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Re: [OM] T20 inop on Auto

Subject: Re: [OM] T20 inop on Auto
From: HI100@xxxxxxx
Date: Thu, 18 Jan 2001 02:40:18 EST
litefoot@xxxxxxxxxxxxxxx writes:

<< Would you mind explaing briefly what and how a quench tube works? How does
 it limit light output if as mentioned, the T20 actually makes a full dump of
 output each time regardless of aperture? 
>>

The point I was making in my followup post was that the T20 does NOT use a 
quench tube it uses an SCR series circuit.

Early style auto flashes using quench tubes were invented and patented by 
Harold Edgerton, a professor at MIT. 

These flashes work like this:
1)  a large capacitor is charged to a voltage usually between 330V and 550V .
2)  this capacitor is connected across the flash tube with a current limiting 
inductor in series . 
3)   when the flash is triggered by the camera, a large voltage (at low 
power) is applied to the outside of the tube triggering a very high current 
to flow through the Xenon gas in the tube and emit light for a period of time 
determined by the inductance in series with the tube and the energy stored in 
the capacitor.
4)  a second "quench" tube somewhat similar to a normal flash tube (with 
lower voltage cutoff ) is connected in parallel with the main flash tube
5) A light sensor detects light reflected back from the subject to be 
photographed and generates an integrated light signal proportinal to the 
accumulated light over time. When a predetermined amount of light has been 
recieved the second quench tube is triggered which diverts current from the 
main tube and hence extinguishes it's light output. Diverting the remaining 
energy from the flash capacitor into the internal quench tube wastes the 
remaining energy making it necessary to almost completely  recharge the 
capacitor from scratch. 

In a more modern "series type circuit" like the T32/20 etc a very high 
current semiconductor switch like an SCR, GTO thyristor or IGBT  is connected 
in series with the flash tube. This switch is switched off (commutated off) 
to interupt the current flow through the tube when enough light has been 
recieved. This saves the energy left in the capacitor for the next flash and 
can greatly shorten the capacitor recharge time.

The trigger signal used to stop the light output is often still called the 
"quench signal" even though flash quenching operates a bit differently in a 
modern flash. 

                  Regards,
                 Tim Hughes
                >>Hi100@xxxxxxx<<

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