shootfirst

First some quick background.

For studio photography, it is typical to use several flash units to illuminate the subject. One of the ways to syncronize all of the flash units is by using an optical trigger also known as an optical slave trigger. The setup works like this - you have one flash that is triggered by the camera - all other flashes are attached to an optical slave trigger so that they will go off in unison with the master (camera triggered) flash. This optical slave trigger simply shorts two leads from the flash when the flash of the master light is detected.

Now the problem/question.

Most of these inexpensive slave triggers reportedly use an SCR (silicon controlled rectifier) that acts as the switch or trigger. Apparently from what I've learned, this SCR must have the current brought to zero in order for it to reset.

This is not a problem for most flash units on the market. But, the top of the line Canon Flash units will not function properly with a standard optical slave trigger. It will flash once, but after that it cannot reset. My guess is that there is some residual current flowing that does not allow the SCR to reset.

My question - is there a circuit I can place between the flash unit and the optical slave trigger that will bring the current to zero long enough so that the SCR can reset without interfering with the general operation of the slave trigger and flash?

The flash circuit is a relatively simple DC circuit - just 2 leads from the flash to the optical slave trigger. When these two leads are shorted together, the flash goes off. I'm not really sure exactly how either the flash or the optical trigger works, but the theory seems simple enough.

There was an individual that I found online that reported that he had placed an RC circuit in the system, but my attempts to duplicate his efforts have been inconsistent and I am unable to contact him.

Any help would be greatly appreciated!

Please let me know if you need any additional information. I have a multimeter and can take some measurements as necessary.

Thank you,
shootfirst

zachtheterrible

You are right, once a SCR is triggered, it will stay on unless current is stopped from flowing through it.

Unfortunately I can't think of a circuit though :cry:

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I'm no electronics god, i just talk too much.

jrz126

:shock: I must say, this is the best post I have ever read, so much detail, and complete sentences too.

Unfortunitly, I dont really know how to help. I know alittle bit about strobes from my failed strobe light project, but not enought to be able to suggest anything that I would be certain of.

Only thing I can think of is that maybe the slave trigger is picking up ambient light which is causing some current flow and preventing the SCR to reset?

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Jeff Zimmerman
To the optimist, the glass is half full.
To the pessimist, the glass is half empty.
To the engineer, the glass is twice as big as it needs to be.

shootfirst

Well golly!

Thanks.

Looks like that 5 years of college was worth something! I just wish I would have paid attention during my Electrical Engineering classes. I sure did hate those classes at the time. Didn't understand much of it either - that's probably why I didn't like it.

Anyway - anyone else have any ideas on how to help me with this circuit?

Thanks,
shootfirst

Roff

Try this:

Connect a 10k resistor and a 10uF nonpolarized capacitor in parallel. Connect this combination in series with one of your trigger wires. It shouldn't matter which one.
I don't guarantee that this will work, but it gave me a short pulse when I simulated it. I don't know how long your pulse needs to be, or what the Canon flash load is, so it was a bit of a SWAG.
It also takes several hundred milliseconds for the circuit to recover before it will reliably fire again. I'm hoping that's not too long.

There are certainly other sources of nonpolar caps. I just found this one with a quick search.

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Ron

audioguru

I suspect that the Canon strobe's recharge circuit supplies a high current so it can recharge quickly. Therefore the SCR in the optical slave keeps conducting the recharging current. Maybe the strobes that can be retriggered don't have such a high recharge current which allows the SCR to turn off.
Ron's suggested 10uF capacitor might need to have a value as high as 100uF to steal enough recharge current away from the Canon strobe in order for the SCR to turn off.

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Uncle $crooge

Oznog

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.

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shootfirst

Thanks Ron and audioguru.

I've had some success with the parallel RC circuit.

Would you mind quickly describing why this works? :?:

The individual who inspired me to try this suggested a 47k resistor and a .1uF cap in the same configuration you described. I tried that combination and also a 47k resistor w/ a 1uF Cap. Do I need to keep a relationship between the R and the C? FYI the voltage running through the trigger wires is about 4.6VDC.

Both solutions work (1uF and .1 uF), but the sensitivity of the optical trigger has dropped way down (w/ either of the caps). The master flash has to be pointing directly at the trigger whereas without the RC circuit on a non-Canon flash, the flash could be pointing anywhere in the room and the trigger would sense the light.

Any ideas on how I can reduce the impact of this RC circuit on the sensitivity of the trigger?

Thanks again for your help!

shootfirst

williB

http://www.botzilla.com/photo/G1strobe.html#wide
is this what is happening...
(2) the first flash will set off your slave units, which will fire and overexpose the meter reading but be unable to recycle quickly enough to make the second, real, exposure.

shootfirst

No - that is another common problem that many people run into. In my case, I am disabling the Canon ETTL system and using the flash in manual mode so there is no preflash. It only flashes once. The cheaper version of the Canon ETTL flashes like the 420EX and the built-in flash on most newer Canon cameras will not let you disable ETTL so you are stuck trying to work around the preflash.

That provides a unique electronics challenge as well. How would you get the slave to ignore the first flash but fire on the second.

I'm not interested in tackling that problem. I'm shooting in full manual and trying to deal with the other issue I described in my first post.

Thanks,
shootfirst

williB

Hmm .. How long between flashes is optimal .??
i mean you said that your slave flashes once.. How many times , or how often do you need to flash ??

shootfirst

Which issue are you discussing?

The first one that I brought up regarding the flash only flashing once and then locking up or the issue you brought up regarding the pre-flash disturbing the sync of the master and slave flash?

The flash may be asked to flash as much as 5-10 times per second for a rapid sequence - but that is not a typical usage and requires that you flash at a reduced power so that there is ample time to recharge between flashes. More likely you will be triggering the flash every 5 seconds or so. At full power a more frequent rate of flash will overheat the unit.

Thanks,
shootfirst

bryan1

Hiya Shootfirst,
On reading your post brought back memories of a project siliconchip did it was called "A smart slave flash trigger" and looking in the index it was back in july 2003. Here is the link to the article http://www.siliconchip.com.au/cms/A_30754/article.html
I reckon it could be just what your looking for and it is avaliable in kit form if you don't want to source all the gear and make a circuit board.

Hope this helps

Cheers Bryan

Edit:- Don't pay for the rest of the article if you like it PM me and I'll e-mail it over to you

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audioguru

I got the entire project from Google. I entered Silicon Chip A Smart Slave.
It doesn't talk about the problem where a strobe can't be retriggered due to the SCR in the optical slave staying on.

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Uncle $crooge

shootfirst

This thread is getting slightly off topic. I am interested in the multiple flash issue, but I'd like to get the flash lock-up (SCR) issue solved first.

I've been able to get the SCR to reset using the parallel RC circuit described above - but the sensitivity of the optical sensor has gone down. I'm wondering if I need to tune the RC to get the sensitivity back.

I'm currently using a 47kOhm R and a 1uF Cap wired parallel to each other and inserted in series with one of the trigger leads. The voltage is 4.6 VDC.

Any ideas if I require a larger or smaller resistor or cap? Do I need to keep a specific relationship between the R and the C?

Thanks!