Capacitor from the electronic flash units of cameras. Typically around three to four hundred volts and designed for rapid discharge.
Photoflash capacitors are a commonly used in coilguns. It is important to note that a protection diode is useful in preventing damage to the capacitors when they are discharging through something inductive, such as a coilgun coil.
From Disposable Cameras
Disposable cameras are often obtainable for free just by asking at a processing outlet.
They tend to have a voltage rating of 330VDC and a capacitance of 80-120 µF, and occasionally 160µF. That comes to between 4.5 and 9J each.
The equivalent series resistance (ESR) of these parts is fairly low which means a parallel array of them can provide very short risetimes.
Experimentation shows that they are not suitable for series connection to increase their voltage rating. This is likely due to the high variability of capacitance, causing some capacitors in series to be charged to much higher voltages than others, resulting in catastrophic failure. Photoflash capacitor banks in series-parallel can be made safely if balancing resistors are used for charging.
If they are used for pulse discharge or inductive experiments (such as a coil gun or rail gun) it may be advisable to inverse parallel a high current diode and series resistor across them in order to limit reverse charging.
It would be a good idea to keep flash capacitors somewhat below their rated voltage for maximum life.
Approximate Occurrence in Sample of 200 used Disposable Cameras
|Temperature Range||-20 C (-68F) to 60C (140F)|
|Reverse Breakdown Voltage||1.7V|
|Rated Max Working Voltage||330VDC|
|Max Surge/Withstand Voltage||350VDC|
|Approximate Lifetime||~100,000 Cycles|
|Capacitive Tolerance||-10% and +20%|