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
Relay Recommendation
Oct 30, 2005
7 Replies
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
time.
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.
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 part.
Thanks for any help!
CB
Relays will never be able to protect against overvoltage
You need something called a tranzorb or TVS (transient voltage supprressor)
Dan
-- Dan Hollands
1120 S Creek Dr Webster NY 14580
I was thinkin that most flash units use thyristors anyway ?
Regards ......... Rheilly Phoull
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
it.
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.
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:
formatting link
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
that's a silly design...
you'll need something faster than that, possibly something with an opto-coupler.
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.
Bye. Jasen
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