[Prev][Next][Index][Thread]
Re: flash tubes
Hi Nick,
>Original Poster: NickandSim-at-aol-dot-com
>
>Hi All,
>I have to disagree with Reinhard over the life of the tubes. All
>commercially made xenon flash/strobe tubes have a third electorde for
the eht
>to ionize the gas, if you read a data sheet for a flash tube it wil
quote a
>minimum trigger voltage, and all commercial strobe or flash units have
>pulse transformer. This is not easily reconisable - it is about the
size of
>5 dimes stacked on top of each other and potted in resin.
>The factor which determines tube life is the size of the discharge cap
(you
>can destroy a tube by putting a big cap on it ie. 680uF will shatter
your
>tube all over the place and leave you picking glass out of your eyelids
>if you were stupid enough to look at it - he,he,he)
SNIPola
Well, this is NOT what I said! I said you donīt NEED a pulse
xformer to FIRE the tube. This is not the way commercial flashes
are built, because firing a X-tube w/o a pulse xformer WILL
shorten itīs lifetime. All I said, was you donīt need a trigger
xformer to ionize and trigger the gas inside a X-tube. A
standard 100Ws flash tube can be easily triggered with a 7.5kV
NST. This only gets you a dim discharge. Adding a 20-30nF cap
parallel to the tube and the NST, will allow you to flash the tube
(self triggering) at "normal" brightness every second or so (due
to the resonant rise, the NST doesnīt need more than 25V on
the primary side. However, this forced self triggering of the tube
puts a gigantic load on the tube and it will get quite hot after only
a few seconds of operation. I never said flash tubes are a
possible exhange for the spark gap. The pressure inside is too
low (and as the gas is easily ionizable, yet only de-ionizes very
slowly, they will quench like sh..) and the unit is not made to
dissapate the heat produced. All the pulse transformer does,
is ionize the gas, so that it will conduct at 300-500V. If you
force-trigger the tube via a high enough input voltage, the
trigger transformer just isnīt necessary. The lifetime of the
X-tube IS NOT soley dependant on the input Joules (cap size).
It is a part of the lifetime factor, but it is not "THE" factor.
Example:
1.)
NST (self triggering setup)
Cap: 25nF
Peak voltage: 8kV
J = 0.5*V^2*C = 0.8J
2.)
Flashtube PSU (triggered setup)
Cap: 330ĩF
Peak voltage: 500V
J= 0.5*V^2*C = 41.25J
The second setup gives you ~ 50 times the Ws, yet
the tube will survive this (i.e: normal lifetime). The
first setup, fires the tube automaticly (self triggering)
every 0.5-1 second. After about 10 sec the tube is
so hot, that it will burn your fingers. If you canīt
believe me, just try it.
Coiler greets,
Reinhard