Trigger voltages and the mechanism that accomplishes this

[purelithium]

How is trigger voltage measured? How does the camera tell the flash when to fire? How does the flash damage this circuitry if it's trigger voltage is too high? Why do older flashes produce too much trigger voltage that a newer camera can't handle? Wouldn't newer cameras be designed so they could work with existing flashes?

Sorry for the questions, but I can't find anything that explains the trigger voltage "no-no". All I've seen is people saying older flashes are bad, because their trigger voltage is too high.

To me, I would think the camera is the one to produce the "trigger", telling the flash to then fire. I don't understand why the flash would need to output any voltage at all...

[PacAce]

How is trigger voltage measured?
Use a good analog VOM or a digital VOM and measure the voltage across the contact on the base of the flash shoe and the contact on the side of the flash shoe. Set the VOM to measure volts DC and watch your polarity.

How does the camera tell the flash when to fire?
There is a switch in the camera that is normally open. This switch can be mechanical or electronic or both. The flash fires when the switch is closed.
How does the flash damage this circuitry if it's trigger voltage is too high?
Electronic switches have certain voltage and current tolerances. If that voltage or current tolerance limit is exceeded, it will burn out the electronic switch.

Mechanical switches are subject to arcing when there is too much voltage applied to it and the switch is closed. Arcing can melt the mechanical switch contacts and render it useless. This arcing is the same principal that is put to use on an arc welder! :shock:

Why do older flashes produce too much trigger voltage that a newer camera can't handle?
Probably because that's the voltage that's readily available. Flashes require high voltage in order to fire the Xenon tube that produces the flash. This circuit is kept open until a flash is required at which time, current is allowed to flow through it via a simple switch. It would require more components to isolate this voltage so that the triggering voltage flowing through the camera is a lot lower then the voltage that's used to light the flash tube.

Wouldn't newer cameras be designed so they could work with existing flashes?They are. The current EOS DSLR cameras can safely handle 250v. It's just a few old flashes that have much higher triggering voltages that you need to worry about.


To me, I would think the camera is the one to produce the "trigger", telling the flash to then fire. I don't understand why the flash would need to output any voltage at all...
The camera does "produce the trigger", by closing it's flash triggering switch on the hotshoe and the PC terminal. This causes the flash to fire. Just like with any electronic component, you need current to flow through the flash circuit for it to work. And in order to have current flow, you need voltage across the circuit to "push" the current through it.

[FlashZebra]

PacAce has a very nice detailed explanations to your specific questions.

Here is a stab at a short answer that does not necessarily address your specific questions as stated and is definitely not comprehensive.

Everything you might connect to a flash unit to fire it (camera, radio slave, optical slave, sound trigger, etc) is fundamentally just an added switch (to short two electrical contacts). With all of these devices you are just adding a switch to a circuit that only lacks a switch to function.

So, flash units are basically circuits in search of a switch to complete their firing function.

This simplicity is very elegant and works nicely. This is very like you turning on an overhead light with a wall switch. That switch just shorts two electrical contacts allowing current to flow. A flash unit is just waiting to be provided with a switch and that switch to be closed.

All the Voltage, all the current, comes from the flash unit. The camera (or other triggering device) just provides the missing switch.

Enjoy! Lon

[purelithium]

Thanks for answering my questions so well, PacAce. You too londuck!


They are. The current EOS DSLR cameras can safely handle 125v. It's just a few old flashes that have much higher triggering voltages that you need to worry about.

125v? Really, According to this chart, it's only around 6 v... http://www.botzilla.com/photo/strobeVolts.html

Hmm... I just really don't want to risk my 300D's hot-shoe circuitry by using an old flash. I have an old Vivitar 2800-D flash, and a Metz Mecablitz 34 AF-3 C

I have no clue if either is safe. I'll try to take a photo of the method i've been using to test the trigger voltage later on.

Thanks again for your great explanations.

[SkipD]

I do believe the 300D is limited to 6 volts through its triggering circuit. The 20D (and newer models, I believe) has a 250-volt circuit - at least for the PC connector. There has been little published by Canon to convince some of us that the hotshoe switching circuit can handle the same voltage. It's usually better to be safe than sorry when the specs are not published.

That said, sometimes I have witnessed triggering problems when a Wein SafeSync was used with flash units that already had a low trigger voltage.

[PacAce]

Thanks for answering my questions so well, PacAce. You too londuck!



125v? Really, According to this chart, it's only around 6 v... http://www.botzilla.com/photo/strobeVolts.html

Hmm... I just really don't want to risk my 300D's hot-shoe circuitry by using an old flash. I have an old Vivitar 2800-D flash, and a Metz Mecablitz 34 AF-3 C

I have no clue if either is safe. I'll try to take a photo of the method i've been using to test the trigger voltage later on.

Thanks again for your great explanations.
You're right. The 300D is limited to 6V. However, I was talking about the current models (the 300D is an older model that has been replace several times already) in response to you question about newer models being designed to handle the higher voltages.

[PacAce]

I do believe the 300D is limited to 6 volts through its triggering circuit. The 20D (and newer models, I believe) has a 250-volt circuit - at least for the PC connector. There has been little published by Canon to convince some of us that the hotshoe switching circuit can handle the same voltage. It's usually better to be safe than sorry when the specs are not published.

That said, sometimes I have witnessed triggering problems when a Wein SafeSync was used with flash units that already had a low trigger voltage.

Thanks for the correction, Skip. I had a lapse in memory there. :)

[olly_k]

I have just converted a Hanimex flash with a simple optically isolated circuit (off the net) so that only 6V appears across the hot shoe! Simple circuit but I would suggest it is for more advanced hobbyists due to the voltages, but mainly the fidlyness of squeezing everything into the flash's body - No room for extra circuit boards!

http://i170.photobucket.com/albums/u270/olly-k/Hanimexinside.jpg

I actually tried asking how to use a flash manually with my EOS 350 but seeed to have had little help :( Maybe we could share some tips :D

[FlashZebra]

I do believe the 300D is limited to 6 volts through its triggering circuit. The 20D (and newer models, I believe) has a 250-volt circuit - at least for the PC connector. There has been little published by Canon to convince some of us that the hotshoe switching circuit can handle the same voltage. It's usually better to be safe than sorry when the specs are not published.

That said, sometimes I have witnessed triggering problems when a Wein SafeSync was used with flash units that already had a low trigger voltage.
The D30 (not 30D), D60, 10D, and 300D Rebel all have a 6 Volt limit on the hotshoe and PC connector, as do many of the Canon film SLR cameras of that era.

The 20D, 350D XT, 30D, and 400 XTi all have a 250 Volt limit on any camera chassis PC connection and very likely (note the very likely qualifier) have this same 250 Volt limit on the hotshoe. But, Canon has not stated these specifications in a clear manner, so nobody is really sure about the hotshoe limit.

This oversight or intentional omission on Canon's part causes a grand amount of problems and confusion, and undue expense for Safe Sync devices that are purchased, but may not actuality be necessary.

Enjoy! Lon

[FlashZebra]

I have just converted a Hanimex flash with a simple optically isolated circuit (off the net) so that only 6V appears across the hot shoe! Simple circuit but I would suggest it is for more advanced hobbyists due to the voltages, but mainly the fidlyness of squeezing everything into the flash's body - No room for extra circuit boards!

I actually tried asking how to use a flash manually with my EOS 350 but seeed to have had little help :( Maybe we could share some tips :D
Are you powering the isolation circuit off the cells already in the flash unit, or adding another battery?

What sort of power is required? If it is 4.8 to 6 Volts that would be great as many flash units have four cells already there that you could jump to (and allow for both alkaline and NiMH cells).

Enjoy! Lon

[olly_k]

Are you powering the isolation circuit off the cells already in the flash unit, or adding another battery?

What sort of power is required? If it is 4.8 to 6 Volts that would be great as many flash units have four cells already there that you could jump to (and allow for both alkaline and NiMH cells).

Enjoy! Lon

Hi Lon, yes the only power required is that used to drive the led part of the opto isolator, and this is derived from the flash internal suply - I should say at this point the cicuit doesn't achieve true isolation as the trigger shares comon ground but I can't ever seeing this being a problem!
I used an isolator nicked from an old PSU but maplin have one which only requires a voltage drop of 1.3V so a dropping resistor is required depending number of batteries within the flash.

I should give credit where credit is due as I based mine on a circuit on the net but can't find it now! I will try to find or draw a diagram when I get a minute!

[FlashZebra]

I should give credit where credit is due as I based mine on a circuit on the net but can't find it now! I will try to find or draw a diagram when I get a minute!
Anything like this?

http://strobist.blogspot.com/2006/10/diy-safe-voltage-circuit.html

Enjoy! Lon

[PacAce]

The D30 (not 30D), D60, 10D, and 300D Rebel all have a 6 Volt limit on the hotshoe and PC connector, as do many of the Canon film SLR cameras of that era.

The 20D, 350D XT, 30D, and 400 XTi all have a 250 Volt limit on any camera chassis PC connection and very likely (note the very likely qualifier) have this same 250 Volt limit on the hotshoe. But, Canon has not stated these specifications in a clear manner, so nobody is really sure about the hotshoe limit.

This oversight or intentional omission on Canon's part causes a grand amount of problems and confusion, and undue expense for Safe Sync devices that are purchased, but may not actuality be necessary.

Enjoy! Lon

Chuch Westfall has already stated that the PC and the hotshoe are both similarly rated. I think I posted a link to the source in one of the thread in this forum not too long ago. I'll see if I can did it up when I get a chance.

[FlashZebra]

Chuch Westfall has already stated that the PC and the hotshoe are both similarly rated. I think I posted a link to the source in one of the thread in this forum not too long ago. I'll see if I can did it up when I get a chance.
I was aware of this statement. But, that is it with Canon.

This testimonial by one Canon USA official has never been codified.

This is important info and there should be a citation in the product specification, it should not hinge on the testimonial of one Canon official, however important that official.

I personally like important factual info like this in print, readily available to all camera users, and very explicit.

By my personal yardstick a testimonial by one Canon official is a bit short of "a clear manner".

Enjoy! Lon

[b1gdaddy]

I should give credit where credit is due as I based mine on a circuit on the net but can't find it now! I will try to find or draw a diagram when I get a minute!


I used this circuit modded a little but same basic circuit.

http://www.carlmcmillan.com/images/Optoisolated_Adapter.GIF

http://photography-on-the.net/forum/showthread.php?t=270311

[olly_k]

Anything like this?

http://strobist.blogspot.com/2006/10/diy-safe-voltage-circuit.html

Enjoy! Lon

Not at all far off, except that circuit completely isolates the camera, from the flash. Also the circuit I used uses the opto coupler to fire a triac.

[olly_k]

I used this circuit modded a little but same basic circuit.

http://www.carlmcmillan.com/images/Optoisolated_Adapter.GIF

http://photography-on-the.net/forum/showthread.php?t=270311

That's it I added a capacitor & resistor to flash side perhaps not required but I think increase compatability.

[FlashZebra]

Not at all far off, except that circuit completely isolates the camera, from the flash. Also the circuit I used uses the opto coupler to fire a triac.
At some point I may give this one a try (one on Strobist website). I can solder, and understand electricity in a reasonable manner, but I cannot make on the fly modifications as you may be able to do.

Do you think the two circuits highlighted on the strobist website could be powered by 4.8 to 6 Volts (this circuit cites 3 Volts). So, I could steal power from four cells already in the flash, and not bother with another power source?

Enjoy! Lon