When the voltage in the capacitor is high enough, current can flow through the resistor to light up the small tube. The capacitor circuit is also connected to a smaller gas discharge tube by way of a resistor. The capacitor is connected to the two electrodes on the flash tube at all times, but unless the xenon gas is ionized, the tube can't conduct the current, so the capacitor can't discharge. Like a battery, the capacitor holds the charge until it's hooked up to a closed circuit. The flash circuit stores this high-voltage charge in a large capacitor. The high-voltage current then passes through a diode, which acts as a rectifier - it only lets current flow one way, so it changes the fluctuating current from the transformer back into steady direct current. This oscillating action produces the high-pitch whine you hear when a flash is charging up.įlash capacitor from a regular point-and-shoot camera The circuit keeps interrupting itself in this way, gradually boosting voltage through the transformer. This voltage in the feedback coil conducts current to the transistor base, making the transistor conductive again, and the process repeats. The burst in current causes a change in voltage in the secondary coil, which in turn causes a change in voltage in the feedback coil.

When the transistor is 'switched on' in this way, a burst of current can flow from the battery to the primary coil of the transformer. Applying current to the base of the transistor allows current to flow from the transistor collector to the emitter - it makes the transistor briefly conductive (see How Amplifiers Work for details). When you press the charging button it closes the charging switch so that a short burst of current flows from the battery through the feedback coil to the base of the transistor.

Flash circuit from disposable camera