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How to reset SCR used for triggering camera flash

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shootfirst said:
If someone has a schematic of a similar optical trigger circuit, I'd love to see it.

I was just realizing that the circuit (multi flash slave) that was shared in a previous post in this thread could possibly be modified in such a way that I could take out the counter and keep the optical trigger portions.

If I wanted to start from scratch, perhaps someone could help me pull the optical trigger portion of that circuit out and ignore the counter.

Just a thought.
 
Was looking into the reset problem this evening. Happened to have googled out this old thread :)
Here's what I understand from the discussion above, from other sources, and from the circuit of a simple optical trigger (SYK-3).

**broken link removed**

If we add into the wire to the hot-shoe an R and a C in parallel, the C should only let through current momentarily. As C charges up, the current through it will quickly drop. R should be large enough to allow through a current less than the thyristor's hold-current. Hence the thyristor will be cut off due to insufficient current, and R will discharge C into heat.

From the current through an R-C circuit (this R is the resistance of the flash as seen by the trigger, not the one in parallel), we know that having chosen R to be large enough, C will determine how quickly the current will fall, hence how quickly the thyristor will be turned off.

I may be terribly wrong - do correct me :p
 
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SCR reset........

Was looking into the reset problem this evening. Happened to have googled out this old thread :)
Here's what I understand from the discussion above, from other sources, and from the circuit of a simple optical trigger (SYK-3).

**broken link removed**

I have made many optical slave flash triggers units for photographers using Canon EX type flash units. All I have to do is to replace my units normal SCR with a high voltage transistor.
Problem solved!
 
Yes- I know this one because I'm working on one right now!

As said the circuit needs a capacitor in parallel with a bleeder resistor in series with the SCR. The cap will support a significant current until it is all charged up and then the current is only the flash cap voltage divided by the bleeder resistance. If this is below the commutation threshold of your SCR you're good.

A question occurs if the tube fails to fire. The flash cap will still be at 350v or so which means a much higher current through the resistor after the SCR has triggered and the cap has charged. If the SCR fails to commutate it cannot fire again unless the main power is cycled off. So, best thing is to size the bleeder resistance high enough so that the full flash cap voltage/R is less than your SCR's "holding current". Be sure to take into account whether the spec sheet is giving you the highest or lowest guaranteed holding current across all rated conditions. Higher temps=lower holding current which is more of a problem. Usually they give the nominal value for the 25 deg C rating, at 105 deg C it's basically half the nominal value.

For example if you have a 300v cap and a 30mA holding current SCR, 10k is too close. At 100 C junction temp it will hold down to 15mA. So 20k+ would be a good idea.


The Canon flashes are only putting about 4.5 volts in the trigger circuit. 20 year old flashes used to put 350-400 volts on those contacts (ala Vivitar 283 - 285). those voltages would kill a digital camera. The 10ufd non polarized capacitor ought to be just fine.
 
Hopefully the person with the problem found the solution five years ago and isn't still waiting. :D
 
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