Not quite. Although most flash bulb circuits do have a capacitor, the circuit is formed by a battery (about 22 volts), a capacitor a resistor and the flash bulb all in series. Obviously no current flows when there is no bulb fitted as the circuit is broken. When a bulb is pushed into its holder it completes the circuit and the capacitor charges through the resistor and bulb until it reaches the same voltage as the battery. When the shutter is fired the camera's contacts connect the capacitor to the bulb allowing the full charge in the capacitor to discharge into the bulb.
Originally Posted by fotch
A simpler circuit would be just connecting the battery to the bulb via the sync. contacts but the battery's relatively high internal resistance would lead to erratic performance.
A capacitor has a lower internal resistance and as such it can discharge faster and with more instantaneous current than the battery leading to more reliable firing.
The resistor prevents sufficient current to flow to fire the bulb as you place it in the holder as that is definitely something you wouldn't want to happen!
Last edited by Steve Smith; 07-05-2010 at 02:36 PM. Click to view previous post history.
Reason: Circuit drawing added.