# Flashlights and Capacitors

Why capacitors are used in flashlights of cameras instead of attaching the light directly to the battery? How this leads more light?

There are 3 main reasons for using a capacitor.

First it stores the energy, so it can deliver a pulse of energy that is far larger than the battery can. Remember it may take several seconds of battery energy to fully charge the flash capacitor. Then the capacitor releases all that in less than a millisecond ($10^{-3}s$) or even just a few microseconds, so the flash bulb gets a massive jolt of energy.

Secondly, the flash capacitor stores the energy at a much higher voltage: we're talking about up to 1000V (typically around 300V), instead of the 6V from 4 AA cells.

Finally, the capacitor is designed so it can deliver extremely high currents, again higher than the battery can deliver by itself.

Finally, the charge can stay on that capacitor for a very long time. Never touch a charged flash capacitor. The energy stored in them can be lethal!

• Thank you hdhondt and freude for helpful answers. I am doing some calculations but I can't get to this conclusions. The capacitor $C$ charged with a battery $\varepsilon$ will release the charge like $\varepsilon C e^{-t/RC}$. $R$ is the resistance of light. The power $P=I^2R=(\varepsilon^2/R)e^{-2t/RC}$ which is not more than the $\varepsilon-R$ circuit power which is $P=\varepsilon^2/R$ ! Apr 26 '13 at 11:15
• About the second reason, how is that done? $300V$ from $6V$? Apr 26 '13 at 11:27
• The charging circuit first converts the 6V DC to AC. Then it uses a transformer to increase the voltage to whatever the flash needs to fire, rectifies this and charges the capacitor. This high voltage, and the lerge current is what gives the flash its power. The same system (but in reverse) is used in powerpacks. First the AC mains is rectified. The resulting DC is converted to high frequency AC. A transformer is used to bring the voltage down, and then it is rectified again. In both cases, the use of high frequencies means a tiny transformer can be used. Apr 27 '13 at 10:48

Because we don’t need lighting, but flash, that means we have to release much energy is a short time interval. Capacitors could accumulated and release energy fast.