How do flashes work?

Discussion in 'Lighting' started by BetterSense, Mar 25, 2009.

  1. BetterSense

    BetterSense Member

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    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. MattKing

    MattKing Subscriber

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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. aluk

    aluk Member

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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. Steve Smith

    Steve Smith Member

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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. archphoto

    archphoto Member

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

    Peter
     
  6. AgX

    AgX Member

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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. Q.G.

    Q.G. Inactive

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    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. Joe VanCleave

    Joe VanCleave Member

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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! :wink:

    ~Joe
     
  9. Steve Smith

    Steve Smith Member

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    Ionize (or is it ionise?). That's the word I couldn't remember so I left out that bit!


    Steve.
     
  10. MattKing

    MattKing Subscriber

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    Thank you for the correction :smile:.

    Matt
     
  11. ic-racer

    ic-racer Member

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    Yes, it shorts the primary on the ignition coil (transformer). The current through the contacts is related to the size and charged voltage of the trigger capacitor and the impedance of the ignition coil primary windings. The older units I'm familiar with don't have a SCR to isolate this circuit.
     
  12. AgX

    AgX Member

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    What is a SCR ?
     
  13. Aurum

    Aurum Member

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  15. Steve Smith

    Steve Smith Member

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    Also known as a thyristor as I used in my description earlier.

    Think of it as a diode which (as you may know, only conducts in one direction) which does not conduct until it is triggered by a voltage on a third terminal. It continues conducting until the voltage drops to (near) zero.

    A variation of this is a triac which is similar in operation but conducts both ways and can be used with AC. A triac is more likely to be used for light dimming than an SCR/thyristor.


    Steve.
     
  16. AgX

    AgX Member

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    Thank you both.

    Thyristor is what it is called over here.
     
  17. Chazzy

    Chazzy Member

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    So who is to blame that some flashes put out a voltage which can damage some cameras? It is a matter of bad flash design or bad camera design? Surely it has to be one or the other, and it seems that the latter is more likely. Why don't all cameras incorporate something like the Wein device?
     
  18. Q.G.

    Q.G. Inactive

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    If anything, bad camera design.

    But it is not as much a matter of bad this or that, but more of evolution.

    The old* mechanical thingies did not mind at all how many Volts were run through them. The only thing that could get damaged was the photographer holding the thing.
    So why would flash manufacturers need to limit the voltage?

    Cameras evolved into thingies that came to mind how many Volts they are exposed too. Flash units that take that into account evolved alongside.

    So can you say that it is bad design that, say, carbide is of no use to me today when i find the headlight on my bike needs fixing?
    Should the lamp manufacturer have made it so that it can still burn carbide without being damaged? Or should the chemists of the carbide supplier have thought up a new type of carbide that would be perfectly usable in modern bicycle lamps?
    Or should i just go find a replacement bulb for the thingy?
    :wink:


    *Still plenty of them around that do not mind. Old, and new.
     
  19. Steve Smith

    Steve Smith Member

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    I would say it was bad camera design.

    In the days of bulb flashes, a capacitor would be charged by a battery and then the shutter contacts would discharge that into the bulb setting it off. The capacitor was used as the battery on its own would have too high an internal resistance, limiting current, which could not guarantee an instantaneous (or nearly) firing.

    These contacts had to be fairly robust to handle the current.

    Then along came early electronic flash with high trigger voltages. Luckily the robust mechanical contacts of the shutters of the time were also able to handle the high voltage.

    For a while, camera design evolved and the mechanical contacts remained allowing the cameras to work with older flash equipment.

    One day, someone must have decided to use an electronic trigger instead and to save money (probably) limited its voltage rating (to 6 volts ?) making it incompatible with most older flashes. Perhaps it was a way to sell new flashes as only the new types would be compatible.

    A good design would have put high voltage capable circuitry in the camera. Imagine a Wein Safe built into the camera instead of used as an accessory. This would have been easy and use a few cheap components but some designers decided not to do it.


    Steve.
     
  20. Chazzy

    Chazzy Member

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    Does anyone happen to know the trigger voltage of a Minolta 360px? Any chance that it could damage a more recent camera?
     
  21. ic-racer

    ic-racer Member

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    Actually what I was referring to was that, for example, a slave strobe will be fired by a SCR (silicone controlled rectifier) which will serve the purpose of activating the ignition transformer or coil, rather than physical contacts in a shutter. The triggering voltage for the SCR will be small, so if one put the SCR circuit between the contacts in a shutter and the ignition circuit, the contacts in the shutter would be protected. Here is an example, further isolated with a opto-coupler.

    [​IMG]
     
  22. Steve Smith

    Steve Smith Member

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    The only problem with that circuit is that it needs an external power supply. It could be derived from the high voltage present on the flash trigger contacts.

    However, look how simple it would be to incorporate it into the camera making it compatible with practically any flash ever made. Actually, with the right choice of opto isolator, R2 and SCR1 would not be needed making it a two component solution.

    There can only be two reasons for not incorporating it:

    1. Cost cutting (but not much).

    2. Marketing. Forcing camera buyers to buy the matching compatible flash units.


    This circuit could equally be fitted into older flash equipment making it safe on any camera.



    Steve.
     
  23. John Koehrer

    John Koehrer Subscriber

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    The design is a result of evolution. From flash powder with a trigger to the PC contact and mechanical switch to the electronic switch.
    The PC connection has proven to be a simple and reliable system. Not much to go wrong & easy to service. The electronic switch works very well as long as there's no polarity or voltage problems.
    The technology has changed. It's nice to suggest that manufacturers could update either camera or flash easily, but face it nobody will for the simple reason that there are millions of units out there and you can be certain the makers won't want to foot the bill and there are relatively so few problems.
    Betcha a dollar to a doughnut that individual equipment owners won't want to foot the bill either.
    The most expedient solution is the safe-sync system. At around $40 it's still less expensive than replacing a main board if you blow it up.
     
  24. glbeas

    glbeas Member

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    Dr. Harold Edgarton invented the electronic flash. He wrote a book on it which you might still be able to find at the library which was pretty interesting. He had a circuit you could build that produced a flash duration of a half millionth of a second using some outrageous capacitors and a quartz tube wrapped with wire leads on the outside and the trigger wire run up the inside. It used air as the flash medium because it quenched far more rapidly than xenon. It was featured in the pages of Scientific American more years ago than I care to count as a project for experimenters.
     
  25. Steve Smith

    Steve Smith Member

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    I wasn't sggesting that manufacturers should update their equipment. They should have made it all compatible in the first place instead of introducing a new, incompatible standard.


    Steve.
     
  26. John Koehrer

    John Koehrer Subscriber

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    It's a good thought, but you can't do that unless you have a REALLY GOOD crystal ball.
    Unless you know what technology is coming down the uninvented/unimagined pipeline you simply can't do it.
    The PC/mechanical switch worked very well for years but the designers can't be faulted for not intuiting modern systems.