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Transistor-triggered photogate

Discussion in 'General Electronics Discussion' started by Mark34, Jun 21, 2017.

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  1. Mark34

    Mark34

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    Sep 23, 2016
    Hello ! I need a photogate to detect a fast passing object and when the light is blocked to trigger a 300V capacitor into a flashtube.
    The circuit in the image does the job but I have no ideea where in the circuit to connect the capacitor and let's assume the capacitor is already charged.
    [​IMG]
     

    Attached Files:

  2. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    I suggest you look up schematics for flash guns. These use tubes which will withstand the high voltage and are triggered with a transformer putting out a high voltage.You could even use the circuit out of a disposable camera if they are still around. I begged a couple a few years ago and was told to be careful, they bite. How right she was.:)
    I have used an SCR to drive a transformer for an electric fence but current will pass at a low voltage. What voltage will your flash tube withstand without a trigger.
     
  3. Mark34

    Mark34

    47
    0
    Sep 23, 2016
    I have already a charging circuit for the capacitor, but i need it to discharge the capacitor when a small ball is passing and the best way is a infrared detector, the question is where to place the capacitor in that circuit?
     
  4. duke37

    duke37

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    Jan 9, 2011
    The way is to charge the capacitor to the high voltage with it in series with the flash gun.
    The SCR triggers the primary of a pulse transformer to set off the flash.
    Look up 'flash gun circuit diagram'.
     
  5. Mark34

    Mark34

    47
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    Sep 23, 2016
    The question was for the circuit diagram attached and I don't see any transformer there, all i need is to add a capacitor and a flashtube, when the infrared light is blocked the capacitor needs to be discharged into the flashtube,(let's consider the capacitor is always charged to 200V so there is no need for a charging circuit for the capacitor), i expect an explication like connect the scr anode to the capacitor cathode and the flastube there...
     
  6. duke37

    duke37

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    772
    Jan 9, 2011
    The tube will not flash with 200V across it no matter how it is connected.
    Put 200V across the tube and put a low volage pulse into a pulse transformer and this will output a high voltage pulse.
    If this is connected to an external electrode close to one main electrode, then a small amount of ionisation will occur and will set off the main flash.
    The pulse transformers are very small, you may be able to get one from a disposable camera.
     
  7. Mark34

    Mark34

    47
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    Sep 23, 2016
    Anyone else ?
     
  8. AnalogKid

    AnalogKid

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    Jun 10, 2015
    No, it isn't. Your basic concept of how a photographic strobe works is incorrect because it is incomplete.

    As has been suggested twice, look up photo flash, strobe, and flash gun circuits. You will see that you also need a trigger transformer. That is the thing that is driven by the signal from the photo detector. The capacitor sits fully charged, with its full voltage across the flash tube, and nothing happening. When the trigger transformer is pulsed, it adds an additional few kilovolts (!) that causes the gas to break down and ionize. This then conducts a much higher current from the capacitor, forming the flash.

    There are many websites with strobe circuits, design rules, examples, kits, etc.

    ak
     
  9. AnalogKid

    AnalogKid

    2,515
    718
    Jun 10, 2015
    Dumb response. You asked the same question three times, and got different variations of the same *correct* response three times. Obviously you don't like the answer because it does not fit your simplistic view of the problem, but why do you think someone else will say anything different?

    ak
     
  10. Mark34

    Mark34

    47
    0
    Sep 23, 2016
    Well the flash tube was just an example in reality i need a capacitor discharge into a coil and i don't need additional kilovolts, so like i said i just need to know where in that circuit diagram to connect the capacitor and coil/flash tube. And lets assume the capacitor doesn't need a charging circuit and its always charged 200V because I already have that and I want to simplify things.
     
  11. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    You would simplify things if you do not complicate matters by asking for advice on things you do not want

    If you have an inductance, just connect it to the charged capacitor and short the other end to common with a SCR.

    You may be able to find suitable circuits if you look up electric fence energisers.
     
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