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Lately, I have been trying to learn how capacitors work and I stumbled across a device called a "USB Killer." From the information I have gathered, the USB Killer uses 5v from the host to charge its capacitors then outputs 10v when they are full. Since the capacitors are being outputted at a higher voltage, then that means the circuit must be in a series. Maybe something like this: Capacitors in a series

However, this confuses me as after capacitor 3 has fully charged, it will not allow capacitor 2 to charge. Lets say that the 5v battery is somehow able to power all the capacitors to 5v, why would the circuit be at 10v+? Capacitors are meant to resist change so shouldn't the circuit stay at 5v max?

Note: This could all be completely wrong as this is just how I imagine the USB Killer's capacitors would be organized.

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  • \$\begingroup\$ If you have 5V then you can only charge to 5V. What you could do is connect the capacitors all in parallel and charge them all to 5V, then switch them all in series. That then gets you the sum of 15V. Still not enough to kill your USB port. USB killers generate a much higher voltage than 15V. \$\endgroup\$ – JRE Jun 3 '17 at 20:59
  • \$\begingroup\$ @JRE Ah, I see. Yes that makes a lot more sense for the capacitors to be switched from parallel to series. \$\endgroup\$ – Kevin Duarte Jun 3 '17 at 21:23
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You basically got everything wrong ;)

These USB-Killer doohickeys use a DC-DC converter to step up the USB 5V to a much higher voltage, something like 100 volts or more.

Like in a photoflash, this high voltage generator charges a capacitor bank. Since caps are readily available at these voltages, there is no need to use a series combination. Several ceramic caps are wired in parallel simply to stuff more capacitance in the height footprint of a USB key. A single electrolytic cap would work just as well, but it would be larger, and thus unable to masquerade as a USB key.

When the caps are charged to the target voltage, a switch (most likely MOSFET or SCR) triggers and discharges the caps back into the USB port, destroying it.

Pretty idiotic... I mean, you can give such gadgets to people and laugh when they zap their computers, but then this raises other philosophical questions, like, do they own a baseball bat or a crowbar, and what will they do with it once they get really pissed?.......

EDIT: about the nature of the voltage booster

Charge pump? Probably not. A charge pump is a simple way to double a voltage, or convert a positive voltage into the same negative voltage, but if you need a high output/input voltage ratio, as is the case here, the circuit becomes large and complex. An advantage of using a charge pump voltage multiplier for high ratios is that the components don't need to be rated to the full output voltage, good if you need 100 kV, but not here.

Photoflash charger? Large step-up ratios are most efficiently done using a transformer, so this would be the best solution, but since the point here is not efficiency or conserving energy, the extra complexity and cost would be unwarranted.

Since they mention using a negative high voltage (in order to make it simpler to switch with a big fat NMOS then the most likely candidate would be the inverting buck-boost converter. It is simpler, uses one switch and one diode, and one inductor, plus it is cheap.

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  • \$\begingroup\$ I learned something and I was entertained. Cheers \$\endgroup\$ – Archaeus Jun 4 '17 at 0:07
  • \$\begingroup\$ Thanks for this informative answer. Yes, USB Killers are pretty idiotic, I was just using it as an example on how to use capacitors to output huge energy. Is there a name for a circuit like this? \$\endgroup\$ – Kevin Duarte Jun 4 '17 at 3:29
  • \$\begingroup\$ This basically a charge pump. \$\endgroup\$ – Andrey Pro Jun 4 '17 at 11:52
  • \$\begingroup\$ See edits to my answer... \$\endgroup\$ – peufeu Jun 4 '17 at 12:34

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