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| Old camera speedlight low voltage trigger conversion |
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| floobydust:
The need for an opto-coupler might be a carry over from the days of mains-powered flashes which are not isolated. I'm in favour of one to block any EMI and ensure the camera body could not get damaged by the flash. Old flashes are usually HV DC with a trigger transformer and trigger capacitor raw to the hot shoe - no thyristor at all. So the camera shutter switch is to discharge HV trigger capacitor into the trigger coil. Best to add thyristor. If you put a scope there, you'll see the SCR retriggers a few times due to ringing with the trigger coil's inductance. It's not a nice single pulse. One question is the configuration of a low voltage camera hotshoe. It seems to be open-collector with a weak pull-up resistor to camera's Vcc? Schematic is of old Fuji FinePix S100FS camera that can drive a HV hot shoe. |
| Zero999:
Even if it's not powered from the mains, isolation is a good idea. Just use a relay. This one is perfect: high coil resistance of 500R, a contact rating of 200V (but will probably take a bit more) 1A and a built-in back-EMF diode. https://uk.rs-online.com/web/p/reed-relays/2919710/ |
| cprobertson1:
--- Quote from: floobydust on June 18, 2019, 10:52:12 pm ---The need for an opto-coupler might be a carry over from the days of mains-powered flashes which are not isolated. I'm in favour of one to block any EMI and ensure the camera body could not get damaged by the flash. Old flashes are usually HV DC with a trigger transformer and trigger capacitor raw to the hot shoe - no thyristor at all. So the camera shutter switch is to discharge HV trigger capacitor into the trigger coil. Best to add thyristor. If you put a scope there, you'll see the SCR retriggers a few times due to ringing with the trigger coil's inductance. It's not a nice single pulse. One question is the configuration of a low voltage camera hotshoe. It seems to be open-collector with a weak pull-up resistor to camera's Vcc? Schematic is of old Fuji FinePix S100FS camera that can drive a HV hot shoe. --- End quote --- No idea about the hot shoe schematic on my camera (Pentax K-50) (wait, I just realised I never mentioned that earlier in the post?) - I was hoping that my modification could be mostly-universal though: and an optocoupler just makes sense for that given that I can't know the specs of the hot shoe it may or may not be attached to. Side note - I'm saving up for a modern flashgun too xD I see a £70 model I like the looks of, though it's still only entry level of course (I'm an entry level photographer, after all!). I still want to get this old one working though - at least the Hanimex TZ7000 (I still haven't tore this one down yet or even probed it for that matter). The older flash (the one I was originally working on) is a Sunpak (I can't remember the model name I'm afraid). The sunpak is a little monster though - it's bit me twice now, and it also has reversed polarity on the hot-shoe (centre-negative) - though that's easy enough to fix just by swapping the wires around internally. It also has a big voltage spike (in the opposite direction) when the flash fires... I might need to add some snubbing in there too - but I'll deal with that bridge when I come to it! --- Quote from: StillTrying on June 18, 2019, 03:28:06 pm ---I'd probably do something like this, ignore all this opt-isolation nonsense. :popcorn: --- End quote --- Ah! Okay! ... Relay then? ;) --- Quote from: Zero999 on June 19, 2019, 09:04:00 am ---Even if it's not powered from the mains, isolation is a good idea. Just use a relay. This one is perfect: high coil resistance of 500R, a contact rating of 200V (but will probably take a bit more) 1A and a built-in back-EMF diode. https://uk.rs-online.com/web/p/reed-relays/2919710/ --- End quote --- Ha! Brilliant! I went to post that up as a half-serious/half-joke suggestion, and as I hit send, I get a message saying there's another post where Zero999 actually using a relay - impeccable timing! :-DD I did consider it, but was worried about how best to power it: see, with an optocoupler I can use a primary cell for a potentially very long time - with a relay the current draw is potentally 10x greater. That said, it's not like we'll be drawing current for a long time - I could probably get away with it. In fact, come to think of it... could I just power the coil from the 4xAA batteries? As I mentioned earlier, I was pensive about the idea as it means things aren't fully isolated and I was worried about loops - but on the other hand, I really don't want to be adding an entire additional battery source into the flash unit if I can help it (not a big one, anyway). Side note: do relays switch fast enough? The part you linked to lists 0.5ms including bounce - so if the shutterspeed is as faster than 1/2000 I may actually miss the flash (plus any other delays in the trigger circuitry on the flash itself)! I mean, it shouldn't be a problem - if I'm flashing I probably won't be using shuterspeeds that high - but it might be fun to play around with all the same! I wonder how that trigger-time compares to the TRIAC circuits, actually - I never thought to work out the trigger-time for them. |
| cprobertson1:
Sorry to come back to this again but I can't wrap my head round this for some reason - or rather, I keep flip-flopping as to whether it's a good idea or not. Powering the optocoupler or relay (coil) from the flashgun's 4xAA batteries would be ideal... but, because we don't have total isolation, I have some fears - which may or may not be well-founded! * Can the HV voltage multiplier form a capacitive loop, causing a high voltage to appear on the hot shoe contacts? * Can EMI cause a HV pulse on the hot shoe contacts? * Under reasonable conditions, can the HV multiplier fail in such a way as to expose the hot shoe contacts to HV? * Am I worrying over nothing? I'm rather annoyed with myself that every time I draw the circuit out/look at the block diagram I've drawn, I end up bouncing back and forth: I feel like this is something I should just know! --EDIT-- The power draw is actually pretty low, even for the relay - and I'm not running it continuously - I reckon I could get away with a number of different primary cells, while still keeping everything neatly isolated from the camera itself - so I may have addressed my own concerns! --EDIT-- Corrected "nearly isolated" to "neatly isolated" |
| Zero999:
The circuit I posted fully isolates the camera from the flash gun, as the relay's contacts are isolated from the coil. A 6V battery or four AA batteries in series, to make 6V will be fine and should last for ages, as the relay will only be activated for short periods of time. I don't know whether it will be fast enough. How fast is your flash gun? |
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