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I need to repair the Nikon's SB-600 speedlight. As I need to access to only the bottom part of the speedlight I skipped the danger of electric shock. However when I tried to disconnect one of the socket I get the shock to my finger. As I have no resistor I put the speedlight in the closet for about 24 hours and now I'm wondering: how likely that the capacitor is discharged to safe level?

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    Chances are that after 24 hours the cap has discharged most of the way. These things are small anyway, so even a full charge may be unpleasant, but unlikely to hurt you. I'd be careful to make sure you don't have a discharge path between your hands as that current will flow near your heart. Anything that a small speedlight can do that stays within one arms is very unlikely to cause anything more than a unpleasant jolt. However, resistors are cheap and plentiful. Get a 10 kOhm resistor already and discharge the thing properly if you're that worried about it. – Olin Lathrop May 9 '14 at 12:43
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    I don't know if this will help, but have you ever had an electronic device where an LED or something stays on when you unplug it? Well more often than not if you then press the power button it will suddenly lose all that power and the light will go off. I have a similar thing with my breadmaker, the screen stays on when I unplug it for about 10seconds but if I press a button before that time, it will go off, like the power has been released. So what I am suggesting is try pressing the buttons to discharge any remaining power. – connersz May 9 '14 at 15:31
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    @connersz the capacitor of the flash doesn't work like that. The items on the screen and the lights are powered by the batteries. The batteries also charge the capacitor, but the capacitor holds charge on it's own, and releases all of that charge to the bulb, but only the bulb, in one go. – laurencemadill May 10 '14 at 10:56
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    Note that one thing you could do is use some part drained batteries in it that mean the recycle time is much reduced. Then fire a test flash and open the battery compartment immediately after. The less time between firing the flash and disconnecting the batteries, and the longer the recycle time from the batteries, the less chance the capacitor is going to have to recharge – laurencemadill May 10 '14 at 10:58
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A properly formated electrolytic capacitor almost has sort of an imprinted charge (like electret condenser microphones) and will lose very little charge by itself since the voltage is what actually causes it to maintain its isolation (which is why you should power up flashes for at least 15 minutes every 3 months or so if you don't use them or the isolation will deteriorate, greatly increasing the danger of breakthrough when finally powering up the flash, and breakthroughs are often terminal). It very much depends on the rest of the circuitry how much charge will stick around if this is a flash in active use (some flashes will discharge controlledly in few minutes, others won't).

Note that using an 1/1 manual flash is not guaranteed to get rid of all charge: flashes cannot utilize every bit of charge and may maintain reserves for preflashes, red-eye flashes, prefocus flashes and what other uses are there.

So in general be very very careful until you actively discharged the capacitor, and don't underestimate the stored energy when doing so as it can act rather destructively. If you have the schematics, there often are terminals reasonably safe for discharging since they have circuitry limiting the surge and dissipating the energy. If you cannot read schematics, are you sure you are the right person opening a flash? Some wand flashes store 90J or similar. A 100th of that energy will give quite a jolt to horses touching a medium-size electric fence.

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Surprisingly, but after three days the voltage on the board connector was still more than 200 Volts! So I have to state that the capacitor have very low self discharge current.

I connected the resistor directly to the board connector as I could not reach the dedicated contacts on the flash head.

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If the flash is set at 1/1 full power, then it's less than on second after you have pressed the test or shutter button.

Unless you are an engineer or good flash tech, then you use a resistor and short the capacitor to ground.

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I have personally pitted screwdrivers on (non-photoflash) electrolytic capacitors that had sat charged but idle for WEEKS.

Self discharge specifications of such capacitors will always be worst-acceptable numbers, since self discharge and leakage currents are usually considered undesirable to an engineer.

Never rely on a big capacitor being safe unless you either a) received it obviously shorted, b) discharged it thoroughly, c) measured the voltage across its terminals.

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