Yes, capacitors can be extremely dangerous. I always use extreme caution as I really don't know when they become dangerous, meaning by capacity.
It is the amps that kill.
Old motorcycle magnetos were always a shocking experience.
We do need to be careful what we advise.
Originally Posted by goamules
Garrett, the OP asked about a solenoid that is almost certainly line-voltage AC or low voltage (<24V). My suggestion is irrelevant in the first case, and, I think, pretty innocuous in the second.
We are not talking about photoflash capacitors here, much less Tobe-Deutchmann low-inductance multi-kilovolt pulse capacitors. In the grand scheme of things, a novice is probably more likely to be hurt with a big lithium battery (or even nicads or a car battery) than with a capacitor that stores less than 22 joules at 24 VDC. It's easy to get badly burned with the former, but unless you have a break in the skin or a pretty large area of skin wetted with electrolyte, you are going to have a hard time electrocuting yourself with a 24 volt capacitor. The current capacity is there, but the voltage to push current through the skin is not.
In addition to the risk of a lithium battery overheating, with any high-current battery there is the risk of a fire or burn from inadequate wiring and the absence of suitable protective devices. Hence the time honored advice to remove jewelry before working on an automobile--a wedding band can do a number on your ring finger if you get it wedged between the battery lead on a starter and ground.
Randy, I can give you a fairly good idea when they become dangerous. It's around 40--50 volts, usually, because that's high enough to produce a "sensible" shock. The size of the capacitor is less important at the low end of the voltage range, because if you can feel something, your reflex actions can lead to worse injury than the shock. Of course, if you are talking really big capacitors then arcing or simple resistive heating come into play, and if there are inductors involved then you can develop higher voltages through magnetic field collapse.
But this is a photography forum, not one on industrial safety. So I will point out that the energy storage in the aforementioned capacitor is comparable to that of a Swedish medium-format reflex camera held at about waist height. Isn't that why we all stick to large format?
Well If I could have found a 5" opening without electric shutter, It would have be the best choice but very large opening packard shutter are rare, and new they cost too much. I think of someone who may have the knowledge to help me, and hope I will be abble to use this shutter with my apo nikkor 1210 mm...
I undertsand that it won't be so easy to make it works... Well thank you for your advice, Thierry M.L.
If the solenoid is of unknown value why not replace the solenoid with a new one of known voltage ?
Should be easy enough to find a one that has the same throw when energized.
That depends on duty cycle. I have been looking into just that and have not yet found one that does a 10 minute hold the shutter open for focus cycle and then works quickly for a 1/30th shutter speed. That's why I am trying RC servo control.
We both would appreciate any suggestion on a solenoid replacement.
Packard shutters are inherently bi-stable devices; you open them, and they stay that way until you close them. The "instantaneous" feature uses spring tension to give the brief opening but in a way they are still bistable, in that there is a stable "closed" state with the cylinder unpressurized, and another closed state maintained by pressure after the exposure. For the one Packard I use with the instantaneous pin, gentle bulb pressure will open the shutter and leave it that way, which is great for focusing.
The obvious way to have both functions is to use a solenoid or motor to drive an air cylinder, and either put a rubber bulb in parallel for focusing or arrange an intermediate "stop" position where the blades are open.
I arrived at the solenoid/cylinder solution because I wanted to be able to do long exposures and didn't want to hack the shutter(s) to put in dual solenoids. An early version of the resulting setup, with all of its electromechanics, looks like this:
It ain't pretty, but it works. Presently, it also has two 68,000 mfd capacitors to buffer the battery, because the 12V gel cell becomes polarized pretty quickly by the large pulsed current draw. The solenoids are energized only to open and close the shutter, so the average power draw is very small. For focusing, I just manually shift the linkage to open the shutter.
The best solution, I suspect, would be to use a gearmotor with an eccentric to drive an air cylinder. Stop switches on a cam would allow either a full cycle for use with the instantaneous pin, or a "halt" at just the point where the blades are open. This would be pretty easy to rig, and would avoid the key problem with solenoids: the ratio of force between the beginning and end of stroke is pretty large, and it is the minimum-force point that dictates the current requirement unless you provide some sort of nonlinear force/position conversion (a cam or multibar linkage, for example).
Most of the photographers I hear from on forums can't calculate the lens speed from the focal length and apparent iris. Many ask how to cut a hole in a lens board. You want to talk about engineering an electrical shutter you're probably talking to yourself or a couple others. Without the proper background knowledge and test equipment no one will ever figure this all out. PM the OP and offer to make his shutter work, or skip it.
Electrical solenoids on LF shutters are a solution looking for a problem. Duty cycles, megajouls, and electron hole flow theory only matter to electrical engineers. Disable the solenoid spring and actuate the shutter manually somehow.
Or use this: http://www.youtube.com/watch?v=pb7OWlVYYRw
Garrett
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Harold, I love this this design! Thanks for sharing!
I never would have thought of the air cylinder you did. We used to use air switching relays to hammer 100 PSI to actuate 2" water valves open and shut for elaborate cooling systems. We needed to switch from -40 degrees to 250 degrees quickly to thermal shock automotive engines. I was the plumber, not the programer, but I did do loop tuning.
Here's my rig, not quite fully realized. Right now it runs on 4 AA battery pack and it's programed to run one shutter cycle when I switch the power on. 1/2 second shutter with flash sync. If I switch the power on and off quickly it leaves the shutter open. The microswitch is not yet functional, I am hoping to program it to reduce actuation lag.
You two need to watch the video I posted. It might give you some more ideas.
Garrett
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What is your solution?
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