Low-Tension Ignition Explained,
Making Your Own Coil,
And Oscilloscope Pictures
Above is a schematic of a typical low-tension (ignitor) ignition system. I've shown it with a "spark saver". Spark savers are generally used only on hit-and-miss engines to conserve battery power. The spark saver is usually a contact on the exhaust rocker arm that closes the battery circuit to the frame of the engine (shown as the ground symbol) when the exhaust valve is closed. When the governor latches-up, the exhaust valve is held open and the battery circuit is interrupted so the ignitor does not draw current.
Also included in the schematic is an indicator which should tell if your ignitor is working. The indicator is NOT necessary to the operation of the ignition. The neon lamp fires (lights-up) at about 60 Volts. A characteristic of neon lamps is that, before they fire, the resistance between the terminals is extremely high. At the time it fires, the resistance between the terminals of the lamp becomes very low. The resistor is in the circuit to limit the amount of current drawn by the lamp circuit to keep it from shorting out the ignitor.
A low-tension coil is usually composed of a core of a bunch of lengths of soft iron wire. Over this core is wound a number of turns of relatively heavy gauge copper wire. This arrangement is called an induction coil or choke.
You Wanna Make Your Own Low-Tension Coil?
First off, you want to figure the amount of wire you'll need for your coil. You can use the following chart:
Here's how you work the chart. Say, you want to wind a coil that will draw a half-Amp at 6 Volts and let's say you have a bunch of 20 gauge copper magnet wire laying around and want to use it for your coil.
To find out what the resistance should be for this, use the formula Resistance=Voltage/Current. Divide the Voltage (6-Volts) by the current (0.5 Amp). This gives 12 Ohms. To find out how many feet of wire you will need, divide the resistance (12 Ohms) by the resistance per foot (0.0119) Ohms per foot for 20 gauge wire). This gives 1,008 feet, which is the amount of wire you'll need.
Now you've got to get some idea of how long the core should be in order to hold this amount of wire. A good guess is about as good as anything. The length or the core and the final diameter of the coil after it's wound don't make a whole lot of difference in how well it works.
When you build your coil, make your core out of any soft steel but a bundle of soft iron "mechanics" wire works really well. Cut and straighten enough lengths of this to make a bundle at least 1/2 inch in diameter (the more the better, up to about an inch or so). Before you bundle the wires, a more efficient core can be made if you dip them in thin paint or some other insulating material and let it dry thoroughly before bundling them together. (Not to get too technical, insulating the core wires minimizes what are called "eddy currents" in the wires.)
The hard part of this project is the actual winding of the wire on the core. Take your time here - this is critical! You first must insulate your core using something like thin plastic sheeting or wax paper (I used a couple of layers of clear packing tape). After you do this, lay the magnet wire against one of the end caps and tape it down so it will not un-spool as you wind the coil. Leave enough wire at the end and secure it so it won't get bent as you wind the core. There's nothing as frustrating as finally finishing your coil and then finding that the starting end has become so brittle that it breaks off flush with the coil!
When you wind the wire on the core, do NOT wind it in a random fashion. If you do, you will surely get a lot of shorted turns because of the pressure of the wire turns against themselves will break through the insulation causing short circuits. Shorted turns are death to a coil! Wind the turns on the core evenly side-to-side as close to each other as you can get them. They can touch each other but CANNOT overlap. When you get to the end of a layer (your coil will have several layers on it), hang on to the loose end and wrap a thin layer of insulation over the layer you've already wound. Now, wind back to the other end and continue winding a layer and insulating 'til you've got all the wire on the coil. Wind in the same direction for every layer.
To finish off your coil and protect the top layer from damage and the entire coil from moisture, wrap it with tape or string. That's all there is to it!
Above is a low-tension coil I made. The core is a 1/2 inch diameter bunch of soft iron "mechanics" wire. After fitting the end blocks over the core and securing it by driving some pointed short wires into the ends of the core, I wrapped the core with black tape. Then, I wound several layers of 18 gauge magnet wire, enough to give a resistance of about 7 Ohms. I separated each layer with cellophane packing tape for insulation. I then wrapped the windings with cotton twine and soaked it in polyurethane. The coil draws about 3/4 Amp at 6 Volts. The lever switch is made of stuff from my junk box and the binding posts are available through a number of sources.
I can use this coil as-is with either a 6 or 12 Volt battery. I can also use a 12 Volt auto brake light bulb in series with the coil to limit the current to keep it from getting hot and to give the battery longer life. If your engine is hard to start because of oil fouling of the ignitor, etc, wind your coil for about 1-1/2 Amps and wire it so the lamp can be switched-into the circuit after the engine starts. This gives an extra hot spark for starting and, after it is running, the lamp will keep the coil from getting hot..
Oscilloscope Pictures of A Typical Ignitor System
The coil used in these tests is a genuine Fairbanks-Morse factory ignitor coil.
The picture above is of the current build-up when the ignitor points close. It takes roughly 10-thousandths of a second for the magnetic field to build-up to 3 Amps after voltage is applied. Theoretically, no spark should be generated when the points close because there is zero current until the points actually make contact and then it's too late for an arc to occur.
(The spark you see when you connect an induction coil to a battery is caused by what is called "contact bounce". When the two wires or contact points first strike each other, they bounce off each other several times and this tiny, repeated gap causes a small spark to occur.)
This picture is of the voltage spikes that occur when the ignitor points snap open. The voltage spikes are over 300 Volts, due to the magnetic field collapsing and turning the coil into a generator when the points open. You will see several spikes. There are actually many more spikes than shown above because they are very close together. The whole process is over when the magnetic field of the induction coil has decayed to near zero and no more voltage is being generated.
Several voltage spikes are generated because the voltage builds-up faster from the collapsing magnetic field than the points can open and multiple arcs occur. The spark that is generated is essentially a very hot plasma with near zero resistance. This plasma shorts-out the coil for an instant until the voltage falls to a point (and the points open to a larger gap) that the arc can't sustain itself. Then the circuit is open again and the voltage rises again, this time to a higher value because the points have opened more in the tiny interval since the last arc.
It looks like ignitor points open fast but this process of generating multiple arcs all happens VERY quickly, much faster than the points can open as you will see in the next pictures.
All of the following pictures look different because they are of different events.
I've expanded the horizontal (time) speed of the 'scope in the above picture so each horizontal division represents an interval of 50-millionths of a second. The spikes are so close together that you still can't see all of them.
I've speeded-up the trace speed of the 'scope in the above picture to 20 millionths of a second per division and you can almost see all of the spikes generated from the multiple arcs at the ignitor points.
This last picture shows the spark voltage spikes at a speed of only 10 microseconds per division. It's no wonder an ignitor spark is hot. Just one of these little sparks contains a relatively small amount of heat energy but, a whole bunch of them in a very short time can make enough heat to easily ignite a fuel charge.
Now, if you substitute a resistor (or light bulb) for the induction coil, you will get a pretty puny spark. Because there is no magnetic field to collapse (or a very small one), the voltage will be no higher than that of the battery and there will be only one or two of these low-energy sparks generated w;hen the points open. I believe that most of the "fire" generated when you short two wires together without an induction coil in the circuit is caused by oxidizing metal and not a plasma arc. Also, it takes a lot more current to make a hot spark without the use of an induction coil.
Anyway, that's how I see it.