The device USB killer is popular nowadays.  Due to its damaging nature to the electronic devices it is popular among people nowadays. Lets know the working of this USB killer.

                                   USB killers consist of an few oscillators, a step up transformer and a few capacitors.  At first the oscillator circuit takes 5-6 volts from the USB port when connected to a USB peripheral(for example USB port on your computer), and it drives the transformer which is step up function.  Normally a step up transformer steps up the input voltage and delivers high output voltage.  The high output voltage depends on the number of turns on the primary and secondary coil.  But the USB killer uses a transformer which steps up up to 200-220 volts form a 5 volt input dc source.  The capacitors thus stores the resulting voltage as a charge.  When it builds a potential enough charge, the oscillator disconnects and the capacitors are shorted across the USB port`s power pins or port. Thus making the port shorted and its dead now.
                                      If the necessary voltage is not achieved the capacitors are disconnected and the oscillator reconnects, and the process repeats!  Though the USB ports are supposed to be able to withstand 2 kV static discharges,  but usually it won`t tolerate even 100 volts which isn't enough to short through the protection circuit but is enough to destroy a usb root hub and dump charge down the power supply lines or motherboard components of your cpu.  Many phones use USB in peripheral mode which doesn't have to provide power over the port,  but does accept power.  This was meant to charge the devices  but not to drive power devices.
                                       But somehow some devices can drive small loads like USB storage media or hard disks which is in popular with nowadays USB ports. Hence as soon as a USB killer is inserted in these ports it will instantly get toasted by the killer. Initially it shows like a USB flash drive is inserted and after few seconds you will see fumes inside your CPU.  In worst case your CPU will burn! So be cautious before inserting any kind of USB.  Its circuit is similar like the mosquito killer bat.  there output is via the aluminium net but here the output is given to the USB port itself.

the usb killer looks like this!

the circuit diagram of a basic killer circuit.

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