Introduction: Ignition Coil High Voltage Display

About: If you've got a problem, Yo, I'll solve it ! -Robert Matthew Van Winkle
A common automotive ignition coil can generate an output voltage on the order of 30,000 volts. This is a sufficient voltage to produce arcs which jump an air gap of an inch or more.  Driven properly they can also be used to create a Jacobs Ladder display

Ignition coils are inexpensive and readily available, and constructing a circuit to drive the coil is straightforward.

This article provides an overview of ignition coil operation, as well as a drive circuit design and system capable of driving the coil to produce arcs for your entertainment and experimentation.

Of couse, as with any high voltage project, safety warnings are in order.  Excecise caution and good sense when working with high voltage.  Remove power from the system before making adjustements.  

Don't touch the arcs.  Be mindful that the potentials generated can jump a significant gap, and that insulation on tools like regular pliers may be inadequate to prevent you from getting a shock.

Keep in mind that some parts of the system can get hot.  The arcs can be hot enough to to ignite paper and plastic, so operate it in a safe location.

High voltage discharges generate Ozone gas, which can cause irritation if breathed in.  High voltage discharges also generate some ultraviolet radiation, so limit your exposure and don't stare directly at the arcs.

First, enjoy some video!

Step 1: Details

An ignition coil produces a high voltage output on its secondary when the current flowing in the primary winding is interrupted. The first step in generating a high voltage from an ignition coil is to store energy in the inductance of the primary winding. That energy is then released, generating the high voltage output.

Energy is stored in the primary of the coil when current is flowing through it. This energy is provided by a DC power supply, usually the 12 volt battery of a vehicle’s electrical system.

The primary circuit is driven by closing a switch to ground, which allows current to flow from the power supply through the primary. When the switch is initially closed, the current in the coil remains zero, as the inductance of the primary does not allow the current to change instantaneously. The current in the primary then increases exponentially until it reaches its steady state value.

The steady state current is the maximum value the current will have. It is determined by the voltage of the power supply and the total series resistance of the primary circuit.

The steady state current in the primary is determined by:

I_steady_state = V_source/R_primary

Rprimary consists of the total resistance of the wire in the primary coil, as well as the resistance of the wires and other connections in the primary circuit.

The length of time needed for the current in the primary to reach its steady state value is determined by the time constant. The time constant is a parameter which is equal to the primary inductance (in Henries) divided by the primary resistance (in ohms), and so it has the units of seconds. In equations it is commonly represented with the Greek letter tau. A smaller time constant means that the current will increase more quickly.

The equation for the current in the inductor is:


The graph shows how the current in the coil increases after the switch is closed. The graph expresses the current as the percentage of steady state value. Note that after 4 time constants have passed, the current is at about 98% of its steady state value, and the after 5 time constants, the current is at more than 99% of its steady state value.

The relevant equations for the primary circuit are summarized in a chart below.

Energy in the Primary Winding

The energy stored in an inductor is a function of its inductance and the current flowing through it. The equation for the energy stored in an inductor is:

Energy= ½ * L * I^2.

Refer again to the graph that shows the current and energy in the primary coil versus the time elapsed. Note that the longer the switch is closed, the less additional energy is stored in the primary inductance. After the first time constant, the energy stored is 38% of the maximum. After two time constants have elapsed, the total energy stored is 73%. After three time constants, the total energy stored is 90%. After four time constants, the total energy is 96%, and so on. After five time constants, the energy stored is essentially at 100% of the maximum possible value for the power supply voltage and primary resistance in the circuit.

Once the primary current has reached its steady state value, no additional energy can be stored in the primary of the coil. If the switch remains closed after this point, the energy from the source will simply be dissipated as heat in the resistance of the primary circuit. It therefore makes no sense to keep the switch closed longer than 4 or 5 time constants, as past that point the energy will simply be wasted.

Step 2: Coil Connections

A typical ignition coil has three terminals.

The first connection is one lead of the primary coil winding, and it gets connected to the power supply (typically the 12 volt battery in a car). This lead will usually be marked with a “+”.

The second connection is common between the primary and secondary. The other lead of the primary winding and one lead of the secondary winding are connected to this terminal. This is the point that is connected to ground when the switch closes. It will usually be marked with a “-“.

The third terminal is the high voltage output terminal. It is located in the center of the coil, surrounded by a plastic shroud. Inside the shroud, the terminal looks like a metal cup with an inner diameter of about 5/16”. Refer to the picture below to see the actual terminals. The center high voltage terminal is usually connected via a special cable to the distributor when used in its normal application.

Refer to the pictures below showing the coil connections.

Arcs are drawn from the high voltage terminal and the common “-“ terminal.

The connection to the high voltage terminal is made via a custom piece. It consists of a piece of 1” diameter nylon rod. One end of the rod is machined out to fit over the shroud on the high voltage connector. The other end of the rod has a hole drilled in it to allow a 8-32 threaded standoff to be tightly pressed into place. An 8-32 screw is threaded into one end of the standoff to make the connection to the HV contact within the shroud. The other end of the standoff allows an easy connection to the HV terminal. The pictures below show the details.

Step 3: Driver Circuit

In normal automotive use the current in the primary is interrupted at the appropriate time to generate a spark at the spark plug to ignite the fuel-air mixture in a car’s engine. In old cars the current is interrupted by a mechanical switch called the points, which are opened and closed by a cam on a shaft that rotates when the engine is running. In newer implementations a sensor sends a signal to an electronic module to cause a semiconductor switch in the module to open and generate the spark at the proper time.

All that is needed to use the coil to generate arcs for experimentation and display is a circuit that turns a switch on and off to interrupt the primary current of the ignition coil. Concerns such as adjustments to the timing to ignite the fuel in cylinder at the proper time do not apply for a display application.

The diagram below shows the basic blocks of a system used to generate acrs using an ignition coil. The general drive circuit consists of a pulse generator that turns a semiconductor switch (a transistor, FET, or in the circuit presented later, an IGBT) on and off at a desired rate. The power supply can be any battery or DC power supply of an appropriate voltage, capable of supplying enough current.

The function of each section of the system is described in detail next. Refer also to the detailed schematic.

+5V Regulated Supply
A LM7805 regulator, VR1, provides the +5V regulated voltage for the pulse generator circuitry. Diode D1 in series with the input to the regulator is used to to prevent damage if the power source is connected backwards. There are electrolytic and ceramic capacitors (C1, C2, C3, and C4) on both the input to and output from the regulator for filtering.

Pulse Generator
As described earlier, energy is wasted if the switch remains on past the time the steady state primary coil current is reached, so the pulse generator circuit must be designed and adjusted with this in mind. Again, the time constant of the primary circuit will determine how long it takes for the primary current to reach its steady state. The inductance and resistance of the primary will vary between different coils, so the maximum on time of the switch will need some adjustment depending on the characteristics of the coil you are using. Some coils are intended to operate with additional resistance in series with the primary coil, and this resistance will need to be added to the other resistance in the primary circuit when calculating the steady state current and time constant.
Two different designs of the pulse generator are described in the next steps. One is based on a PIC microprocessor, and the other based on a 555 timer.

Pulse Generator Based on Microprocessor
A PIC microcontroller based pulse generator circuit is used to generate the signal used to turn the IGBT on and off. Using a microcontroller gives the flexibility to control both the on time and the frequency independently. The software allows the spark repetition frequency to be set between 10Hz and 400Hz, and it allows the on time of the pulse to be set between 0.5 and 2 milliseconds.

The maximum current can be limiting by adjustment of the on time. If the on time is set to less than about four time constants, then the current in the primary will never reach its steady state.

The circuit uses two potentiometers, R3 and R7. One is used to set the on time, and the other to set the repetition frequency. The potentiometers are read by the microprocessors A/D converter. R4 and C5 and R6 and C6 are used as low pass filters on these analog inputs. The software generates the output pulses based on the on time and frequency readings.

One processor input is used to read an enable switch. When the switch is pressed, the input is brought low, and the microprocessor generates pulses on the output with the on time and frequency as set by the potentiometers. When the switch is released R1 pulls the input high and the output is turned off.

The program flow chart diagram, *.asm source code file, and *.hex file for programming are included.

Alternate Pulse Generator Based on 555 Timer
This 555 timer circuit can also be used for the pulse generator, but it can’t generally let you control the on time and frequency independently. Refer to the separate schematic for the 555 timer version of the pulse generator.

The schematic shows a 555 timer circuit which uses diodes to create separate charge and discharge paths. The on time is determined by how long C1 takes to charge through R1, potentiometer R2, and diode D1. The off time is determined by how long C1 takes to discharge through R3, potentiometer R4, and diode D2. By separating the charge and discharge paths, the on and off times can be controlled independently, allowing the user to set the on time so that it is not so long that it results in wasted energy. Note however that changes to either the on time or the off time will result in a change to the pulse repetition frequency, unlike the microprocessor based version of the pulse generator.

The ON time, OFF time, and frequency of the output are determined as follows:

Ton = 0.693 * (R1 + R2) * C1
Toff = 0.693 * (R3 + R4) * C1
Frequency = 1/ (Ton + Toff) = 1.44 / [ C1 * (R1 + R2 + R3 + R4) ]

Pushbutton switch SW1 connects the reset line of the timer high, enabling the pulse generator output. The output goes low when SW1 is released, turning off the HV output.

The switch used here to control the current in the primary is an International Rectifier IRGB14C40LPBF IGBT. This IGBT is specifically designed for automotive ignition system applications.

When the switch in an ignition system opens to disrupt the current in the primary, the voltage across it rises up to hundreds of volts. This IGBT is rated to handle this voltage. It also includes internal clamping diodes to protect against overvoltage. Refer to the internal diagram of the part shown below to see how the internal protection is implemented.

Proper heatsinking is needed for your switch. This is especially true if you ever plan on overdriving your coil with a higher power supply voltage.  Exact heatsinking needs require some calculation of the actual power dissipated in your switch.  To err on the side of caution mount to a piece of aluminum and also use some forced air, such as from a small fan.  In the system shown here, the heatsinking is provided by a large copper area of the PCB.

Circuit Protection
It is beneficial to provide protection to the driver circuit to avoid damage from transient overvoltage “spikes”. MOV1 is placed across the power supply input to clamp voltage spikes. MOVs (metal oxide varistor) are protection devices which are designed to be off when the voltage across them is below their clamping voltage, but then turn on and conduct once that voltage is exceeded. When it is conducting, the voltage drop across it is relatively constant. The MOV therefore “clamps” the voltage across the power supply bus to a safe level, so that it does not rise high enough to damage the driver circuit.

The MOV must be selected with a clamping voltage that is greater than the power supply voltage; otherwise the MOV will be conducting all the time and it will overheat. For example, the MOV I chose for use with a 14 volt power supply clamps at around 20 volts. Transient spikes greater than 20 volts will then be clamped so that the power supply bus will not see a voltage greater than that.

It is also useful to have a larger of capacitor (C7) on the power supply input. A couple thousand microfarads worth of electrolytic capacitors works well. The capacitors provide a low impedance path to high frequency spikes on the power supply. Make sure that the voltage ratings of the capacitors you use are greater than the supply voltage and greater than the MOV clamping voltage.

Step 4: Overdriving

Ignition coils can be overdriven to increase the power output.

As mentioned earlier, the energy stored in the primary is related to the current flowing in the primary. Also recall that the length of time required for the current to increase to a certain point is related by the time constant and the steady state current. The time constant is unchanged by a change in drive voltage, but the steady state current does increase with a greater drive voltage. So, driving with a higher voltage allows the primary current to reach a given level of current in a shorter amount of time. Overdriving therefore increases the output power by allowing more pulses of the same energy to be released in a given amount of time (a higher drive frequency) compared to the case where a lower power supply voltage is used.

When overdriving the coil with a voltage greater than that of the electrical system it was designed for (usually 12 volts), it is possible to damage the primary if the switch is left on too long. The current in the primary will increase to a level beyond what the coil was designed to carry in the automotive electrical system, and so excessive power will be dissipated in the resistance of the primary winding. This must be taken into consideration when setting the maximum on time produced by the pulse generator circuit.  By properly limiting the on time of the switch, the coil can deliver more energy to the HV output without increasing the power dissipation in the primary winding or the switch.  This is possible because the power dissipated in the primary is determined by the RMS current in the primary.  Using a higher power supply voltage and properly limiting the ON time allows the RMS current in the primary to be kept the same while the energy throughput is increased.

Overdriving risks drastically increasing the power dissipated in the semiconductor switch if the on time is not limited. A switch that is adequate for 12 volt systems may easily be destroyed when overdriving, if care is not taken. The on time of the switch will have a significant effect on the power that is dissipated in the switch. It becomes even more important to use adequate heat sinking for the switch when overdriving. The image below shows the remains of an IGBT that burned out when overdriving a coil. Keep the on time propely limited to avoid burning up your driver circuit!

If you are overdriving your coil, also keep in mind you will also have to make sure that the circuit protection (MOV and electrolytic capacitor) are rated for the higher supply voltage.

Step 5: Coil Damage Due to Overvoltage

In typical automotive use, the coil only needs to generate a short spark to ignite the fuel-air mixture in the engine. A higher voltage output allows the generation of a sufficient spark, even if the spark plugs have deteriorated and become fouled. It is generally not suggested to run with the distributer disconnected, as the output voltage of the coil can then rise high enough to damage the coil internally. In normal operation the coil output voltage is limited due to the shorter gap of the spark plug. Even with wear and fouling, the output does not rise high enough to damage the coil. Many automotive diagnostic sites and articles list poor high voltage connections as a possible cause of ignition coil failure.

I damaged one coil in this manner. Initially, arcs as long as 1.5 inch could be generated, but after a relatively short length of time producing arcs of that length the output reduced to the point where the arcs were only about one half that length. This kind of damage is caused when a pathway of carbonized material forms internal to the coil.

This kind of issue can be confusing, as measurements of the primary and secondary winding resistances may look the same as they did before the damage. The meter will not produce a high enough voltage on its resistance measurement setting to cause the carbonized pathway to conduct, so the resistance measurement may look normal.

The picture below shows a coil with a visible scar on the center HV shroud, cause by arcing. The carbon track then provided a lower resistance path to the HV output, so that the arc would no longer jump across the desired spark gap, but instead jump directly from the coils HV output to its “-“ terminal. This coil operated properly again after cleaning up the carbon residue and covering it up with tape, but if the damage had been internal the coil would be irreparable. The possibility for damage of this nature is more pronounced when overdriving the coil, as the arcs tend to be hotter.

Step 6: Results

The pictures below show the complete ignition coil system mounted on a board. The pulse generator circuit and IGBT switch are on a small custom PCB. The input power jacks, enable switch, and the on time and frequency controls are mounted on an aluminum angle bracket.

I'll repeat the safety warnings here. 

Excecise caution and good sense when working with high voltage. Remove power from the system before making adjustements.

Don't touch the arcs. Be mindful that the potentials generated can jump a significant gap, and that insulation on tools like regular pliers may be inadequate to prevent you from getting a shock.

Keep in mind that some parts of the system can get hot. The arcs can be hot enough to to ignite paper and plastic, so operate it in a safe location.

High voltage discharges generate Ozone gas, which can cause irritation if breathed in. High voltage discharges also generate some ultraviolet radiation, so limit your exposure and don't stare directly at the arcs.

The first picture is a short time exposure showing the arcs from the high voltage output when used with a power supply of 14 volts. Note that they arcs are distinct blue streams, as opposed to hotter arcs.

The arcs become much hotter when a greater power supply voltage is used along with a higher driver frequency. The subsequent pictures show operation using higher power supply voltages. Notice that the arc is now a hot and yellow.

The video shows how the output arcs are affected by changes to the repetition frequency, switch on time, and power supply voltage.

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