Winding Coils, Part II
Hello again, everyone. This month we will go through part two of David Metzs' letter on coil winding. Last month he explained Ohm's Law and how it applies to ignition coils. This month he discusses resistance, measurements and tolerances. Next month we'll get back to normal and I'll have more on the Gade build. Is this hobby great, or what?
David Metz writes: Let us now go back to the beginning and look at wire resistance. The resistance of a conductor is directly proportional to its cross-sectional area. My wire chart doesn't give figures for 18-1/2-gauge, but it does give the following for standard 18-gauge enamel covered wire:
Ohms per 1,000 feet = 6.3860 ohms
Current carrying capacity in bundles: 10 amps
To obtain more resistance for a given wire diameter, you have to use more wire. Or, to obtain more resistance (and more inductance) you can use a smaller diameter wire. Let's look at 20-gauge wire:
Ohms per 1,000 feet = 10.1280
Current carrying capacity in bundles = 7.5 amps
Thus, with a smaller wire, you would have more resistance for a given volume of wire, and since you can fit more turns on the core, you would have more inductance, giving you more "kick" to make your spark.
Measuring low resistance can be difficult. Many ohmmeters do not work that accurately at low values of resistance. There are two ways of dealing with this problem. First is to simply ignore it - electrical engineers routinely work with tolerances of 5 to 10 percent. "Close enough" is often all you need.
Or, you can measure the resistance indirectly using Ohm's Law. To do this, you need to be able to accurately measure the voltage applied across the coil and the current flowing through it. That is, you place an ammeter in series, close the contacts and measure the current. While doing this, record the voltage across the coil using a second meter.
It's important to note that battery voltage often drops under load. Thus, we have to monitor the voltage while making the critical current measurement.
A word of warning: If your meter is connected across the coil when you interrupt the current flow, a high voltage will be applied to the meter terminals, possibly damaging the meter. To prevent such damage, a "snubbing" diode should be placed across the coil to short the pulse.
Personally, if I wound your 1,200-turn coil and got a reading of 5.6 ohms, I would be quite happy with the result. The numbers fall right where they should on the wire tables I have.
Your use of insulating paper between the layers is the proper practice as well. After winding, the whole coil should be soaked in varnish to hold the turns in place and prevent moisture from penetrating it and causing a breakdown at high voltage. Ideally, the core should be made from insulated "soft iron wire" to prevent eddy-current losses.
Be sure your engine holds the igniter contacts open until just before top dead center. Only then should they close just for an instant to allow the magnetic field to build. Then when they open you will get your high voltage pulse. Keeping them closed all the time except for firing only heats the coil and wastes battery life.
Have a tip other model makers should know? Send it to Rusty Hopper at Gas Engine Magazine; firstname.lastname@example.org