Camera flash trigger

Discussion in 'The Projects Forum' started by Bob123456, Sep 9, 2014.

  1. Bob123456

    Thread Starter New Member

    Aug 14, 2013
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    I was trying to replace the contact switch on a camera flash (from a single use camera) with a mosfet (so I could trigger the flash with an electrical signal). The mosfet is rated to about 600V 30A for the drain-source.

    The drain is attached to the top bit of the switch (higher voltage), the source to the bottom, which is also attached to the ground of the power supply used to trigger the gate. When I measured there was 200V across the open switch.

    For some reason this doesn't want to work, am I missing something basic here?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Do you have the gate connected to the source when you make the measurement? You can't leave a MOSFET gate floating.
     
  3. Bob123456

    Thread Starter New Member

    Aug 14, 2013
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    Yeah the gate is connected to the source across a 10K resistor.
     
  4. Bob123456

    Thread Starter New Member

    Aug 14, 2013
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    Untitled.png
    An image incase it helps
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Wich mosfet do you intend to use?
    In your schematic, it looks like a logic level one.

    Bertus
     
  6. Bob123456

    Thread Starter New Member

    Aug 14, 2013
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    I'm using an FCP11N60F N channel mosfet. The one in the schematic was just the 1st one I found.
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Looking at the datasheet, the mosfet would need 10 volts to conduct.

    Bertus
     
  8. Bob123456

    Thread Starter New Member

    Aug 14, 2013
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    Oh ok. I was looking at the gate threshold voltage where it says 3-5V, my bad. I did manage to get it to run an LED with 5V though, I'm guessing it wasn't putting through enough current to run the flash though. I'll try it out tomorrow with 10V and let you know how it goes.
     
  9. Bob123456

    Thread Starter New Member

    Aug 14, 2013
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    No luck I'm afraid. First I tried the mosfet across the switch like before (with 10V at the gate) and nothing happened.

    Then I manually tied the contact switch together and put the mosfet in series with the flash itself (-ve terminal of the flash into the drain, and the source connected to the point where the -ve flash terminal went back into the board) but this didn't work either.

    In both cases the capacitor still had all it's juice left for when I manually discharged the switch. Any ideas?
     
  10. BReeves

    Member

    Nov 24, 2012
    412
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    I use the NTE3046 SCR Optoisolator to fire a flash with high voltage trigger circuits. Don't have a schematic handy but it works well and the input is an LED which can be triggered with the voltage you were first trying to use. It also isolates the high voltage from whatever the circuit is you are triggering it with.

    This may help, from a previous project..
     
    Last edited: Sep 10, 2014
  11. Bob123456

    Thread Starter New Member

    Aug 14, 2013
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    Thanks allot for the schematic. Just checking are the square waves used both +5V? Also I'm a bit confused what the second square wave (from the ttl strobe trigger) is for if the first one triggers the switch.
     
  12. wayneh

    Expert

    Sep 9, 2010
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    Something is goofy here. I can't believe the switch needs to pass more current than the ~10mA required to light the LED. With the MOSFET in place, does the flash fire if you short across the drain and source pins?
     
  13. Bob123456

    Thread Starter New Member

    Aug 14, 2013
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    Yes.
     
  14. wayneh

    Expert

    Sep 9, 2010
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    And yet, when you pulse 10V (between source and gate) onto the gate pin (which is otherwise pulled down to source by a 10K), it does not fire?
     
  15. BReeves

    Member

    Nov 24, 2012
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    Just showing the polarity of the trigger pulse to help someone understand how it worked back when I drew it.
     
  16. Bob123456

    Thread Starter New Member

    Aug 14, 2013
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    It's alright guys, I was only using the flash to test if I could get the circuit's capacitor to discharge, I gave up on using the contact switch and just directly connected to the capacitor (very stupid of me I should have thought of it earlier) and that worked.
     
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