Why diodes in series with xenon flash tube?

[grizedale]

We have a 15W xenon flash tube (in a flash lamp) which has two diodes in series with it.

The 5W version does not have diodes in series with the xenon tube, so why does the 15W version?

The xenon tube is connected across a 250V capacitor.

One diode (S3G type silicon diode) is connected from the cathode of the tube to the negative terminal of the capacitor. (obviously: cathode of diode to negative termial of capacitor.)

The other diode (again: S3G type silicon diode) is connected from the anode of the tube to the positive terminal of the capacitor (obviosuly: diode anode to capacitor positive terminal)

Why have the designers added these diodes?

The 5W version does not have these diodes.

The only thing i can think is that the stray inductance of the connections to the xenon tube may be causing oscillations and sending the current the wrong way in the tube which would damage it?

 

[hshah8970]

Your assumption is correct. These are flyback diodes, almost always connected across big inductive loads.

For readers who want to know more about flyback diodes, click here.

I would imagine that with the 5 W flash, the inductance was not high enough to warrant adding extra components.

 

[FvM]

Series diodes (one would be sufficient) are used to apply a higher anode voltage during ignition, e.g for stroboscopes with wide frequency range. Apparently this isn't the case in your circuit.

It can be also a feature for tube test during production. Or just meaningless. The stray inductance oscillation point sounds absurd. If there woul be actually an oscillation, it doesn't hurt the tube.

 

[hshah8970]

"The xenon tube is connected across a 250V capacitor."

My mistake. I thought the diodes were connected in parallel to the capacitor with the tube.

 

[grizedale]

One of our xenon flash tubes has a datasheet which recomends the following circuit for triggering..........it has a diode in series with the tube, which ive no idea why its there....do you know?.................

https://i47.tinypic.com/11k9wex.jpg

 

[FvM]

The operation of this kind of circuit is explained in the Excelitas application notes. It doubles the anode voltage during trigger, allowing the flash lamp to trigger with a lower main capacitor voltage. I already mentioned it in post #3.Alternatively an anode voltage booster circuit can be used.

 

[godfreyl]

The diode (and the capacitor connected to it) double the voltage between the anode and cathode of the tube when it's triggered.

btw: It's not clear in the picture but the two 0.1uF capacitors, the 1M resistor and the thyristor should all be connected.

edit: Oops, sorry - cross posted with FvM.