# Calculating resistance of xenon flash tube

## Main Question or Discussion Point

Hi,

I am trying to build a high-speed flash, based on the PCB from a disposable camera, with the aim of producing a flash with a duration of 5μs - 20μs. The main circuit works, firing four xenon flash tubes from a 330v capacitor (charged to 300v).

This is the schematic for the disposable camera PCB:

http://img526.imageshack.us/img526/8117/kodakpcb.png [Broken]

The only differences with my circuit are that I'm using four third-party flash tubes (datasheet here: http://www.produktinfo.conrad.com/datenblaetter/525000-549999/540866-da-01-en-BLITZROEHRE_200_450V.pdf" [Broken])

I have found two ways of cutting the discharge off early - one using two SCRs and the other an IGBT:

http://img705.imageshack.us/img705/711/2xscrswitch.png [Broken]

http://img577.imageshack.us/img577/9936/igbtswitch.png [Broken]

I am unable to select the right components because I can't calculate the minimum resistance for the flash tubes and therefore the maximum current for the main discharge circuit, i.e. the max current either the SCR or IGBT would have to handle.

I have found two documents describing how this can be done: http://www.excelitas.com/downloads/APP_PFL.pdf" [Broken].

The first is the source of the SCR/IGBT schematics and, not surprisingly, defines the minimum resistance of a flash tube as being just under the maximum voltage of the main discharge capacitor divided by the maximum current. The second document uses a different equation based on the properties of the flash tube but still requires the maximum current in order to provide the minimum resistance.

I need either the minimum resistance in order to calculate the maximum current or the maximum current in order to calculate the minimum resistance but I have neither!

The first document notes that the minimum resistance of the flash tube is likely to be between 0.1 - 5Ω. This is a huge range! If, for example, the minimum resistance of my flash tubes were 0.5Ω then the maximum current from a 300V capacitor would 600A. I have four tubes which means the current through either the SCR or the IGBT would peak at 2400A! If, on the other hand, the minimum resistance were 5Ω then the maximum current would be 60A * 4 = 240A. This is still enormous.

As is probably obvious, I don't really understand the effect on a transistor of very brief surges. The maximum surge current in most transistor datasheets is given in the order of mS or s. Say, for example, the datasheet states that the device can handle x current for 1mS but my circuit will discharge for only 1/50,000S, can I multiply x by 50 to get the safe maximum surge current?

Any help would be much appreciated!

Moocrow.

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anorlunda
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Ping @berkeman , component related question.