Combination of serie trigger and transistor for a flashlamp

Discussion in 'General Electronics Chat' started by matters_100, Apr 12, 2016.

  1. matters_100

    Thread Starter New Member

    Mar 30, 2016
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    0
    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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  2. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    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.
     
  3. matters_100

    Thread Starter New Member

    Mar 30, 2016
    15
    0
    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.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    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.
     
  5. matters_100

    Thread Starter New Member

    Mar 30, 2016
    15
    0
    Indeed, but don't transistor turned off have a Collector to Emitter Breakdown Voltage ?
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    Yes.
    But, as I explained, the maximum collector-emitter voltage seen by the transistor is the from the Capacitor Charging Supply, not the trigger voltage.
     
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