Xenon flash tube with black deposit on inside of glass near cathode..?

[grizedale]

Hello,

What types of misoperation would be likely to cause this?

We can see this deposit on a 15W xenon flash tube.

Is it caused due to:-

Under-voltage triggering?
Too low anode voltage of the xenon tube?
Too high temperature in the flash tube enclosure?
First trigger pulse of wrong polarity?
Current going the "wrong way" in the tube due to oscillations from stray inductance in the wires leading to the falsh tube anode and cathode?

We are using "capacitive external triggering" as per page 4 of the following.

 

[rfredel]

Hello,

normaly if electrodes going black, the current will be too high. If you have a high following frequency of flashes, you have to limit the burning current.

For your example of your 15W flash tube, the 15W is the max.power. The the average value must be much lower.

Can you give the compl. circuit and the type of the xenon lamp. The it's possible to help you more.


Regards

Rainer

 

[grizedale]

in fluorescent tubes, such deposits are caused by having an average dc current in the tube, i wondered if its the same with xenon tubes?

 

[rfredel]

Hello,

the functional principle of flurescent tubes and xenon tubes are the same.

In flurescent tubes is a mercury gas with low pressure inside. This gas gives UV radiation which is displayed by flurescent material on the glass. In earlier time neon bulbs were used as stroboscope. They also don't use trigger transformers.

Inside of xenon tubes is Xenon gas with high pressure, which lights white. Also you need a higher voltage for ignition. That's the reason to use a trigger transformer.

Regards

Rainer

 

[FvM]

The black deposit is sputtered electrode metal. In a first order, it represents the accumulated energy that runned through the tube, similar to the "blackening" of CFL lamps. If you review the datasheets, you'll notice that flast tubes have rather finite life times.

I don't think that any of the listed "causes" has considerable influence, apart from the fact that some of them are mainly speculative. I would expect the current waveform and particularly the peak current to have a certain effect on it. So waveform shaping elements (series inductors) can possibly reduce but not stop it.

 

[grizedale]

FvM

I would expect the current waveform and particularly the peak current to have a certain effect on it.

This is worrying for us, because the peak current is not specified in the xenon tube datasheet
This is the only info given in the xenon flash tube datasheet, and theres no reference to the maximum current thats allowed to flow in the tube.
....
https://i45.tinypic.com/243f9jo.jpg

 

[FvM]

Please notice that I didn't say the black deposit is caused by high current, I just said I expect a certain influence.

If the tube specification doesn't require a series inductor, you can assume direct connection to a standard (non-electrolyte) capacitor as intended operation. In this case, peak current and pulse duration are only set by the tube characteristic, anode voltage and capacitance. Regular capacitor ESL and circuit stray inductance won't have a significant influence on the current waveform.

In other words, the life time expectation in the datasheet is assuming a standard circuit and operation conditions without waveshaping means.

 

[grizedale]

.though even the low power (1 to 5W) xenon flash tubes specify cpacitance of ~22uF....................and nobody can afford 22uF of film capacitance at 250V surely?

we have to (must) use electrolytics?....there is no other realistic proposition?

 

[FvM]

O.K, I didn't consider the capacitor size in your application, I'm used to stroboscope applications with higher repetition frequencies and lower capcitance values. Non-electrolytic is however the worst case, peak current related. The additional ESR of electrolytic capacitors further reduces the current, no point to worry about.

 

[rfredel]

Hello grizedale,

you can use electrolytic capacitators, but take care that they are for switching applications.

Look at this datasheet.

Maybe there are other firms, but this was the first one I find in an offer of my dealer. 10pcs. for together € 1,95.

Regards

Rainer