To any of you with a bit more experience in electronics, I have a few questions regarding this:
1. Will any capacitor rated at 330V work fine, or do I need a specific type that will cope with the rapid discharge?
Yea and no. Any cap rated to at least 330V will work, but it won't necisarily work "fine". Photocaps are a special breed of electrolytic cap with a couple of unique properties. (1) They are designed to be short circuited without damaging themselves. (2) They have somewhat lower internal resistance (ESR; equivalent series resistance) than typical electrolytic caps.
2. If I use a cap that can't handle it, will it just fail to work, or will it burst apart spraying it's contents in my face?
Mostly likely problem with a cap with too low a voltage rating is that it just won't charge up to 300+ volts. When you overcharge a cap it starts to leak across its internal insulator. Apply too high a voltage at low current (like a photoflash charger circuit) and it'll just leak voltage once it is to the rated voltage. If you had a high current high voltage supply you might be able to get the cap to burst. With a photoflash board that isn't going to be a problem.
3. Does it matter if I get a cap that is rated to a much higher V than 330V? Ie will a 600V cap behave just like a 330V cap if they are both only being charged to the 300V the camera circuit can deliver?
Any cap rated to at least 300V will be charged 300+V.
4. Should somwhere like Dick Smith's Electronics stock suitable caps, or will I have to look elsewhere? Basically, how hard is it to find caps that would work for this purpose?
Photocap's are pretty unusual. I suspect you will have to order them online since RadioShack and the like are unlikely to have more than one type (and it'll probably match what you all ready have from the disposable cameras).
A couple things about high speed photography with a flash.
1. Typical flash units have a flash duration of about 0.1 to 1 millisecond, 0.5mS is pretty common. That puts a pretty serious limit on your ability to freeze action. At 300 FPS an 0.1mS flash means the object moved 0.36 inch during the exposure. At a 1mS flash it moved 3.6 inches.
2. It is pretty simple to measure the flash time. Get a photodiode or phototransistor (or even a cadmium sulfide cell or photocell) and connect it to the MIC input of a PC. Start an audio recording and flash the detector with your flash unit. Use something like Audacity to display the signal and measure the flash's duration. I suspect that the typical disosable camera has a flash duration of about 0.5mS.
3. Flash duration can be estimated from the equation of a capacitor discharge through a resistor (the resistor is the xenon phototube), an RC circuit. In an RC circuit the voltage will drop to about 40% of its initial value in R*C seconds (R in ohms, C in farads). You really can't do much to change the R of the xenon tube but you can change the C of the capacitor. Cut the cap's capacitance by ten and you'll shorten the flash duration by ten (using a very simple minded model of the discharge
). In the real world it won't work out quite that simple. The tube has resistance but so does the photocap. Often the cap's resistance dominates the circuit and it is it's R that should be used in the RC equation. Generic caps have internal resistances (ESR) values of perhaps 1 to 0.01 ohms. High speed photo caps have resistances of perhaps 0.01 to 0.001 ohms. There are caps with even lower resistance. They will give much shorter flash durations but they are rare and expensive. You already have a bunch of these very low ESR caps since there is one on each of your photo boards. The low ESR cap is used to supply the power to the trigger coil. Their low ESR means they'll dump their energy very quickly through the trigger coil. Since transformers work based on the rate of current change (dI/dT) the faster you can dump the cap the higher the output voltage of the trigger transformer is. The trigger cap is charged to 300 V just like the big-ass photocap. It is dumped through the very low resistance of the trigger transformer and creates the 5~10KV trigger voltage that triggers the flashtube.
So you have some high speed caps of the correct voltage rating. Unfortunately, their capacitance is probably too small to be of use. IIRC, they are something like 0.01MFD. So that cap only holds ~0.01% as much energy as the 120 MFD photocap.
A smaller cap value should give a faster flash. If you wire caps in series the capacitance goes down; 1/Ctotal = 1/C1 + 1/C2 + 1/C3 + ...
The RC equation says the lower capacitance should give a shorter flash duration (and less energy and brightness in the flash). Unfortunately, as you wire caps in series you have to add their ESRs. So the C goes down but the R goes up. Net result is that RC doesn't change all that much, and the flash duration doesn't change much.
You can buy low ESR high capacitance caps but they are pretty pricey. You might want to look at Mouser.com or Digikey.com.
Kind of a long shot, but you might scavenge a bunch of the trigger caps from your photoboards. Wire them in parallel. That adds their capacitances. In parallel the reistance is 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + .... If they all have about the same ESR then the final resistance is Rtotal = R<sub>one cap</sub>/(number of caps). Ten caps will drop the resistance by 10 fold. The capacitance goes up ten fold. If the trigger caps are 0.01 MFD it'll take quite a few to get the energy for a sufficiently bright flash.
There are "simple" circuits that can turn off the cap's discharge before it turns itself off. So it might be possible to get a truncated flash with a duration of perhaps 0.01mS (0.036" movement at 300 FPS). Google might help you find something helpful. I suspect that this would be the best way to try to get a shorter flash duration.
One last thing, the 60V (or so) you measure across the shutter trigger on the board is actually the same valtage as is on the trigger and photocaps, 300+V. You only read 60V or so because the switch is in series with a 1 megaohm resistor.
http://www.repairfaq.org/sam/kflashm.gif is a schematic of one type of Kodak disposable camera circuitry. The low ESR trigger cap is C3. The photocap is C2. C3 is charged to the same voltage as C2 but since it holds so little energy when you measure the voltage you actually are measure the photocaps (C2) voltage through a voltage divider. The divider is the 1 megaohm resistor (R2) and the internal resistance of you volt meter (probably 100K to 10M ohm).
(no, seriously, I have played reversi on it))
