25

So whenever we want to shoot our flash before taking a photo. we have to charge it first.

What is the point of the charging our flashes? Aren't their power directly supplied by the battery of our camera?

Please answer for the built in flash on the 2000D and the traditional hot shoe Xenon flashes.

Perhaps these hot shoe xenon flashes have their own batteries charged by the slow hot shoe port. Who knows?

  • 3
    Just a comment regarding [...] for the built in LED on the 2000D: AFAIK, the EOS 2000D's flash is not a LED. In fact, I only know of video LEDs and modelling LEDs (so continuous light that might be placed onto a flash unit), but no flash unit comprised of LEDs. – flolilo Feb 17 at 15:16
  • 2
    Also, even a pulsed LED light strong enough to be useful for more than a few meters would likely involve a pre-charged capacitor - once you need a few amperes, you'll run into issues like creating brownouts, ground bounce, battery stress if you power that straight off the camera battery.... – rackandboneman Feb 17 at 18:51
  • 3
    Also, a standard hot shoe, and most brand-specific hot shoe layouts, haven't any charging power available. Sony MIS/iISO offers access to the camera battery, but AFAIK it's not used to power any full size flashes... – rackandboneman Feb 18 at 0:07
  • 1
  • "Why do we charge our flashes?" Because they don't have any cash? – Michael C Feb 21 at 18:59
74

The peak power at work when a flash is being discharged is extreme.

Example: a classic Metz 45CT (a large but still handheld unit) on full manual power delivers around 90 watt-seconds of electrical output to the flash tube in 1/300s. That means there are 27000 watts at work for a short time.

A 27000 watt lightbulb (BTW, your flash is much brighter than that - more efficient!) would need the ampacity of around eight european household sockets (230V, fused at 13 or 16 amps) to operate, since it would draw around 120 ampere from them. At the voltage levels xenon flash tubes typically operate at, around 80 amperes will flow; if you wanted to pull that energy "live" from a 5-6V battery, you'd need to pull around 5000 amperes AND instantaneously convert them to around 80 amperes at high voltage. Batteries that could deliver 5000 amps - while not wasting most of the power as heat generated across their internal resistance - for a short duration and live certainly exist - you are looking at starter batteries for big lorries, which are rather inconvenient to lug around. Also, a 5000 ampere current pulse is quite a magnetic nuisance if there is anything either ferromagnetic or intended as an electrical conductor near the conductors used.

Your average "big" speedlite, btw, will be around half the power described here (but might in some cases reach the same or higher peak currents in order to discharge even faster), an in-camera flash will be a fraction of these numbers - but still inconvenient to build any other way.

If you need a short, massive current pulse, the best energy source available in engineering is a previously charged capacitor (metal-paper or electrolytic type, not dual-layer "supercap"). This energy storage device is large for its storage capacity if you compare it to a battery - but able to respond far quicker to high current demand, since a) there is no chemical energy conversion involved in its operation, b) the relevant current path only involves metallic, very conductive parts instead of ionic conductors (aka electrolytes. An electrolytic capacitor, btw, does use its electrolyte for chemical self-upkeep, not for chemical energy storage or electrical connectivity).

Also, since the charge in a a capacitor is limited, it allows portioning total energy to what the flashlamp can safely take. This allows a flash in manual mode to just completely discharge the capacitor until it is nearly empty (when the remaining charge can only deliver voltage below the quench voltage of the flashlamp) without complex control circuitry - and at the same time gives you a safety fallback when working with dynamically controlled power levels (telecomputer or TTL mode), leaving no way for catastrophic circuit failure in combination with several kilowatts of constant power available turning into a massive safety hazard. A handheld-sized efficient electrical near point light source with 27000W input, driven constantly, would first severely damage your eyes and then self destroy violently (think of a welding arc - dangerously bright, and it melts metal).

  • 11
    Is this still "Photography" StackExchange or this is the "Physics" section :o) – Rafael Feb 17 at 19:02
  • 17
    @Rafael:If you dig deep enough . . . EVERYTHING is Physics! :) – Binary Worrier Feb 18 at 12:51
  • 2
    In practice, every flash that has some kind of power control (TTL, telecomputer, power steps...) does have the facility to either stop the discharge (via an IGBT or Gate Turn Off Thyristor (GTO), or divert it into another flash tube (aka quench tube) that is kept in the dark inside the unit (that is how the earliest controllable flashes actually did it!)... Still, one would not want to rely on any of that as a safety mechanism. – rackandboneman Feb 18 at 15:06
  • 1
    @J... why not make that a second answer - different angle, same truth. Also, said Metz flash (GN45m@ISO100@28mm) takes 6 AA batteries and won't charge from empty in 5s unless you have very fresh and very good batteries. – rackandboneman Feb 20 at 8:23
  • 1
    @rackandboneman Well, the Metz is a dinosaur with awful NiCd cells. I think most people who are still using them have replaced the stock battery packs with something better. 3s recycle at full power is fully achievable with NiMH retrofits. – J... Feb 20 at 12:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.