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How does a bug zapper work?

To my understanding a capacitor can only be charged to the voltage it is supplied with. Well then how can a bug zapper circuit step up 3v (2 x AA batteries) to nearly 3000V. Wouldn't the capacitor only be charged to 3v? This may be a very simple question, and im sorry if it is, i would just like to learn!

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-max-2 years ago

It uses some fancy boost converter to boost the voltage up to several hundred volts. Most often, there is an inductor (coil of wire) or transformer involved.

If it uses a simple boost converter, the 3V positive goes though an inductor, though a switching device like a MOSFET, to the negative or ground. That electronic switch is flipped ON and OFF really really fast (you can sometimes hear it).

When it is ON, electricity begins to flow through the inductor, and as the current increases, a magnetic field is made in the coil, and it begins to look more and more like a short circuit. However, by that time the transistor is turned back OFF immediately, but in the same way that electricity was slow to get moving, it is also slow to stop, so electrons keep trying to force their way through the transistor. A traffic jam of all those electrons happens and it WILL overpower the transistor if there is no other path for all that huge buildup of electrons! (in physics that is what happens when that magnetic field collapses.) So we need a way to store them electrons! We can add a diode and capacitor, and that way, those electrons can now flow through the diode and 'through' the capacitor. When current 'flows' through a capacitor through, the voltage across it will increase because it is charging. Since the diode keeps the electrons from going the other way back down the transistor when it is flipped ON, the capacitor just keeps charging. Every time the transistor is abruptly turned off, the current that used to flow though it now HAS to go somewhere, so the voltage will rise until current can flow through the diode and capacitor and charge it to higher and higher voltages. If it is allowed to go past a few hundred volts though, those electrons will make a (destructive) path to ground through the transistor, destroying it. To prevent that, there is another special circuit that detects the voltage and when it gets too high, it tells the switching circuit to stop.

If you want even more voltage, then another method is needed. A transformer is similar in looks and construction, but works a little differently. There are actually 2 coils, a primary and secondary. When it is switched on or off or oscillated in some way, the magnetic field created also fluctuates, and that causes a nearby secondary coil to turn that magnetism back into electricity. The ratio of turns is basically proportional to the change in voltage, so you will see inside these circuits, the first coil will have like 10 turns of wire, and the secondary coil will have several hundred to several thousand turns of really thin wire. Now the voltage is boosted up to maybe 1000V AC, we can connect it to a cascade voltage multiplier or voltage tripler circuit (lots of diodes and capacitors) to get out many thousands of volts! you can google how those work, it is quite magical!

The capacitor isn't a 3V capacitor. The circuit is a boost converter taking the 3V from the batteries and stepping it up to 3kV. In the process the amperage gets dropped significantly. So if the circuit is drawing 3V @ 2 A from the batteries (assuming 100% efficiency which it never is) then it will output 3000V @ .002A.

Jaxon0077 (author)  mpilchfamily2 years ago
Thank you for the reply! I was aware that the capacitor wasn't 3v. The part that really confuses me is the "boost conversion" process. Again, thanks in advance! (If you know a website that goes in depth with it that would also help)

Look up flyback driver, flybacks and disposable camera flash they work the same as a bug zapper.

joe

Google Boost converter. Lots of great info out there.