. Within an ignition coil, a magnetic field is set up in a wide set of wire loops every time a kick passes through that part of the circuit. When the current stops (the wave passes), the magnetic field collapses. Tesla thought to put a second coil inside the first, so that this collapsing magnetic field has somewhere to go. Just as running current through a coil created a magnetic field, running a dying magnetic field through a different coil will create a current. But say that I’ve made my second coil out of 100 turns of wire, while my original coil only had 10. The voltage induced on my second coil will be 10x my original voltage, and my current will be 1/10 it’s original value. In ignition coils, this ratio is more like 1000:1, allowing me to take a modest input voltage and make a nice, long spark.
It’s important to note that the two coils have no electrical connection between them– the magnetic field literally reaches one coil from the other and transfers the energy (a concept that is the basis of radio communication). Thus ignition coils have four terminals: a primary coil where you can connect a battery, and a set of secondary terminals that are the high voltage outputs.
There’s just one caveat: the magnetic field has to begin to die in order to be resurrected on the second coil. This means that the current that created the field has to come on and off as well– a requirement perfectly suited for AC current. The trouble is that a car battery is DC, and so all it does is pour electrons, rather than make waves.
Your car solves this problem by turning the DC on and off really, really fast using a device known as a distributor, which causes the current to stop and start several times each time a wheel rotates. Each cycle makes a new kick of energy pass through the circuit and thus creates a new ignition spark in the engine each time (and thus creating a steady, continuous set of rapid combustions that allows the car to run smoothly). This used to be done mechanically, but recently integrated circuits have largely replaced physical distributors.
For my project, I faced a similar issue of creating adequate waves in my ignition coil. If I attached the coil to a 12V lantern battery and then rapidly connected and disconnected my leads, I would get sparks, but only as often as I was able to manually connect and disconnect the switch. I considered using modern integrated circuits to do the switching for me, as occurred with distributors in cars, but I realized at the time that I barely knew how to send an email, let alone program an integrated circuit.
A trick I found online is to use what’s known as an electromagnetic buzzer, or a circuit that keeps trying to disconnect itself. Common devices known as N.C. relays have four terminals: two for control, two for output. The control terminals are electrically connected, as are the output (although the latter have a special magnetic switch connecting them). If I connect DC power through the two control terminals, it activates an electromagnet that then pulls open the output switch. If I disconnect power, the field goes away, and so the output terminals close up again. Thus in an ambient state, the N.C. relay is Normally Closed—anything connected to the output terminals will have a continuous circuit through the terminals.
A good way to consider this is that the N.C. relay is a logical NOT gate: if I have power through the inputs, then power is not allowed to run through the outputs. If the inputs lack power, then it is possible for power (in a separate circuit) to run through the output terminals.
The electromagnetic buzzer uses an NC relay to set up a paradox: what if a circuit were to run into one input terminal, out the other, back in one output terminal, and then out the other and into a circuit? Running power through this configuration would definitely provoke the electromagnet, which would then open the switch- and thus break the circuit. Since the magnet then turns off, the switch closes again—causing the magnet to turn back on, the the cycle to repeat. This means that, if I wire a common relay up correctly, I can end up with an electromagnetic paradox, a circuit that constantly strives to turn itself off.
This is the basis of the electromagnetic buzzer, which gets its name from “buzzing” between on and off. If I were to plot the shape of the current in the circuit over time, I would get square waves, where the current runs for a short period at a constant DC value (making a straight line on the plot at this value), and then stops for another regular short period (making another straight line at zero).
Returning to the ignition coil, this means that all I need to put AC through my coil is my battery, the N.C. relay (correctly wired as a buzzer), and my coil. You can see in the video that this process creates a constant spark at a fairly low frequency (you can see it jumping, for example). In true Tesla coils, the sparks jump so fast that they give the illusion of being animated or continuous (at which point they are called “streamers” because they appear to stream out of the terminal).
For an even higher output voltage (3-5 inch sparks), another coil can be connected in reverse parallel with the original coil primary terminals (ie, in parallel with wires crossed). Sparks will jump between the output terminals of the two coils because they will have opposite polarities due to the reversal.