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Photoflash SCR question

Discussion in 'Electronic Basics' started by Al Adrian, Nov 13, 2003.

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  1. Al Adrian

    Al Adrian Guest

    Here's an interesting problem.. I have a couple old photoflashes laying
    around here and I want to make use of them as studio flashes.

    I've looked on the web and figured out that there are high voltage flashes..
    and newer low voltage ones.. (I have one of each here, I'm working with the
    low voltage one)

    Anyway, I can discharge my low voltage flash and the SCR in my Camera.. (and
    a component SCR I have here to play with) works a treat when I have the
    flash set to full discharge manual. The switch line voltage drops to 0 and
    it's all ready to go again when the flash recharges)

    The problem is if I want to use the Flash's automatic mode, where the
    on-flash light sensor cuts off the flash discharge at a lower point when
    it's gotten a certain amount of light. there is something on the flash that
    only allows the switch voltage to drop to about 0.8V.. not enough to reset
    the SCR. It stays at about 0.8V using 7mA of current until I press the
    manual button on the flash (which shorts the switch line to ground, resets
    the SCR and current drops to 0)

    Any ideas on how I could go about making this flash work without opening it
    up? I have no idea as to what's inside. The flash is designed to work with
    old cameras which use metal switch contacts on the shutter to short the
    switch line to ground. (shorting the switch line to ground does work when
    it's in auto mode).. I think it's just the SCR's internal voltage drop in
    combination with the way the switch line works (voltage source?) that is
    keeping the SCR from re-setting.

    Obviously anything I put inline with the trigger signal can't cause much

  2. Maybe a relay and a relay driver transistor could take care of the
    problem. I'm not sure what you need from the description, but a relay
    contact would have nearly zero V drop.
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