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.