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Applying Ohm’s Law: V = voltage, I = current in amps, R = resistance in ohms.

Hello again, everyone. Well, my experiment building a coil, which I discussed in the September 2005 issue, caught the attention of model engine fan David Metz, Muscatine, Iowa. David, who seems to have a lot more experience on the subject of wiring, was nice enough to send me a long letter detailing what I need to know. This is good, basic information I know we could all benefit from. So here’s the start of David’s letter, and we’ll finish his thoughts next issue. Is this hobby great, or what?

Ohm’s Law

David Metz writes: Rusty, my opinion is that you did everything right. You had the right wire, the right number of turns, the right core and the right insulation between layers of windings. My guess is that your only problems are in your measurements. Read on:

You made the comment that you were attempting to reach a value of 2 volts on the contacts of the igniter using 12 volts on the coil. I think what you wanted to say is that you wanted to have a current of two amperes flowing through the coil and battery in a simple series circuit. The relationship between the voltage and current in this circuit can be defined with Ohm’s Law (P = power in watts, E = voltage, I = current in amps, R = resistance in ohms):

R = E/I: Voltage divided by amperage = resistance in ohms
I = E/R: Voltage divided by resistance (ohms) = amperage
E = I x R: Amperage times resistance (ohms) = voltage

And for determining power:
P = I x E:
Amperage times voltage = power dissipated in Watts

So, with 12 volts across the coil (contacts closed) and a 6-ohm winding resistance, you will have a current flow of 2 amps.

What we are trying to do is convert current flow through the coil into a magnetic field and store electrical energy as a magnetic field. Once the field has built up (it takes microseconds), no more energy can be stored. This is important, as keeping the contacts of the igniter closed more than needed simply wastes electricity.

In this case 2 amps multiplied by 12 volts would cause our coil to dissipate 24 watts of heat. The coil would get quite hot with that much heat being created inside of the windings. Note that this is pure resistive heating. You would get the same dissipation of heat if the same amount of wire were stretched between two poles.

With the contacts open, current flow drops instantly to zero. The magnetic field that has built up around your coil collapses and the magnetic energy is converted back into electrical energy making your contacts spark as they open.

The more voltage you apply across your coil, the stronger the magnetic field and the greater the energy stored in the magnetic field.

Thus the coil designer has some trade-offs to deal with. First, the coil resistance must be high enough to limit the closed contact current. This is done both to reduce coil heating due to resistive losses and to increase battery life. Second, the amount of energy stored in the magnetic field must be great enough to make sufficient spark when the contacts open. Third, the inductance of the coil must be great enough to produce the desired amount of inductive “kick” to make the spark. The more inductance, the more spark the coil will generate when the magnetic field collapses.

After thinking about this for awhile, I began to imagine some engineer, back around 1900, spending long hours snapping an igniter and winding coils trying to get a practical working system together.

Next month we’ll continue with David Metz’s discussion on windings and wire resistance.

Have a tip other model makers should know? Send it to Rusty Hopper at Gas Engine Magazine;

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