Flash Trigger Voltage Real World Experience

[John Seaman]

<p>We're always conscious of the dangers of using flashguns with high trigger voltages on electronic cameras. The excellent Botzilla website is the source of much useful information here:</p>

<p>http://www.botzilla.com/photo/strobeVolts.html</p>

<p>My thought is, how real is the risk? I'm thinking that although the open circuit voltage can be disturbingly high, the current must be extremely small. So, has anyone actually experienced, or reliably learnt of, a camera damaged by excess trigger voltage?</p>

 

[wayne_f1]

<p>No experience here with damage, but it was probably a real hazard, simply due to the availability of the Wein Safe Sync shoe protectors - they must help something. :) The old flashes with high sync voltage rarely exceeded 200 volts (however some say early Vivitars could be 250 volts). The Safe Sync $50 price surely exceeds the value of any flash that needed it (the money could be better invested).</p>

<p>But today, sync voltage in anything modern is about 5 volts. No Nikon speedight flash (gong back 30 or 35 years) was ever more than about 5 volts sync voltage. </p>

<p>But as one example, all of the Nikon DSLR manuals say the cameras are rated to 250 volts. You'd think anything would be rated similarly, but as another example, a few of the earlier Canon EOS manuals actually said DO NOT USE ANYTHING HIGHER THAN 6 VOLTS. The ISO spec for hot shoes allows 45 volts, so I don't know how real or precise that was - it was followed by saying Use Only Canon Flashes. :)</p>

[Jochen_S]

<p>I touched aproximately 110V on a sync cable & it hurt.<br>

I do not know what ruined the contacts in my user-beater M3 but I had them refurbished. <br>

These bits aside I am too cheap to do further testing / disaster provocation. - I rather email camera tech support to ask them what my bodies migt be able to stand. - Schould do so for Fuji X series next. My other digitals are low Voltage for sure.</p>

[wayne_f1]

<p>I've been curious how the old high sync voltage flashes were built? Modern stuff is just a resistor between the port pin and a 5V source. Open circuit sees the 5v, but when the camera shorts the pin to ground, it's zero volts of course. No current or damage due to the protective resistor, but the zero volt level triggers a semiconductor trigger circuit. SCR probably. The cameras today use a SCR to short the pin. The isolation is a bit like a relay in that regard. :)<br /> <br />Old flash stuff though... making this part up, but before modern electronics, grounding that pin must complete the actual circuit to produce current in the flash tube coil that ignites the tube?</p>

[John Seaman]

<p>Hmm I suppose there must have been a measurable current when the flash contacts in the camera closed, otherwise nothing would have happened. Also I think that one reason for the covers over flash terminals. was to shield the operator's fingers from possible high voltages from the flashgun, via the flash wiring in the camera.</p>

[peter_simpson1]

<p>Electrical engineer here. When a mfr says there's a 25V (or whatever) limit on the flash connector, he has a reason to say that. For example, if the trigger is a FET or a transistor, that part has a maximum rated voltage it will switch. Exceed that, and the part <em>will</em> be damaged.</p>

<p>Now, a modern all-electronic camera is unlikely to have a set of contacts to trip the flash. So, it's probably using some kind of transistor device. If you exceed the rated voltage, that part will cease to function. Even if you only exceed it once. Trust me, I know this from personal experience (in a circuit other than a camera flash trigger).</p>

<p>It's really easy, BTW, to build your own "safe sync" device and install it in your favorite old flash. A handfull of parts from Digikey and a custom PCB from ExpressPCB and I had my own: http://ka1axy.blogspot.com/</p>

[John Seaman]

<p>Peter I too am an electrical engineer, albeit of the heavy current persuasion, and I totally accept what you say. However I would still be finally convinced by an actual instance of this happening in a camera.</p>

[pete_s.]

<p>Mostly old wifes tale I'd say. Nikon have had 250V capable hot shoes since forever and says so in their manuals as well. Chuck Westfall from Canon said that the Canon 10D (I think) had mechanical contacts but anything more advanced and newer and all EOS 1 series cameras could handle 250V both in the hot shoe and pc sync.</p>

<p>They use triacs/thyristors to trigger flash over pc sync and hotshoe and if you look for specs on these, they can handle at least 400V so in reality 300-350V would work fine on a 250V capable hotshoe.</p>

<p>For any other camera manufacturers I don't know for sure. Mechanical contacts is not a problem per se. Those that manufactured leaf shutters like Proctor and Compur used the pc connector (those two companies invented it) and they were of course 100% mechanical.</p>

<p>With a very few number of exceptions I think all modern cameras use a triac or similar to trigger a flash in the hot shoe. It's cheap and just too much hassle to do anything else and possible have customers returning their broken cameras for warranty repairs. It would be almost impossible for the manufacturer to prove that the users put a high voltage flash in the hotshoe and that's why it broke. In the same way it's also lot easier to recommend using low voltage flash units and not specify at all what the hot shoe actually is capable of. There is probably also an iso standard or similar that they can refer to. So that's why there are no reports to be found on cameras that has been destroyed by old flash units.</p>

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[peter_simpson1]

@PeteS. You're probably safe with that one :-)

 

I'm slowly adding the safe sync circuit to all my HV strobes. Then, I know I'm safe.

[rodeo_joe1]

<p>It doesn't matter how small the current is John, electronic components are voltage sensitive, and even a static discharge can cause permanent damage to them. The issue is that a high voltage can puncture the extremely thin insulation that forms a barrier between N and P type silicon layers, or between device features.</p>

<p>Take those "measurements" on the botzilla site with a big pinch of salt. They're obviously mostly taken by people with little or no electronics knowledge, and with a wide variety of measuring instruments. This is probably the cause of the wildly varied readings.</p>

<p>The common trigger circuit used in older flashes has a very high resistance in series with the trigger pin. This means you can't get a sensible reading of its true voltage, even with a digital multimeter. A device called an electrometer should be used, or you can take two readings with different meter resistances and then use simultaneous equations to calculate the open-circuit voltage. Either way, not many people have the equipment or skillset to do this.</p>

<p>The simplest thing to do is assume that any flash whose trigger voltage is listed at over 200 volts will actually have an open circuit voltage in the region of 320 to 350 volts; this being the common high voltage used across the main storage capacitor. Even the comparatively recent Sunpak AZ3600 has a trigger voltage of 320 volts unless a remote hotshoe sensor is fitted. Likewise some old Mecablitz hammerhead units.</p>

<p>Most cheap Chinese-made radio triggers will withstand 400 volts, and even if you manage to "kill" a receiver, it's no big deal at the price. Only a few ancient studio strobes will exceed that 400v limit.</p>

<p>I use radio triggers as a matter of course, so never experienced a camera malfunction caused by flash. However I did manage to blow one trigger device by connecting it to an old (very old) Bowens 400 monolight.</p>

<p>BTW. The circuit used in old flashes is still used in disposable cameras. There's a CCT diagram here:<br>

http://www.privatedata.com/byb/rocketry/Ignitor/cached_griesser_ignitor/fa_schematic.gif<br>

You can see that the whole of the Xenon discharge voltage appears on the trigger cicuit, and is only limited to a non-fatal current by the 3.9 megohm resistor in series with it.</p>