Ignition Coil High Voltage Experiment

Electronics has always been in my blood!

Electronics has always been a passion of mine. In grade school I used to make circuits with batteries, switches, and motors. In 7th grade science class we actually got to build a single wet cell battery and perform different experiments with it. I chose Vocational Electronics as an elective in my Junior and Senior year of high school. I also experimented in electronics workshop, located in my parents basement, which consisted of an folding table, soldering iron, Radio Shack Micronta 8 Range Mulitmeter, and an old Eico Model 460 Oscilloscope my Uncle, who was an Electrical Engineer, gave me.

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The GM HEI Ignition Coil

Plasma and electrical discharge has always fascinated me, so I jumped at the chance to salvage the HEI Ignition Coil when my father replaced the Distributor Cap, with integrated HEI (High Energy Ignition) Coil, in my mother's 1975 Chevy Monte Carlo.

The HEI Coil was designed by the Delco-Remy Division of General Motors and was used in all GM Engines from 1975 through 1980. Unlike most Ignition coils of the time that only generated 18,000 Volts, the HEI Coil was able to produce 40,000 Volts resulting in a more complete burn in the combustion chamber.

I was already aware of the High Voltage potential of the GM HEI Coil from reading automotive repair manuals, which is why I chose it as the basis of my experiment.

The Experiment

I decided in 1986, my Junior year in High School, that I was going to build a High Voltage Project using the salvaged HEI Ignition Coil. Below is my completed project. I will go through each aspect of this project in this blog. 

Below is the GM HEI Ignition Coil as it looks removed from the Distributor Cap. It is secured by nails at the corners to the wooden base. The center of the HEI Coil is the High Voltage output. I use part of a spring metal battery clip to secure the wire from the High Voltage output to the electrode in place.

I used an IRF250 N-Channel MOSFET Transistor, in a TO-3 style package, to pulse the direct current to the primary winding of the HEI Ignition Coil. Originally I was using three 2N3055 NPN Bipolar transistors in parallel to do the job but found that the specs varied slightly with the transistors and one was doing more work than the others and getting very hot to the touch. I found that the IRF250 MOSFET did a better job than the three bipolar transistors and didn't get as hot due to its very low resistance between Source and Drain when switched on.  The picture below shows the three transistor mounts on the aluminum heat sink with only the single IRF250 MOSFET Transistor remaining.

A 1N1007 Rectifier Diode is placed in parallel with the Source and Drain of the IRF250 MOSFET. Normally it is reverse biased but it does conduct the counter EMF current caused by the magnetic field collapsing in the Primary winding of the HEI Ignition coil.  The 1N1007 purpose is to protect the IRF250 MOSFET from this large counter EMF Voltage. This counter EMF Voltage is several magnitudes larger than the input Voltage.

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My Ignition Coil High Voltage Experiment is powered by conventional 120 Volts AC line current. The transformer below drops the line Voltage from 120 Volts to 24 Volts AC. 

The Rectifier and Diode assembly below take the 24 Volts AC and convert it to roughly 30 Volts filtered DC, under the load of the HEI Ignition Coil. The HEI Ignition Coil was designed for 12-15 Volts input Voltage and I am pushing the envelope by doubling it.

The perfboard on the right contains a separate Capacitor/Diode assembly used to convert AC to a filtered DC to run the Oscillator perfboard located to the left. The Oscillator board drives the IRF250 MOSFET Transistor used to pulse the DC to the Primary winding of the HEI Ignition Coil. The Oscillator perfboard consists of the 555 Timer IC as the Oscillator, an LM324 Voltage Regulator used to provide a consistent 5 Volts DC to the timer circuit. In addition, there is a driver transistor at the bottom used to control IRF250 MOSFET Transistor and not load down the Oscillator circuit.

A second transformer, circled below, converts conventional 120 Volts AC line current to 6.3 Volts AC which is used to power the Oscillator circuit.

I created a little control panel out of a piece of scrap steel bent at right angles, then primed and painted it green. The control panel consists of an on/off slide switch that controls power, a neon power light, and a potentiometer that controls the oscillator frequency from 20 to 20,000Hz.

Below is the "Test Chamber" I created. The distance between the electrodes are adjustable by adjusting the brass nuts. At one time the Test Chamber had a lid on it but it melted when the material, that I was testing its insulating properties at High Voltage, caught fire and melted it. I shouldn't have used a plastic box as the Test Chamber but it was the only insulted box structure I had at the time.  

Striking a plasma arc in the test chamber! The plasma arc does strange things when the input Voltage to the primary is modulated at 15,000 Hz. At this frequency, the electrical arc leaps out and forms circular and even "U" shaped arcs. I speculate at around 15,000 Hz I reach resonance of Ignition coil/electrode circuit. Typically you need an LC (Inductive and Capacitive) circuit to achieve resonance. I suspect the capacitive aspect of the circuit comes from either the internal capacitance between the copper windings in the HEI Ignition coil or the capacitance between the electrodes.  I am wearing thick blue insulated gloves and safely glasses during initial operation as I don't know what to expect from this project that has been dormant for 29 years!

My Vintage Ignition Coil High Voltage Experiment in action!