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I have been doing some research on building my own bug zapper circuits for use in an art project (just for arcing electricity, I'm not killing any bugs), in particular finding a good source for transformers. I have found several companies selling transformer based spark igniter kits which also sell the transformer individually. Looking at the circuit for these spark igniters, I see that the two designs are quite similar in their functionality:

Spark Igniter: Spark Igniter Circuit

Bug zapper:

Bug Zapper

Both circuits are based around using a transformer in flyback mode, and feature some sort of transistor based oscillator at the input, which create the AC voltage necessary to build up charge in the transformer coil. In one case this is through a 555 oscillator, in the other it's using the inductance of the extra transformer tap to toggle the transistor on and off.

The difference is mainly at the output. The bug zapper includes a filter network to create a differential DC charge between the two sides of the coil, whereas the spark igniter just has an open circuit. The difference in result is that the bug zapper produces an occasional pop, when the DC large becomes large enough to jump the dielectric gap, whereas the spark igniter produces a continuous stream of electricity at the frequency of the input oscillator.

Would modifying the spark igniter to work as a bug zapper be as simple as adding the output circuitry from the bug zapper, or are there additional factors to take into consideration?

Thanks for your help, this subject is not very well documented online so any information is much appreciated.

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Converting the igniter circuit to output DC is not as simple as you'd think.

The drawing for the spark igniter shows it putting out 20000 volts. That'll jump about 6 or 7 millimeters through air. The gap is usually less than that, so it sparks continuously when operating.

The bug zapper puts out only a few thousand volts. Your diagram doesn't explicitly give the output voltage, but with that 2000V rated capacitor on the output side you can't expect more than 2000V out of the transformer. The thing with the diodes and capacitors is a voltage doubler. It converts the 2000VAC from the transformer to DC at probably around 4000V.

4000VDC will jump an air gap of a little over one millimeter. The bug zapper doesn't zap bugs through the air, it zaps them when they get really close to the grid on both ends.

A bug zapper isn't supposed to spontaneously zap when there's no bug there. It does so when a piece of fuzz drift through and completes the circuit. If the grid isn't spaced wide enough then it might spontaneously arc over if nothing is zapped for a long time - the voltage can creep up if not discharged, and it might get high enough to jump the air gap.

You can't easily convert an igniter to DC because the AC voltage on the output side is crazy high. You'll need diodes rated for like 40000 volt peak reverse voltage. That means either expensive parts or lots of cheap ones in series.

I wouldn't bother changing the igniter output to DC. I'd just turn it on for short pulses. It will always arc, so just turn it on and off to get a zap/pop.

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This question is ambiguous, which is perhaps why it has not been answered. A spark ignitor is designed to produce spark when the power is applied, which is accomplished by generating sufficiently high voltage to break down the dialectric between a spark gap - every time. A spark igniter that did not spontaneously spark would be a failure for the purpose of causing ignition. A bug zapper is designed to apply high voltage to a grid that is designed to be less than the voltage needed to break down the dialectric between the gap, so that it is not just continually sparking, which would be an undesirable nuisance. But when a bug flies or is scooped to be between the gap of the grid, its body artificially bridges the gap to be less than what is needed for a dialectric breakdown, which allows the spark to take place and zap the bug.
Your question about adding the output changes to convert the ignitor to a zapper, goes against your stated use-case of creating spontaneous arc (discharge) for art. But, in an attempt to answer your question, the design difference that the output diodes in the zapper bring to bear, is to rectify the output to be pulsed DC which then can charge the additional HV capacitor to allow for a higher current capacitive discharge into a bug when they bridge the gap. The resulting spark from a capacitive discharge would be with greater current, resulting in more effective kill strength, I suspect. But to the question of polarity of the output, it is also kind of a misnomer because the oscillator of both circuits produces a pulsed DC rather than a true AC input signal which is essentially a repeated half-wave. This creates only single magnetic polarity on each cycle which will also only create single polarity flyback voltage in both circuits. The existence of a spark or discharge is solely based on breakdown voltage for a given spark gap and whether the circuit meets or exceeds this value. Polarity and current are additional characteristics that may be concerns or affects that also could be maximized or minimized depending on desired purpose. FWIW

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