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Old 02-09-2005, 12:21 AM
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Doug_Kerr Doug_Kerr is offline
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20D - flash sync circuit voltage limit

There was a post on this a little while ago, but it did not produce a complete answer.

Do we know for sure what is the voltage limit for the sync circuit on the 20D hot shoe?

Th 20D manual says (p 98):

- Also, do not connect to the camera's PC terminal any flash unit requiring 250 V or more. [They presumably mean "having a sync circuit voltage of 250 V or more". This electrical engineering stuff is tough for camera manual writers!]

- Do not attach a high-voltage flash unit on the camera's hot shoe. It might not work.

I'm not sure what a "high-voltage" flash unit is. And the warning is about not working, not about danger to the camera. (Perhaps the result of some overvoltage protective circuitry.)

Note that the hot shoe and the PC terminal persumably have separate interfaces (i.e., are not just in parallel), so we can't jump to the conclusion that the ratings are the same. The clue to that (also from P 98) is:

- A Speedlite connected to the camera's hot shoe and a flash unit connected to the PC terminal can be used at the same time.

That would not be a reasonable statement if the two connections were merely in parallel.

Best regards,

Doug




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Old 02-09-2005, 08:46 PM
KirkDarling KirkDarling is offline
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Re: 20D - flash sync circuit voltage limit

I have found somewhere a definitive word that the PC and hotshoe are, indeed separate circuits (for the most part), but I can't say where that was. You probably noticed, too, that the PC contact has no polarity. By the way, it's a Nikon screwlock. I've read of people breaking the PC contact with tight-fitting regular PC cords. I strongly advise using a Nikon screwlock cord so that you don't have to depend on a tight friction fit--with its attendent stress and wear attaching and detaching it.

I've wondered what "It might not work" means. Does that mean the circuit is self-protective and will block or shunt away very high voltages? It doesn't seem to say "the camera may be damaged."

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Old 02-09-2005, 09:05 PM
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Doug_Kerr Doug_Kerr is offline
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Re: 20D - flash sync circuit voltage limit

Hi, Kirk,

[ QUOTE ]
I have found somewhere a definitive word that the PC and hotshoe are, indeed separate circuits (for the most part), but I can't say where that was.

[/ QUOTE ]
Well, for one thing, I concluded that (earlier today) from the comment in the manual that two flashes could be operated at the same time - one from the PC terminal and one from the shoe. That would not be at all reliable if the two were just in parallel.

[ QUOTE ]
You probably noticed, too, that the PC contact has no polarity.

[/ QUOTE ]
I hadn't known that, but that's nice.

[ QUOTE ]
By the way, it's a Nikon screwlock.

[/ QUOTE ]
Hadn't noticed that yet. Good to know.

[ QUOTE ]
I've read of people breaking the PC contact with tight-fitting regular PC cords. I strongly advise using a Nikon screwlock cord so that you don't have to depend on a tight friction fit--with its attendent stress and wear attaching and detaching it.

[/ QUOTE ]
Sounds like a good idea. Anything that mnakes the world's worst electical connector behave better is worth doing!

[ QUOTE ]
I've wondered what "It might not work" means. Does that mean the circuit is self-protective and will block or shunt away very high voltages? It doesn't seem to say "the camera may be damaged."

[/ QUOTE ]

That's my guess. Likely some type of "crowbar" circuit or such.

Maybe Chuck Westfall will appear and let us know the whole answer on this. I sure miss his regular particpation here.

Best regards,

Doug

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Old 02-12-2005, 12:43 AM
KirkDarling KirkDarling is offline
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Re: 20D - flash sync circuit voltage limit

Something from the knowledge base at Euro-Canon:

"The trigger circuit voltage (TCV) rating for any EOS digital SLR is the same on the hotshoe as it is on the PC terminal (if the camera has one), but the acceptable TCV level varies according to the camera model. For the EOS-1D, EOS-1Ds, and EOS-1D Mark II, acceptable TCV is 250V. For EOS D30, D60, 10D and 300D, acceptable TCV is 6V.

The main reason for the difference is the way the x-synchronisation signal is generated. With the 250V cameras, the x-synchronisation signal is generated electronically. With the 6V cameras, the x-synchronisation signal is generated mechanically. It would be great to have electronic x-synchronisation in all EOS digital SLRs, but it costs more. "

http://www.cps.canon-europe.com/kb/d...jsp?faqId=1015

This of course makes little sense compared to the instruction manual.

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Old 02-12-2005, 11:11 AM
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Re: 20D - flash sync circuit voltage limit

Hi, Kirk,

Yes, I had seen that. We must note, however, that it does not mention the 20D (it was presumably prepared pre-20D), and there is always the possibility that the 20D design does not follow the same plan as the other "big" cameras.

One member of the dpr forum had an extensive discussion with Canon tech support, where a technician summoned by the agent assured him that the limit for the shoe was 6 V. (Of course, even on the 10D and 300D, what that actually means has been susceptible to considerable misunderstanding, which Chuck Westfall was been able to nicely clear up.)

It is hard for me to believe that, however. Probably the only source of a so-called "6V limit" would concern with erosion of the shutter flash sync contact where that directly feeds the the sync circuit in question. But if theh shoe in teh 20D is driven from the shutter sync contact, what feeds the electronic switch that drives the PF terminal? It probably can't be the same shutter contact, or there would be interference with the PC terminal driver input when there was also a flash being driven from the shoe (and the manual assures us that we can simultaneously drive one flash from the shoe and another from the PC terminal). Would there be a second sync contact on the shutter? Could be, but that doesn't seem likely.

Of course another possibility is that the PC terminal driver circuit isn't fed from a shutter contact at all, but from timing signals generated by the processor as it controls the shutter by way of its operating magnet(s). (This is how it works in my Fuji S602, as near as I know.) But then why have a shutter contact at all? It would of course save the cost of a second "line driver" for the shoe, but I would think that a shutter contact would cost almost as much and would of course give another point of failure.

(Excuse me of thinking like a forensic engineer, but that is my profession at the moment!)

I am hopeful that Chuck Westfall may come forth and give us the real scoop.

As a matter of interest, by way of comparison, in the Fuji S602, Fuji didn't originally state a voltage limit for its sync output, but after some pressure stated it to be 250 V. Reverse engineering of an S602 cadaver showed the sync output (shoe, also the PC terminal in the "Pro" version, which has such) to be driven by a transistor switch with a rated brakdown voltage of 400 V.

Best regards,

Doug

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Old 02-14-2005, 02:58 AM
Phil_Aynsley Phil_Aynsley is offline
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Re: 20D - flash sync circuit voltage limit

One of the tech guys from Canon got back to me today re the voltages
question.

"For the 1D MkII, 1Ds MkII and 20D the PC socket voltage can be up to
300V without any damage. Again please note that is connected via the PC
socket."

He also said that there are no mechanical contacts in the MkII's and I didn't ask about the 20D.

I'm not sure about via the hot-shoe. Also the consequences of
accidentally touching one of the four "control" connections on the
hot-shoe (which would certainly not take any high voltage) with the X-sync pin while
mounting/dismounting an external device doesn't bear thinking about.

He later confirmed that the limit for the hot shoe is 6V. This may be a reflection on the above danger rather than an actual circuit limitation?

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Old 02-14-2005, 11:30 AM
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Doug_Kerr Doug_Kerr is offline
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Re: 20D - flash sync circuit voltage limit

Hi, Phil,

[ QUOTE ]
I'm not sure about via the hot-shoe. Also the consequences of
accidentally touching one of the four "control" connections on the
hot-shoe (which would certainly not take any high voltage) with the X-sync pin while
mounting/dismounting an external device doesn't bear thinking about.

He later confirmed that the limit for the hot shoe is 6V. This may be a reflection on the above danger rather than an actual circuit limitation?

[/ QUOTE ]

A good point, which I had previously overlooked.

Of course, if that is in fact the sole basis of the "limit" on the shoe, once could perhaps avert it by being certain that the flash unit was turned off when being mounted. However, there is no certainty that when the flash is charged but then switched off that the sync input would be immediately dead. (It is dead in that situation on my Vivitar 2800, currently the only general-purpose flash unit in the house with a high voltage sync input - about 142 V.)

Incidentally, we can't be certain that the four "control" contacts of the 20D shoe haven't been provided with overvoltage protection.

Thanks for your insight into this aspect of the issue.

Best regards,

Doug

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