Normally, it would take an extremely high voltage to cause the xenon
gas inside a flash tube to ionize. Once the tube is conducting, though, it will maintain discharge until the storage capacitor is discharged. To get the tube started, a third trigger electrode is used. On most xenon
tubes, this is a wire wrapped around the glass, or a conductive trace silvered onto it. Cheap disposable cameras just have a piece of copper foil stuck next to the tube. This electrode is wired to a small trigger coil. The trigger coil is a small high-voltage autotransformer
that will step up a pulse of voltage off of the strobe
or flashgun's storage capacitor to about 1,500 volts. A large storage capacitor connected in parallel with the tube's electrodes is charged to 200-500 volts by a small converter. The trigger pulse is carried across the glass by capacitive coupling
, and ionizes the gas enough to start the discharge. The tube will continue to conduct until the capacitor
is mostly discharged.
The trigger coil is fired by a set of contacts attached to the shutter (on a camera flash), or by a small mercury vapor filled bulb which flashes over and becomes conductive once suitable voltage has built up on the storage capacitor to fire the tube.
The schematic for a camera flash trigger circuit is shown below. C1 is the main storage capacitor, and C2 is a small capacitor used to prevent the trigger coil primary from discharging C1. S1 is the shutter switch, which would be replaced by the mercury vapor bulb in a strobe. T1 is the trigger coil.
from power | | T1 | |
supply --- C1 $|| | |flash
--- +--$|| | |tube
| | $|| | |
| --- $|| || |
| C2 --- $|| || |
| | $--------|| |
| / S1 trigger | |
Note that I said it mostly
discharges the capacitor - if you plan to mess with the innards of a flash gun or strobe light, discharge it first with a test lead or screwdriver. Otherwise, you may get a nasty little surprise