Zappers use a positive offset squarewave voltage (Pulsed DC) to kill pathogens. Typically 5 to 15 volts. I have a variable frequency one which is more effective because you can go to the frequenciy that kills the organism even without a positive offset. I now prefere the Beck Protocol Devices for infectious disease.
It depends on what you are using the device for. Different electrode size also makes a big difference.