Need help solving a mystery surrounding a bug zapper!

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Roflberry Pwncakes®

Joined May 18, 2021
So I had an electronic fly-swatter with a broken switch, so I tore the guts out to play with and see how 2 AA batteries can produce enough juice to make flies explode.

So the first thing I did was short the power switch with a solder bridge and connect my benchtop power supply and set it for 3V. Then I set my multimeter for Vac, because it has a transformer in series with a diode and a girthy capacitor of unknown capacitance value (because it's stamped with 2 different strings that could be the value indicator, so it's either 0.53uF or 82uF) rated for 400V, so I suspected it was acting an oscillator. It's been 15 years since I went to school, and I've been out of the game and I'm actively reeducating myself to get back in the game, but I haven't gotten arround to brushing up on RLC circuts yet.

Anyway, as soon as I touched my test leads to the output terminal, which is that fatty capacitor in parallel with a 22MΩ resistor, the backlight on my computer monitor went out. I know it was the backlight, because if it was a total power loss the OEM logo would have been displayed when the power came back, and it wasn't a loss of input signal because the monitor would have displayed a message saying there was no HDMI input detected. I've reproduced the effect several times, as soon as I touch my meter leads to the output, my monitor's backlight goes out. My power supply shows that it's drawing a constant 450mA. I then noticed that my meter was detecting Vac with the leads in proximity to the zapper and was reading 500mVac with the leads less than 6" away.

The voltage reading would decrease as I moved the leads away from the zapper. So I'm guessing the transformer is producing an electromagnetic field large enough to induce a reading on my meter. I'm puzzled by why my monitor's backlight goes out every time I touch my leads to the zapper's output terminals... is it making some sort of EMP? Tried building this in Multisim to get understanding, but neither the transformer or the transistor are in the component database for some reason; the minimum number of secondary windings is 3 in Multisim, but this transformer has 2 secondaries.
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Joined Jan 23, 2018
You will certainly be able to damage your computer that way, probably far beyond any repair. The bug zapper uses an ocillator circuit and a step-up transformer arrangement to zap the bugs. It maybe AC or DC, both will work.


Joined Sep 21, 2010
This is the circuit & whats in those bug zapers. Quite capable of creating a contious spark across 2mm gap.BUG ZAPPER CICUIT.JPGBUG ZAPPER.3.JPG


Joined Jan 23, 2018
Thattransistor can do 5 amps, but not for a long time. And 40 volts but not for much time switching 5 amps. And since it is a sort-of-linear application it should not be run for a long time. Of course, you will not get 5 amps from a pair of AA cells.