An ignition coil (also called a spark coil) is an induction coil in a vehicles ignition system which transforms the batterys 12V (6V with ballast resistor coils) to the thousands of volts needed to spark the spark plugs. When the contact breaker closes, it allows a current from the battery to build up in the primary winding of the ignition coil. The current cannot flow instantly because of the inductance of the coil. Once the current has built up to it's full level, the contact breaker opens. Since it has a capacitor connected across it, the primary winding and the capicitor form a tuned circuit, and as the stored energy oscillates between the inductor formed by the coil and capacitor, the changing magnetic field in the core of the coil induces a much larger voltage in the secondary of the coil. More modern electronic ignition systems operate on the same principle, but some rely on charging the capacitor to around 400V rather than charging the inductance of the coil. The wire which goes from the ignition coil to the distributor and the wires which go from the distributor to each of the spark plugs are called spark plug wires or high tension leads.
In modern systems, the distributor is omitted and ignition is instead electronically controlled. Much smaller coils are used with one coil for each spark plug or two spark plugs for one coil. A large ignition coil puts out about 20K. These coils can be remotely mountd or they may be placed on top of the spark plug (coil-on-plug or direct ignition.) Where one coil serves two spark plugs (in two cylinders), it is through the wasted spark system. In this arrangement the coil generates two sparks per cycle to both cylinders. The fuel in the cylinder that is nearing the end of the compression stroke is ignited, whereas the spark in its companion that is nearing the end of the exhaust stroke has no effect. The wasted spark system is more reliable than a single coil system with a distributor and cheaper then coil on plug.
Testing Ignition Coils
Specifications
Coil #1 No. Lucas 16C6 Coil #2 No. Diamond F-088
Coil #1 Voltage: 6V Coil # 2 Voltage: 12V
Coil #1 Primary: 1.3-1.5 Ohms Coil #2 Primary: 0.72-0.88 Ohms
Coil #1 Secondary: 7-10K Coil #2 Secondary: 10.89-13.31K
Coil Test Results
Coil #1 Primary: 1.3 Ohms Coil #2 Primary: 0.72 Ohms
Coil #1 Secondary: 7.63K Coil #2 Secondary: 12.85K
Coil #1 Earth Leakage Test: OL Coil #2 Earth Leakage Test: OL
Wasted Spark Coil Pack
Coil #1 Secondary: 12.5K Coil #1 Primary: 0.7 Ohms
Coil #2 Secondary: 12.53K Coil #2 Primary: 0.8 Ohms
Coil #3 Secondary: 12.67K Coil #3 Primary: 0.7 Ohms
Testing Ballast Resistors
Specs Measured Resistance
Ballast Resistor No. BR1 Ballast Resistor ohms: 2 Ohms
Ballast Resistor ohm spec: 2 Ohms
Wiring a ballast resistor in series with my Lucas 1C6C coil I measured the voltage drop across the ballast resistor and the amount of current draw through the circuit.
Current draw: 3.39A
Coil Calculated Voltage Drop: 1.3 x 3.39 = 4.407V
Measured Voltage Drop: 4.77V
Ballast Resistor Calculated Voltage Drop: 2 x 3.39 = 6.78V
Measured Voltage Drop: 4.9V
My result was not to my calculations. When I connected the Power Supply to the Coil, the Voltage supply dropped to 10V. I realise now that this was due to the inductance in the coil.
Wiring up igntion systems
Our next task was to test different types of igntion coils such as wasted spark, coil on plug, Kettering type ignition and building an ignition circuit with transistors. This test was to teach us how different ignition systems worked on cars. We also used a function generator that we could dial up and down to increase the frequency of the spark.
Using a function generator, I wired up a ignition coil with a sparkplug and electronic ignition module.
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Then I did the same experiment, but instead of a function generator, I used a magnetic distributor. So when I rotated the shaft on the distributor, I would get a spark from the sparkplug.
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Then I wired up a ignition system using wasted spark and a function genrator.
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Then I wired up a circuit using Coil on plug.
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Now it was time to build a simplified ignition module with two NPN transistors that replaced the Electronic Igniton Module. A resistor was also used.
Video deleted due to PDF copy
At first our circuit would not work. We checked for open circuits or shorts on our leads and could not find any, so we then checked our components. We then found out our transistors had shorted, and needed replacements. We installed our two new transistors, but our circuit would still not go. We then realised because the primary ignition winding was drawing so much current, that our transistors couldn't operate with the added resistance from the resistor. We then took out the resistor and the circuit worked fine.
If I was going to build this circuit again, I would consider replacing the function generator with a digital distributor. That way I can see that depending on how fast I spin the distributor shaft, I can see the transistors switching on and off.
