trigger value

[ian_humphrey]

<p>Does anyone know the trigger value for flash through pc sync connection for Hasselblad 202fa camera.<br>

Many thanks<br>

Ian</p>

[paul_loveteck]

<p>Ian, can you explain what you mean by "trigger value " ?</p>

[q.g._de_bakker]

How big a voltage the flash unit is allowed to put over the PC contact without frying the camera's electronics, Paul.<br><br>I had a look but can't find any mention of it, Ian. But using any of the modern flash units, with low voltage, should be o.k.

[ian_humphrey]

<p>My sunpack thyristor auto zoom 3600 puts out 197 volts so the camera needs to have a trigger value of at least that. My canon d7 can take flash up to a voltage not above 250, but I can't find a figure for my hasselblad fa 202 camera. It would be useful to know the maximum trigger value for the camera.</p>

[rodeo_joe1]

<p>Ian, as I said in reply to your previous thread, that figure of under 200v for the Sunpak trigger voltage is mistaken. That's what it reads if measured with a cheapish multimeter of only 10 Megohm input resistance. If you measure it with an electrometer or something with a much higher impedance, you'll find that the P-C connector actually has closer to 330volts on its centre pin. I was prompted to re-measure my AZ3600s by your post, and rigged up a meter with an input resistance of 110 megohms. It confirmed that the trigger voltage really is in excess of 300 volts, which isn't surprising since it's a simple circuit tapped straight off the main capacitor terminal voltage. Even allowing for some tolerance, I definitely wouldn't connect it to any camera rated for only 250v.</p>

<p>(For anyone interested, I've sketched out below a typical time-honoured trigger circuit as used in older flashguns.)</p>

<p>WRT your Hasselblad: The 202FA has an SCA 300 compatible input, and the SCA standard allows for up to 25V on the trigger pin. So you can be sure that anything offering below 25v will be safe on the hotshoe or SCA connector. The P-C connector <em>might</em> be a simple mechanical switch closed by the shutter mechanism, in which case there's little danger of it being damaged, but if I were you I'd email Hasselblad for confirmation.</p>

<p>In any case that AZ3600 needs taming in some way to make it safe for use on a modern digital camera. Either by means of a Sunpak hotshoe adapter as I previously suggested, or by using a radio trigger or a Wein SafeSync. The SafeSync is straightforward but probably the most expensive option of the three.</p><div></div>

[steve m smith]

<p>Whilst the voltage might be high, the stored energy in the little 10nF capacitor is very low and will not be a problem for mechanical contacts in a camera.</p>

<p>These things were designed to be used together. There wasn't this paranoia before digital existed. People just bought a camera then bought a flash and connected them together!</p>

[paul_loveteck]

<p>Ian,<br>

The PC connector of the 202FA (the one on the side of the camera) is purely mechanical and can sustain any flash voltage with no problem, just like the PC connector of your Zeiss lenses.<br>

The SCA connector however is electronic and, as mentioned by others above, must be used with care.</p>

[doc444]

<p>Here is a site listing trigger voltages.<a href="http://www.botzilla.com/photo/strobeVolts.html">http://www.botzilla.com/photo/strobeVolts.html</a></p>

[rodeo_joe1]

<p>Steve, I agree that modern flash systems are sometimes too sophisticated for their own good, but times move on, and people demand convenience at the expense of robustness these days. Medium format cameras are no exception, and most newer models boast some form of electronically controlled TTL flash.</p>

<p>To Ray and others, I must reiterate, because it doesn't seem to have sunk in yet: the trigger voltage figures given on that Botzilla site are <strong>unreliable!</strong> Most of those readings will have been taken using a cheap digital multimeter by people with little knowledge of voltage measurement protocols or flash trigger circuitry. So I'd take any reading above 100v on that site (and given elsewhere on the web) with a big pinch of salt.<br />The issue is that those old high voltage trigger circuits always have a limiting resistor in series to prevent shock and injury to the user. This resistor is usually of the order of several megohms, and gives rise to a very large error when a standard 10 megohm multimeter is used in an attempt to read the open-circuit voltage. The result is that the true voltage is grossly underestimated.</p>

<p>One way around this is to take two readings; one direct with the meter and another through an additional high value series resistor (e.g. =>10 megohm). Those two readings can then be plugged into simultaneous potential divider equations to find both the value of series resistance and the real open-circuit voltage of the system. Unfortunately not many people are going to bother doing that, and will simply publish their erroneous voltage readings got by just poking a multimeter on the P-C terminals.</p>

<p>I'll repeat again that the error is usually not trivial, and can easily result in a camera-maker's specification for maximum trigger voltage being unwittingly exceeded. The situation with old studio flash gear is much the same, except that the voltage and error involved can be even higher.</p>

[thirteenthumbs]

<p>In the sample circuit posted above the 1500uf capacitor is the main charge capacitor that supplies the power necessary to fire the flash tube. The 5 meg ohm resistor isolates the capacitor from the PC socket so that a user does not get knocked on their posterior or worse if they should touch both contacts of the socket at the same time <strong>and</strong> provides the trigger voltage to the trigger transformer when the PC socket contacts are connected, usually by the camera. The 10nf capacitor isolates the 5 meg ohm resistor from the common line preventing the charging capacitor from being discharged through the 5 meg ohm resistor.<br>

When the contacts of the PC connector are connected by the camera or some other conductive material the trigger voltage is applied to the trigger transformer resulting in a much higher trigger voltage being applied to the trigger contact of the flash tube causing it to fire discharging the charging capacitor.</p>

<p>The high voltage that can damage a camera is only present for a very brief period of time when the flash is fired. Voltage spikes lasting less than a nanosecond can destroy electronic devices.</p>

[rodeo_joe1]

<p>"The high voltage that can damage a camera is only present for a very brief period of time when the flash is fired." - Actually Charles, the full trigger voltage is seen by the camera as soon as the flash is connected and charged up. It's the current pulse to the transformer that only flows when the flash is triggered. In the circuit above the camera contacts would have the full potential of 330 volts across them for almost as long as the flash was connected and switched on.</p>

<p>Semiconductor electronic devices, even heavy duty thyristors and the like, are very sensitive to over-voltage damage. If a device's internal insulation layer is punctured by a voltage spike it usually leads to a current avalanche that can destroy the insulation permanently or fuse an interconnect. Whichever, the result is a blown and useless device that needs expensive replacement.</p>

[paul_loveteck]

<p>A minor precision to the very good explanation by Rodeo: yes, the voltage is always present when the flash is "on" although it varies from 0 to the maximum (330V in the diagram) during the time it takes to the capacitor to charge. The current flowing through the PC connector is not only very brief, but also very small. In the diagram posted, the current would be roughly 70 micro amps and last only the time it takes to the Xenon gaz to be ionized by the very high voltage (3KV to 10KV) on the secondary of the transformer applied to the tube trigger plate. After the ionization, the resistance of the tube becomes negligible compared to the 5MOhm resistance and all the current flows through the tube, not the PC connector.</p>

[douglas_fairbank]

<p>The flash of the 200 series camera is NOT purely mechanical, the input from the flash goes to a thyristor and not directly to the shutter and based on some of the individual components an upper limit of 200 volts might be possible but may still be less than this figure.<br>

The cost of repairing the camera if a mistake was made is just not worth the risk!</p>

[paul_loveteck]

<p>Douglas,<br>

I have the original Hasselblad schematics of the 202FA in front of me.<br>

The PC connector goes to a purely mechanical switch, there is NO thyristor (and no need for one).<br>

Can you tell us where you get your information from.<br>

Or could it be that you do not understand that this camera has TWO flash connectors and you are confused between the SCA and the PC?</p>

[douglas_fairbank1]

<p>Paul,<br>

I have 38 years of experience as a technician with Hasselblad and I stand by what I say. I have also repaired cameras that have suffered from excess voltage. Do you need any more details?</p>

[paul_loveteck]

<p>Douglas,<br>

Yes, I need something more concrete. If my Hasselblad manual is wrong, I want to know! I need the proof that the 202FA does use a thyristor in the PC synch system (NOT SCA, I repeat). If you send me the proof, I will be glad to update the manual I have and to edit my comments on this discussion.<br>

Do you have a manual different from mine?<br>

Do you have pictures of a 202FA PC mechanism being fried?<br>

Although I respect your 38 years of experience, this is no proof that my manual is so wrong.<br>

Please, note that this discussion is about the PC synch of a 202FA and not "cameras" in general. I know that some cameras can be damaged by over-voltage. I have never heard of a 202FA being damaged by a flash connected to its PC connector.</p>

[q.g._de_bakker]

Paul,<br>Given the context, you raise an interesting question: what is the value of you never having heard of a 202FA being damaged by a flash connected to its PC connector? Why would that mean anything to any of us?

[douglas_fairbank1]

<p>Paul,<br>

As requested here is a copy of my certificate and also a picture of a circuit that clearly shows that the PC socket and SCA are connected in parallel. If you have a schematic that shows anything else it is wrong!<br>

You will just have to take my word for there being no circuit diagrams available and at least one of the components having no more than 200 volts rating and that does not mean the whole circuit has that rating, it may well be less.<br>

I cannot show you a circuit that is 'fried' because as you probably know it is often the case that there is nothing visible.<br>

I am not looking for any battle of words, I only wanted to correct a misleading statement and I am sure that you meant well but any schematic you have that shows the PC and SCA as separate circuits and/or going directly to mechanical contacts is wrong. My source of information is the OFFICIAL factory manuals and I have the printed and more recent e-copies.<br>

To the originator of the thread please do not use a flash with a high voltage or if you must then use a well designed slave.<img src="https://dl.dropboxusercontent.com/u/5279928/certificate%20205.jpg" alt="" /><br>

<img src="https://dl.dropboxusercontent.com/u/5279928/20130804_104515.jpg" alt="" /></p>

[paul_loveteck]

<p>Douglas,<br>

First, thank you so much for your nice answer and the time you spent for the picture.<br>

You are right: your picture indeed shows the PC connector being connected to the thyristor as the SCA is.<br>

This did not show on the schematics I have.<br>

Do you happen to know which thyristor the board is using? Most thyristors can easily handle voltages in the range of 400V in the Anode/Cathode and currents much larger than the current produced by a flash in the synch system. In fact, this is their main reason of existence. A too high voltage to the gate could damage the thyristor, but this is not where the PC is connected.<br>

</p>

[douglas_fairbank1]

<p>As you can see it is not visible on that side of the board and dismantling it is not something that I want to do right now as there are some tricky switches to dis-engage and re-engage. I do recall that it was a common place component.<br>

I actually believe that the main reason for the use of thyristors is to clean up the switching operation because many of the Metz flashguns (among others) that I was involved with were sensitive to noise on the sync contacts and today we see digital backs like the Phase units are also sensitive to noise. When I service Hasselblad lenses I always test the sync dynamically for noise so that they are reliable with modern computer flashguns and digital backs.<br>

So in my opinion the main advantage of thyristors is to ensure clean sync with low voltages. It just does not make any sense for safety reasons for any flash unit to have more than a signal voltage on the sync connection.</p>

[rodeo_joe1]

<p>Not to drag this discussion on further than needed, but the voltage issue with some cameras (not necessarily the 202FA) is with the ancilliary surface mount components, the proximity of tracks and the fragility of insulation in the interconnect wiring. The fact that a thyristor is rated for 400V doesn't mean that the tracks and other SMT components it's wired to won't flash over at less than that voltage! The rated limiting voltage for <a href="http://www.welwyn-tt.com/pdf/datasheet/pcf.pdf">Welwyn's 1/8th watt SMT resistors</a>, for example, is 150v.</p>

<p>Also Paul, the current discharged through the P-C contacts by that veteran trigger circuit most certainly will exceed 70 microamps. May I draw your attention to the 10nF 400v trigger capacitor connected between 0v and the P-C centre pin? It's that capacitor that supplies current to the trigger auto-transformer, meaning the instantaneous current is only limited by the primary impedance of the transformer, and may well amount to hundreds of milliamps, even if just for a few microseconds. Also note that the supply to the flash tube is connected to the trigger capacitor via a 5 megohm resistor, therefore the tube voltage is effectively isolated from both the trigger coil and its driving capacitor. Consequently the ionising of the Xenon tube has little to no effect on the current flowing in the trigger circuit until some time has passed.</p>

[ian_humphrey]

<p>Sorry to get back on this subject. My sunpack thyristor auto zoom 3600 puts out 197 volts so the camera needs to have a trigger value of at least that. My canon d7 can take flash up to a voltage not above 250,so I am informed by Canon. Are they correct and is it safe to use the flash unit with my Canon 7d.</p>