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Trigger a disposable Flash

Discussion in 'Electronic Design' started by Terry, Sep 4, 2003.

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  1. Terry

    Terry Guest

    I am looking for a very simple circuit for shorting the trigger wires from a
    disposable flash from my parallel port. I do this all the time with diodes,
    etc for my DC circuits - but with this I need to isolate the 300 - 400v of
    AC that is on the trigger wires. (I have the flash trigger itself all built,
    I just have to short the trigger wires - and poof!)

    Is there a simple optoisolator circuit out there or will I also have to add
    a SCR or Triac?

    Thanks!
     
  2. Norm Dresner

    Norm Dresner Guest

    I don't know what your current requirements are, but there are plenty of
    optoisolators with triacs (and IIRC SCRs) available to choose from

    Norm
     
  3. Terry

    Terry Guest

    I am guessing the current is very small, but I am not sure at this point. I
    am just experimenting at this point. I am not very familiar with Triacs so
    a little help would be appreciated.

    I was looking at using a MOC3052 - one end to the PC and the other directly
    to the trigger. Does this sound reasonable for this application? I think
    the triac can sink 1A, I know it would not exceeded that.
     
  4. The typical disposable flash trigger circuit contains a 22nF capacitor
    charged to 330V and it gets discharged through a low impedance pulse
    transformer primary through a mechanical switch. Although it is true that
    average trigger current is way less than 1A no matter how fast you trigger
    the xenon strobe, the peak pulse current exceeds 100A, possibly even 200A.
    As such, you may wish to use an SCR or TRIAC which is rated for this type of
    operation, such as the MCR22-6 [
    http://www.onsemi.com/pub/Collateral/MCR22-6-D.PDF ] (assuming you can find
    a supplier). Although I've never used a MOC3052, I would hazard a guess
    that it would be only useful for triggering a larger SCR or TRIAC and would
    explode on the very first trigger if used by itself.

    Howard Henry Schlunder

    in message
    news:iWG5b.3207$...
     
  5. I would use a reed relay, maybe a 6V or 5V one. If you're worried
    about the high current needed for the coil, well, reed relays don't
    take that much current, only a few dozen mA, or less. But you can put
    a couple hundred uF cap in series with the coil, with a few k bleeder
    resistor across the cap. When you charge the cap thru the coil, it
    will pull the contacts in momentarily, long enough to trigger the
    flash. Then the coil current will go down to only a mA or so.


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  6. GrahamH

    GrahamH Guest

    Have a look at AQV251 photomos, stock code 171-8623 at www.rswww.com. Micro
    current opto switch. Just add one series resitor to limit LED current. I
    have also used these to switch from the handshake lines of a PC serial port.

    Graham
     
  7. I suggested using a reed relay to trigger it. I didn't think that the
    peak curent would be that high, but it makes sense, it's a CD, a
    capacitive discharge system. I guess a reed relay wouldn't explode,
    but after the contacts fused, the experimenter would get the hint and
    use something a bit heavier. :p

    One thing I didn't notice on the contacts on the displosible flash I
    have is any pitting or arcing of the pushbutton contacts. But then
    they're a lot heavier than a reed relay.
    --
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    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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