Photoflash capacitor

A photoflash capacitor is an electrolytic capacitor used in flash cameras, professional flashes, and also in solid-state laser power supplies. Their usual purpose is to briefly power a high-voltage flash tube, used to illuminate a photographic subject or optically pump a laser rod. As flash tubes require very high current for a very short time to operate, photoflash capacitors are designed to supply high discharge current pulses without excessve internal heating.

The principal properties of a capacitor are capacitance, working voltage, equivalent series resistance (ESR), equivalent series inductance (ESL), and working temperature

Compared with electrolytic capacitors usually used for power supply filtering at power frequency, a photoflash capacitor is designed to have lower ESR, ESL, and capacitance manufacturing tolerance, but does not need as high a working temperature.

The light energy emitted by a flash is supplied by the capacitor, and is proportional to the product of the capacitance and the voltage squared; photoflash capacitors have capacitance in the range 80-160 microfarads (µF) and voltages from 180–330 volts for flash units built into small disposable and compact cameras, increasing for units delivering higher light energy.^[1] A typical manufacturer's range includes capacitors operating at 330–380V, with capacitance from 80 to 1,500 µF^[2] Photoflash capacitors are not subject to the high temperatures of cased electronic equipment in continuous operation, with nearby components and sometimes the capacitors themselves dissipating heat; they are often rated at a maximum operating temperature rate of typically 55 °C, compared to 85 °C–105 °C or more for capacitors for continuous use in electronic equipment. In most electronic applications an electrolytic capacitor can have a capacitance much larger than its nominal value without detracting from circuit performance; general-purpose electrolytics are often specified to have capacitance between 20% below and 80% above rated value, although tighter tolerances are available. The light energy of a flash is proportional to the capacitance and large variations are not acceptable, typical tolerance is -10+20%.^[2]

Photoflash capacitors are designed to deliver a brief pulse of very high current, and are consequently sometimes used in railgun and coilgun designs.

References

1. ↑ Forrest M. Mims, III, Forrest Mims' Circuit Scrapbook II, Howard W. Sams & Co., Indianapolis IN, ISBN 0-672-22552-2, page 149. Mims gives the rating of the Kodak Disc camera flash as 160 µF and 180 volts.
2. ↑ 2.0 2.1 Hitachi Photo Flash Aluminum Electrolytic Capacitors