Hope I'm in the right subforum. Have not found one about DIY.
I intend to build my own chemical flashbulbs. It will be an oxygen filled and sealed glass recipient with aluminium foil snipplets.
What I don't know is how to ignite the aluminium.
I thought about running two electrodes with a tiny gap between them inside the recipient and connect them to a loaded condensator (best a high capacity goldcap) to produce a spark. Was this done so in the industry produced flashbulbs? Or was the ignition by incandescent filament?
What I know is that in the industry produced flashbulbs a chemical ingitor (a priming paste or powder) existed inside the bulb that got hot and was sputtered around to ignite the aluminium or magnezium foil or wire.
Can somebody point me to information what the chemical used as primer was, how it was applied and how it got ignited and provoqued to sputter?
Don't use a goldcap, the internal resistance is too high. Use a standard electrolytic capacitor with low resistance, e.g. one designed for switch mode supplies.
Don't know anything about the involved chemistry.
A standard flashbulb firing circuit would connect a battery, a capacitor, a resistor, the flashbulb and the shutter contacts all in series. When the bulb is inserted it completes the circuit and the capacitor starts to charge. The resistor is included to limit the current flow so that the bulb does not fire as soon as you plug it in.
The shutter contacts are wired such that when they are closed, the capacitor is connected directly across the bulb therefore dumping its full charge into the bulb.
It is done this way rather than simply connecting a battery via the contacts as a battery has a relatively high internal resistance which would give a bit of variance on the point in time in which the bulb fired. The capacitor's lower internal resistance allows it to deliver a short high current pulse which will set off the bulb more consistantly.
Steve, I know the circuit, it is how I build mine too.
Andreas, didn't know about the high impedance of the goldcaps. Just thought goldcaps have the highest capacity at the lowest physical volume. Speaking about goldcaps, I thought about using them for another application too. I wanted to use the spark to perforate thin alufoil to make pinholes. Do you think I am better of with normal electrolytics for this application too?
I believe flashbulbs had some sort of combustible material coated on a filament (sort of like the sparklers seen around here at holidays). The material ignited rapidly in a burst of very high temperature which then ignited the foil. Don't know what the stuff was, but trying to buy it today would probably put me on a watch list here! I would guess it probably involved powdered magnesium and an oxidizer in some sort of binder.
That's the later "refined" versions; I'm pretty sure the earlier units that used two or three C or D cells were just plain brute force. A D cell can pump out many amperes (briefly). Going to the capacitor types allowed use of a battery that was much smaller physically, and often 15 or 22 volts. (I sort of lived through that transition. :whistling: )
Originally Posted by Steve Smith
I think many flash bulbs used zirconium foil if I remember right. Not sure if thats any better than aluminum and probably way harder to find.
ever used model rockets? Try nichrome wire.
Have you ever pushed a 9 volt battery into wire wool?
Once it gets going, it continues to burn. I think flashbulbs are similar.
Actually, rocketteers who 'roll their own' hunt down ag1 flash bulbs as a relaible ignitor.