Combination of serie trigger and transistor for a flashlamp

Thread Starter

matters_100

Joined Mar 30, 2016
18
Hi,

I've attached a picture of a flashlamp circuit. It uses series trigger (i.e. high voltage to ionize the gas, directly in the circuit), and a transistor which control the length and current of the discharge.
However, as the trigger voltage is usually a few kV, how can any transistor (Mosfet, IGBT, ...) handle this high voltage as they are usually rated to 1kV max ?
Or is there another way of achieving it ?

Thks.
 

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crutschow

Joined Mar 14, 2008
23,803
The transistor is turned on before the trigger is applied, so it doesn't see the trigger voltage.
It doesn't conduct any current at the point since the flash lamp looks like an open circuit.
After the trigger the transistor is then turned off when you want to terminate the flash.
 

Thread Starter

matters_100

Joined Mar 30, 2016
18
From my understanding, in this circuit, the transistor is turned off so after the trigger voltage, the ionisation is maintained for a short time by the simmer supply, and then, the transistor is turned on so that the "proper" arc can occur in the center of the lamp (increasing its lifetime).
The problem is that the trigger voltage is usually more than 10kV, so this really high voltage will destroy the transistor ? I suppose that I'm missing something because apparently this circuit is quite common. Could anyone enlighten me ?

Thks.
 

crutschow

Joined Mar 14, 2008
23,803
Okay, I understand the circuit better.
The high voltage appears across the flash lamp and simmer supply, not across the transistor to the charging capacitor supply negative terminal.
So the transistor only has to handle the charging supply capacitor voltage.
 
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