Q. Electronic Photo Flash?

A simplified schematic diagram of the electric circuit of an electronic photo flash typically used on cameras is shown in Figure 7.7.1. By passing a high current through the flash tube, a bright flash of light is to be produced while the camera shutter is open. The power level is quite high, as much as 1 kW. The total energy delivered, however, is only on the order of 1 j, since the flash lasts less than 1 ms.

The electronic switch alternates between opening and closing approximately 10,000 times per second. Energy is delivered by the battery over a period of several seconds and stored in the capacitor. The stored energy is extracted from the capacitor whenever needed. The battery  source causes the current in the inductor to build up while the electronic switch is closed. Recall that the current in an inductor cannot change instantaneously.When the switch opens, the inductor forces current to go through the diode in one direction, charging the capacitor. Thus the diode allows the capacitor to be charged whenever the electronic switch is open, and prevents the flow in the other direction when the electronic switch is closed. The voltage on the capacitor eventually reaches several hundred volts. When the camera shutter is opened, another switch is closed, enabling the capacitor to discharge through the flash tube. Nowadays several practical electronic circuits employ diodes, BJTs, FETs, and integrated circuits.