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| Old camera speedlight low voltage trigger conversion |
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| cprobertson1:
Good afternoon ladies and gents! I've just discovered an old speedlight that I want to use in my modern DSLR. Sure, I could, just buy a new one, but this is more fun! The only problem is that the sync/trigger voltage on the speedlight sits very comfortably at 380V - and modern DSLRs use SCRs that are spec'd at less than 24V (and in the case of Canon cameras, less than 6v, for some reason.) I had a plan to use a zener and a TRIAC to trigger it while keeping the voltages less than 5.6V, and a series of high value (megohm) resistors to keep the current low - but the more I look at it, the more I think that if something fails, it will bite the camera. What I'm worried about is the failure mode in which a zener fails open circuit - the voltage will raise to the flashgun's switch voltage of ~380V = I have a single point of failure, and it doesn't necessarily fail safe. Sure the current/power may be low, and it may fail closed (in which case it will just retrigger the flash continually) - but if it fails unsafely and I hook that up to my camera it won't be a happy camper! I considered adding redundancy in the form of multiple zeners in parallel - if one fails open, the rest will stop the voltage rising - however, if the failure is repeatable (i.e. if something is going wrong and will consistently cause zeners to fail open), then it fails silently until the last one, at which point it then fails hot and the voltage goes up again. My next idea was to add an optocoupler between the camera and triac - but that leaves me with the problem of powering it. I could either power it from the flashgun itself (using the batteries) or add a small external battery (I don't believe I can borrow any power from my hot shoe for this one I'm afraid... do correct me if I'm wrong though!) So - my questions! * Are the zeners reliable enough to outlast the flash gun under these circumstances (especially if redundancy is added)? * Is powering the optocoupler from the 4xAA batteries a bad idea (I'm thinking along the lines of ground loops and capacitive coupling from the voltage multiplier) * Erring on the side of caution, is there an easy way to indicate that a high voltage (exceeding, say, 6v) on the sync terminals so that I can test before plugging it into the camera? Maybe a simple zener + transistor >> LED to indicate an overvoltage condition? (powered form the flashgun AA batteries) * Is there a better way of doing all of this that I'm missing? (not including buying a modern flashgun!) Ps - the attached circuit is borrowed from a circuit seen on a number of flashgun/strobelight-fan forums (some fun reading there! Looks like some of it is back from the usenet days too!) Thanks for the help folks! |
| Zero999:
You must use an opto-coupler, to isolate the lethal voltage in the flash, from the camera. Zener diodes and a TRIAC just won't do. You could try an opto-isolated TRIAC, which should work, assuming the leakage isn't a problem and the trigger current required by the flash isn't too low. Ensure there are sufficient creepage and clearances, between the hazardous and non-hazardous voltages, on the PCB. |
| cprobertson1:
--- Quote from: Zero999 on June 17, 2019, 02:30:47 pm ---You must use an opto-coupler, to isolate the lethal voltage in the flash, from the camera. Zener diodes and a TRIAC just won't do. You could try an opto-isolated TRIAC, which should work, assuming the leakage isn't a problem and the trigger current required by the flash isn't too low. Ensure there are sufficient creepage and clearances, between the hazardous and non-hazardous voltages, on the PCB. --- End quote --- Optocoupler it is! I'm afraid I don't have any optotriacs to hand, but I have a number of optocouplers from an old project I can use. I'm going to need to figure out a way of powering the camera-side of the optocoupler... an external cell would be best, but I'm not sure where I could fit a cell-holder on the existing enclosure... not without securing it to the outside at least! How is this for an idea: SPDT switch that charges a capacitor from the AA batteries of the flash gun - when switched it completely disconnects the AAs and powers the optocoupler instead: this way the circuits are always kept mostly isolated (though, only by the dielectric strength of the switch)... maybe that's not such a great idea - single point of failure and we potentially lose the isolation. Right - back to the button-cell idea... wait a minute - optocouplers don't use that much power do they - and it's not like my camera is keeping it on for that long... I might be able to get away with just using an internally mounted cell and not worrying about it until it drains, rather than treating it as a consumable... I'll look into that too! Any other suggestions would be most welcome too! |
| StillTrying:
Have a search for your camera, most of the 6V are really 250V. I've been using a 270V flash on a 6V/250V camera for years. http://forum.mflenses.com/canon-flash-voltages-t15474.html I wouldn't recommend 380, 270, or even 250V, but I don't think you have to worry too much about 6V. If it's powered by it's own internal batteries I don't think galvanic isolation is needed at all, just arrange for a 5 to 12V pulse to trigger the 400V triac or even thyristor. You could do with the internal circuit of the flash to see where to get the 6V trigger from, rather than using the 380V. YMMV :) |
| NiHaoMike:
Just get a wireless flash extender. |
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