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Relay Recommendation

Discussion in 'Electronic Basics' started by Captain Blammo, Oct 29, 2005.

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  1. I'm trying to put together a protective device for the hotshoe on my digital
    camera. All that triggering an external flash requires is connecting its two
    terminals together through the camera, but most digicams will fry if exposed
    to more than 6v fed down the line by the flash unit.

    I'm thinking that one or two AA batteries and a relay connected to the
    camera would be just fine to protect against overvoltage, but the relay
    needs to operate as close to instantly as possible and be able to pass
    voltages from, say, 3 to 1000v on the flash unit side with no ill effect.

    I had a look in the digikey catalogue, and just ended up getting confused by
    the vast array of relays and mystery variables listed with them. Since I'll
    be staking the life of the irreplaceably expensive camera on this, I was
    wondering if someone could do me a huge favour and recommend the appropriate

    Thanks for any help!

  2. Warren Weber

    Warren Weber Guest

    I bought one of these ready made from B & H Photo. For my digi camera so I
    can use any type or voltage output flash. Warren
  3. John Fields

    John Fields Guest

    Yow!!! Where's that 1000V coming from? Surely the flash unit
    doesn't expect the camera to discharge its (the flash unit's) caps
    through the hot shoe and hold that voltage off until it's flash

    Also, if you want to fire a relay from the hot shoe you need to know
    how much current for how long a time the hot shoe can handle.

    Basically, you need to look at the spec's for the camera and the
    flash unit and, if you can't figure out how to hook them up with
    that documentation in hand, post the spec's and we'll tell you how
    to do it.
  4. I was thinkin that most flash units use thyristors anyway ?
  5. Dan Hollands

    Dan Hollands Guest

    Relays will never be able to protect against overvoltage

    You need something called a tranzorb or TVS (transient voltage supprressor)


    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
  6. Yow!!! Where's that 1000V coming from? Surely the flash unit
    I've no idea why the older units use so much voltage. I think I've only
    really heard of them hitting 400v or 500v, but I thought I'd be on the extra
    safe side :)
    The Canon aren't very forthcoming with that information, which is rather
    annoying. Information gleaned from the web, however, reliably indicates that
    anything under 6v is fine. Actually, I found a diagram for an optoisolated
    circuit, so I think I'll give that a try:
    Unfortunately, all I know is the 6v limit. What kind of voltage would it
    take to bust up thae above referenced circuit to the point that it got to
    the camera? Something insanely high, I should think. I really don't to take
    any significant risk, but I also refuse to pay $50 for an item that I'm
    fairly positive uses <$5 worth of components when I'm as poor as I currently

    I think the biggest risk with that circuit, though, is that it won't fire
    rather than fry my camera. That's acceptable! Please do let me know if you
    think otherwise.

  7. John Fields

    John Fields Guest

    I wasn't aware that the trigger voltage on flash units could get so
    high, so I stand corrected on that one.

    The circuit looks OK, and with a maximum forward voltage of 1.5V
    across the diode, it looks like the current the camera contacts will
    have to handle will be:

    Vcc - Vf 6V - 1.5V
    I = ---------- = ----------- ~ 14mA
    Rs 330R

    The MOC3010 is guaranteed to fire its internal switch with 15mA
    through the diode, but it's specified to do that with a 150 ohm load
    fed from a 3V supply across its internal TRIAC. I don't know how
    that would change with a higher voltage across the TRIAC, so I
    suppose the best thing to do would be to put one together and try

    Also available are the MOC3011 which is rated to fire with 10mA of
    diode current, and the MOC3012 with 5mA, so I'd be tempted to go
    with the 3012 if I wasn't sure about how much current the camera
    contacts could take. OTOH, depending on the camera's leakage
    current out of the hot shoe in the OFF state, it might keep the 3012
    ON, so perhaps a 3011 would be a better choice. ???

    All the parts are rated with 7500V of isolation between the LED and
    the TRIAC, so no matter what (if the thing was wired right ;)) the
    camera would be safe.

    The only other concern I'd have would be that the absolute maximum
    the MOC's TRIAC can stand off is 250V, so if the flash unit's
    trigger voltage rose above that it could/would fry the MOCXXXX.
  8. Jasen Betts

    Jasen Betts Guest

    that's a silly design...
    you'll need something faster than that, possibly something with an
    it's likely none of the relays will be fast enough unless they're
    solid-state relays.

    I'd just use the hot shoe as is unless it's documented somewhere that this
    is a bad idea... if the camera fails demand a refund.

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