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  1. #1
    BetterSense's Avatar
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    How do flashes work?

    A simple title for a simple question. I'm curious.

    It seems like most flashes will fire, in at least some mode, on most cameras. Even my batteryless TLR has a flash sync socket. I also hear that some flashes will damage some camears, that cannot take the voltage of the flashes.

    So, how does this work? Does the camera simply short out the flash when it fires, either by shorting the coaxial flash sync socket or shorting between the center pin and the hotshoe contacts? I tried firing my SB-28 by shorting the center pin to the side, but it didn't work.

    Also, why is it called a PC sync?

  2. #2
    MattKing's Avatar
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    IIRC, PC comes from "Prontor-Compur" - an early and popular shutter that included a flash synch terminal of the type we refer to as a PC synch terminal.

    Flash synch terminals (whether PC, hot shoe or otherwise) are essentially switches that close the circuit between the flash condensor and the flash bulb or tube.

    Many older flashes feature high voltage and significant current capacity in that circuit - newer cameras tend to have electronic or lighter duty switches in the flash trigger circuit which can be damaged by those higher voltages and current capacities.

    The older cameras had mechanical switches which tended to be more robust.

    Your SB-28 may have a safety interlock that prevents accidental discharge. Is the central contact spring loaded? If so, it may only fire when mounted in a hot shoe.

    Don't ever try to disassemble an electronic flash unless you are confident you know how to safely discharge the circuit - they can seriously injure you if you are not careful.

    Matt

  3. #3

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    Yes, I once took apart a disposable camera when I was a child, ignoring the "danger, high-voltage" warning. Smart-ass that I was, I figured I could just take out the battery and be fine - I had no idea what a capacitor was. It was not a pleasant experience, to say the least.

  4. #4
    Steve Smith's Avatar
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    Basically:

    The flash has a circuit called an inverter which uses a high frequency oscilator to switch the battery voltage through a step up transformer. This creates a high voltage AC supply which is then rectified to give a DC supply of about 250 - 350 volts. This supply is used to charge a capacitor. Connected across this capacitor is the flash tube.

    The flash tube does not conduct current until it is triggered. For this, a much smaller capacitor is charged through a resistor from the high voltage supply. Because of this, the sync voltage is always around the same value as the main high voltage supply.

    When the camera's shutter is fired, this small capacitor is discharged into the trigger transformer which passes a short pulse of very high voltage to the trigger terminal of the flash tube. Once triggered the flash will conduct until the main capacitor is fully discharged transfering the energy stored in the capacitor into light energy.

    Once the flash has been triggered, the capacitor starts charging again and waits for the next trigger pulse.

    This is the description of a very basic flash with no means of controlling output power. Flashes were then improved to shut off the flash when enough light had been detected using thyristors.

    Initially one thyristor was used to short out the tube after enough light had been output effectively wasting the remaining power. An improvement on this circuit uses a second thyristor to turn off the main thyristor so saving the unused charge in the capacitor leading to shorter recycling times.

    There.... that was probably more information than you wanted!


    Steve.

  5. #5

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    Thanks, Steve, well done.
    It saves me the writing and my enlish isn't as good as yours.

    Peter

  6. #6
    AgX
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    I like to add that the pulse from the trigger circuit will ionize the gas in the flash-tube turning it conductive for dischargingt the main capacitator. The gas turns into a plasma emitting light.

    This ionizising takes place as the trigger voltage is brought to the flash-tube via a metal strip or a conductive coating. Furtheremore the actual trigger voltage is higher than the voltage of the main capacitator. Voltage is taken from the same source that feeds the main capacitator, but not sent to trigger directly, but via a coil (like in a car ignition), Thus a pulse of much higher higher voltage triggers the flash tube.

    This however is the classic approach. Current models have a voltage of only a few Volts at the trigger contacts shortcircuited by the camera.


    By the way, we are talking about an electronic flash. There still are flash bulbs. There a real incineration process (of metal) takes place.

  7. #7

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    Quote Originally Posted by MattKing View Post
    IIRC, PC comes from "Prontor-Compur" - an early and popular shutter that included a flash synch terminal of the type we refer to as a PC synch terminal.
    Not quite.

    It does indeed come from Prontor and Compur.

    They were shutters made by two separate German companies (Alfred Gauthier Feinmechanische Werkstatt, in Calmbach, and Friedrich Deckel in Munich).
    They set the standard for the european flash synch terminal.

    Both companies were part of the Zeiss group of companies. Prontor (the Gauthier company is called Prontor now) still is.

  8. #8
    Joe VanCleave's Avatar
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    For even more interesting information on high voltage triggered tubes, check out this link, which describes devices with names such as Krytron, Sprytron and Thyratron. These are used in other, more complex, devices which produce really big flashes of light!

    ~Joe

  9. #9
    Steve Smith's Avatar
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    Quote Originally Posted by AgX View Post
    I like to add that the pulse from the trigger circuit will ionize the gas in the flash-tube turning it conductive for dischargingt the main capacitator. The gas turns into a plasma emitting light.
    Ionize (or is it ionise?). That's the word I couldn't remember so I left out that bit!


    Steve.

  10. #10
    MattKing's Avatar
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    Quote Originally Posted by Q.G. View Post
    Not quite.

    It does indeed come from Prontor and Compur.

    They were shutters made by two separate German companies (Alfred Gauthier Feinmechanische Werkstatt, in Calmbach, and Friedrich Deckel in Munich).
    They set the standard for the european flash synch terminal.

    Both companies were part of the Zeiss group of companies. Prontor (the Gauthier company is called Prontor now) still is.
    Thank you for the correction .

    Matt

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