How camera Flash works

Thread Starter

Jayccob

Joined Oct 26, 2015
7
So I am playing around with some old disposable cameras and one thing I had no clue what the point is the end of a transformer connected to the back plate of the flash tube. What is this for? I couldn't find anyone explaining it just briefly saying it exists. I haven't notice anything difference when I disconnect it. Here's a picture I borrowed from google search. xsbint_strobe2.gif
The spot in question is the T2 white wire in the bottom right. What is the purpose of this?
 

DickCappels

Joined Aug 21, 2008
6,380
When the "fire" button is pressed a high voltage pulse is applied to the plate. The result is an intense electric field through the gas to the two internal electrodes. This electric field is strong enough to strip some electrons from the gas to ionize it, thus, as Dodgydave pointed out a second ago, causing the flash to start.
 

Thread Starter

Jayccob

Joined Oct 26, 2015
7
So I am still missing something here, so the pulse never actually touches the electrodes just the plate and the created field starts the xenon flash? If that is the case why can I disconnect thee wire leading to the plate and it still works?
 

Thread Starter

Jayccob

Joined Oct 26, 2015
7
So correct me if I am wrong but what I am coming up with is that in a sense the flash tube is acting like a capacitor in it's own right. When the plate is charged with the really high (+) volts it creates differential across a insulator, like a capacitor, and rips the (-) electrons from the Xenon gas creating the necessary ionization?
 

dl324

Joined Mar 30, 2015
10,718
So correct me if I am wrong but what I am coming up with is that in a sense the flash tube is acting like a capacitor in it's own right.
A capacitor stores energy and then releases it. The flash tube doesn't really store any energy.
 

AnalogKid

Joined Aug 1, 2013
8,443
Yeah, sorta kinda. Anytime there is a potential difference across something there is some capacitance, but thinking of a flash tube as even an approximation of a capacitor puts it in the wrong category of basic electronic components. A better approximation is a spark gap. You can increase the terminal voltage up and up until it exceeds the breakdown voltage of the insulator between the terminals; in this case, pressurized xenon gas. But the breakdown voltage changes with temperature, pressure, and ambient light. One way to stabilize and control the exact moment of breakdown is to have the terminal voltage very high but not high enough, then trip things over by injecting enough energy into the gas that breakdown is assured, the electrical equivalent of a small spark igniting large gas torch. In a flash this is done with a 2000-4000 V electric field pulse.

ak
 

DickCappels

Joined Aug 21, 2008
6,380
The spark gap analogy is good for the two electrodes that supply the energy for the flash but the analogy (no, this is not a pun) breaks down when it comes to the triggering voltage, which is capacitively coupled through the glass wall of the tube.

A broader view of capacitance: There is capacitance among all conductors that are not electrostatically shielded from one another. For example, the metal watchband on my wrist has capacitance to the Sun until it goes below the Sun horizon.
 
@Jayccob

FWIW a passive 'variant' of the phenomenon is manifest in the observation that 'strike' EMFs of linear florescent tubes are reduced in the vicinity of ground planes -- In ether case the 'dynamic' is increased EMF gradients...

Best regards
HP:)
 
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