10.5. Trigger voltages

Electronic flash units work by pumping high voltage electricity through their flash tubes. These powerful electrical discharges are fired by a triggering voltage, and many older flash units connect the trigger circuitry directly to their controlling switches. That means when a camera's flash contacts close, there may easily be anywhere from 50 to 200 volts flowing through them. This isn't a problem restricted to AC powered studio flash—even battery-powered flash units have high-voltage internal circuits. The battery units use special step-up circuitry to transform the low voltage from the batteries into the high voltage needed to power the flash tube.

Old mechanical cameras handle such high voltages with aplomb, but many EOS cameras employ delicate flash sync contacts. Some EOS shutter switches are only rated to six volts and can be destroyed by high voltages. The damage isn't usually instantaneous but cumulative with repeated use, as arcing and pitting slowly destroy internal components. So it's vitally important to check both the trigger voltage of a flash unit and the maximum trigger voltage of a camera before plugging in any non-Canon flash unit in.