I've always thought it would be funny to build scale-size exploding grain silos for a model train layout. I've also had problems recently with pilot flames blowing out in some of our larger blow-something-flammable-through-something-on-fire projects. Both of these things made clear to me that I needed a good source of high-voltage sparks. So I built a buzz coil, a project derived from the ignition on a Model T that you can toss together to satisfy all your sparking needs with a just a few common automotive parts.
The basic transformer theory is laid out in my previous posts about Variacs and Automotive Ignition Coils. To understand the buzz coil, the key point is that a changing current generates a collapsing magnetic field in a transformer. That collapsing magnetic field induces a voltage in the secondary winding of the transformer. In the case of an automotive ignition coil, we're talking about very high voltage.
That's why the basic design for the buzz coil is lifted from the ignition system of antique engines, like the ones in a Model T Ford. More recently, the design was tossed around by these guys on an Old Marine Engine forum. I decided to build their buzz coil.
How it Works:
Things like momentary switches and relays use terminology like Normally Open (NO) and Normally Closed (NC). Normally Open means that the electrical connection is not made in the default state; no current will flow through those terminals when the switch or relay is in its idle state. Normally Closed means the opposite; current flows unless the relay is activated.
In this buzz coil, we make use of the fact that the Bosch 5 pin relay has both NO and NC terminals. (The more common 4-pin relay has only a NO terminal.)
Our relay is wired such that power to the relay's coil is connected through the Normally Closed circuit. This means that as soon as power is connected, two things happen: Power goes to the positive terminal of the ignition coil, which is also connected to the NC terminal, and the relay is activated. The relay, now activated, moves its internal arm to make the NO connection, thereby breaking the NC connection. Breaking that connection disconnects the power to the relay's coil. When the relay's coil de-energizes, the spring loaded arm returns to its default position. In this position, the NC circuit is completed once again and through it, the battery power is connected back to the positive terminal of the ignition coil and the relay's coil is re-energized. For as long as power is connected (or until the relay catches fire), the relay oscillates between these two states as fast as it's mechanical components will allow. This rapid oscillation means that the 12V power from the battery is being connected to and then disconnected from the positive terminal of the ignition coil over and over again, very quickly. The negative terminal of the ignition coil is connected to ground. Connected in this way, the ignition coil experiences a rapidly changing current through it's primary, which results in a high voltage being induced in the secondary winding. That secondary is connected to the spark plug.
As the relay turns power on and off to the ignition coil over and over again very quickly, the changing current turns into a series of high voltage spikes at the spark plug. It is very similar to what happens in a points-style automotive ignition system.
One seemingly extra part here is the condenser. The condenser is connected across the Normally Closed terminals of the relay just as it is connected across the points in an automotive ignition system. It helps to prevent sparking across the points as they open (inductors like the winding of the ignition coil tend to resist rapidly changing voltages and don't take kindly to the points opening).
How to Build It:
Connect the ground terminal of the battery to the housing (the threaded part at the base) of the spark plug to complete the circuit.
Follow the schematic below. If you look closely at the relay in the schematic, you'll see that the terminals are numbered. While the relay will not physically look like the one in this schematic, you will see those same numbers next to pins on the bottom of your relay.
Ignoring those words of wisdom, here is what I did with mine:
Vin attempts to simulate a grain silo explosion with flour and a high voltage spark generator. He fails and has to resort to carburetor clearner.
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.