If a camera flash is essentially a capacitor, how is it able to discharge so quickly, when it takes so long to charge? Aren't the charge and discharge transients symmetrical?
by 
PerpetualAFK
ANSWERS: 5
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My friend said that it is due to the internal resistance of the battery, which causes the battery to have a maximum output current. This makes sense, but if that were the answer, then battery manufacturers would be marketing their batteries by max current output: "Will charge your flash faster than any other battery!" I think this is only part of the answer. Anyone else have any input?
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One of the advantages of a capacitor over a battery is the ability to discharge it's capacity quickly without damage. When discharging a battery at a high level, it will heat up quite a bit due to it's internal resistance. There were many times when I could not safely remove the battery from my radio-control car after running it; the sustained 20-30 amp draw made it dangerously hot. Discharging a capacitor is different. It can dump it's charge nearly instantly without immolating itself. However the recharge circuitry is the weak link. Where does the capacitor get the energy to recharge? The capacitor itself CAN be recharged instantly, but it takes a lot of power to do it safely, without burning up something else. Now, if you want to carry around a large battery pack with many amp-hours so that it can safely sustain the amperage draw of a quick recharge then be my guest. However if you want to use AA batteries then just deal with the slow recharge.
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The camera flash is not only a capacitor, it is a fast high voltage switch, the actual flash lamp (gas discharge lamp), and the said capacitor. The energy that goes to the lamp is stored in the caps at a few hundred volts (roughly Joules of electrical energy, depending on size of flash). The time transient to discharge is mostly determined by the inductance of the circuit (the ohmic resistance of both the ignited lamp and the switch are quite low). The charging circuit, otoh, is a HV source and a battery, the HV source mostly a transformer and an oscillator (you can hear that high pitch during charging?). To deliver the Joule to the cap, a current of a few ten milliAmps for a few second is drawn from the battery. So the charging and discharging are VERY different!
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It is all about energy. To 'flash' the camera strobe takes a LOT of energy, and that energy needs to be used in a very short time. Batteries are able to provide to total amount of energy needed, just are not able to produce it in the short timeframe needed by the flash. Here is where the capacitor comes in. It has the ability to release energy very very quickly. So a charged capacitor is used to provide the energy to the flash in the small fraction of a second needed. On the other side, a battery is used to recharge the capacitor. And given a few seconds, the battery is able to provide the total amount of energy needed back into the capacitor. This is why it takes time to recycle between flashes. BTW: Different batteries have different abilities to provide energy: In this case, NiCad batteries will 'give it up' a little faster then disposable alkaline batteries. So, if you use Nicads in your flash, you will be able to cut down the recycle time a bit.
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Camera flashes are essentially a type of tesla coil. The rising pitch you hear is an ocillator created by the coil and the cap; as im sure you probably already know. capacitors take only what you give them and then cut off current once its full. If you hooked up a AA battery to it, it would stop charging at 1.5v; albeit instantly. In order to create enough voltage/curent, the power from the battery must be stepped up. The current takes a few seconds as the charge builds up between the cap and the coil. Current is converted to AC at thousands of cycles. every cycle completed adds up with the voltage of the next cycle as the capacitor's transient is slower than the ac being applied to it. The cap discharges quicly because the flash itself requires all of the current stored immediately in the cap. This is as i understand it... keeping in mind that i can be totatly wrong.
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