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NE555 ignition coil driver

Project description

This simple circuit is one among commonly used ignition coil drivers. Ignition coils are a kind of high voltage flyback transformers used in automobile ignition systems. Much like HV flyback transformers in cathode ray tube screens, they are driven by voltage pulses provided from a DC power supply which is interrupted by a single switch. After the current that has developed in the primary is abruptly interrupted, inductive action of the primary creates a large voltage spike which is reflected to the secondary, multiplied by the turn ratio of secondary and primary coil. This allows for much higher output voltages than those available with ordinary transformers. Both ignition coils and flyback transformers have an air gap in their magnetic path, which is essential for storing energy during the on-pulse which is then released after turn-off. An important difference is in the core material – flyback transformers use ferrite cores, and are capable of operation at much greater frequencies and power densities than ignition coils, which use iron laminations.

In a typical automotive applications, older type ignition coils (like used in this project) were usually triggered by a mechanical contact which opened in the moment of ignition. A careful experimenter will notice that if the switch was used alone, it will produce relatively poor output on the ignition coil and relatively large arc in the contacts. Placing a capacitor across the contacts increases the output dramatically – the capacitor slows down the rate of current drop and slowing the voltage rise, allowing enough time for the contact to open without arcing over. This snubber action of the capacitor is an important feat in power electronics.

If mechanical switch is replaced by a silicon one, such as a MOSFET, snubber is still required; otherwise the voltage would again rise very quickly over the switch breakdown rating and energy would be lost as heat. If there is no current drawn from the output, the snubber capacitor allows recycling of the stored energy back to the power supply.

In this particular circuit, an IRFP450 MOSFET is driven by a NE555 timer IC, with frequency and duty cycle programmable by trimmer resistors. Such arrangement only allows operation at constant duty cycle regardless of the output load. When the secondary is loaded with an arc, it is effectively short circuited, causing a DC voltage component across the winding, which is then reflected to the primary. This DC component may cause the primary current to integrate into unsafe values over a period of multiple cycles, unless enough time is left between the pulses for the DC component to dissipate on primary resistance and secondary voltage drop. The same occurs if the output is rectified and used for capacitor charging.

Pulse duration and repetition rate is also limited by frequency characteristic of the coil, which due to losses in iron limits the frequency to lower kHz range and pulse duration to minimum of about a hundred microseconds.

Project construction

A decision was made to put the whole circuit onto a single printed circuit board that is mounted compactly onto the ignition coil itself. To shrink the size and height of electronics (which is important due to proximity to high voltage output), surface mount components were used. Due to it’s size, the resonant capacitor is mounted separately. An IRFP450 14A 500V mosfet is mounted directly onto the PCB, with only a little heatsinking from the copper layer on the board under it. The circuit is only ran up to about 30V supply for the coil primary – high voltage rating of the mosfet is required to handle the flyback pulse. The power output is low, less than 50W. The NE555 IC drives the mosfet gate directly, and it’s duty cycle and frequency are controlled by trimmer resistors on the opposing side of the PCB.

Conclusion and possible improvements

Low power and low-frequency pulse nature of this circuit’s output limits it’s uses. It may be used for applications like insulation testing, or, if the output is rectified, small high voltage loads like lifters and perhaps even small Tesla coils. The output current is usually only several mA peak while voltage may reach over 40kV, causing insulation problems with the ignition coil. Pushing the input voltage further tends to cause surface arcs from ignition coil output terminal to one of it’s low voltage terminals, and subsequent carbonization may render the plastic conductive and the coil unusable. Some people circumvent this by submerging the coil in the oil, and sometimes even extracting the innards from their oil-filled case and putting them into a larger oil filler reservoir, in order to be able to push the coils to higher output voltages. Inevitably, though, most coils tend to get destroyed at relatively low power levels due to extremely high voltages developed by these kind of drivers. In attempt to circumvent this and attain more power from the ignition coil, while keeping the voltage lower, I designed the Half Bridge Ignition Coil Driver.

Links and references

[1] NE555 timer IC datasheet

[2] IRFP450 data sheet

[3] A typical high voltage enthusiast ignition coil driver. Many similar circuits can be found that utilize bipolar transistors instead of power MOSFET’s – I strongly recommend against those, due to prevailing superiority of modern MOSFET and IGBT devices.

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