scherbi Posted June 16, 2008 Share Posted June 16, 2008 Sorry if this has been asked before. I couldn't find it... I have a 40D. What is the spec of the flash sync (PC) terminal? Is it a dry closure, or doesit emit a pulse or what? Is the PC sync a standard that is the same for all cameras and all flashes? Thanks! Link to comment Share on other sites More sharing options...
jason_hall4 Posted June 16, 2008 Share Posted June 16, 2008 What kind of spec are you looking for? The camera just completes the circuit coming from the flash triggering it. There is no pulse from the camera. The are two wires from the flash (+ and -), the camera just complete that circuit, with older cameras, it was a physical contact, today it is with transistors. >>>"Is it a dry closure"<<< Not sure what that means... The PC is sort of a standard, more of the most common, many flashes use it but not all. There are many types of sync plugs out there, some proprietary. PC is actually one of the least reliable, yet one of the oldest and again most common. If a camera is going to have a sync plug, it will be PC as far as I know (In the world of Canon for sure). Jason Link to comment Share on other sites More sharing options...
jason_hall4 Posted June 16, 2008 Share Posted June 16, 2008 Should also mention that some older flashes can have high voltage on the syc circuit. I know the voltage limit on the 40D hotshoe is 250V, I would think the same is true for the PC sync, but I am not 100% sure. We mosly rather see the syc voltage at around 6V. So if you plan to use an older flash (you can bet newer units are "digital safe" but it is worth looking at the specs) you should check the voltage your self or find its specs. Jason Link to comment Share on other sites More sharing options...
scherbi Posted June 16, 2008 Author Share Posted June 16, 2008 I was looking for how to interface with it, DIY, without wrecking my camera. I've externalized the test contacts for a small flash and would like to trigger it from the pc port. There is high voltage across those points, but it rolls a bit above 250 with fresh batteries. I thought the pc port was max 6 volts, but I've been unable to find a true spec for this port. A an example of a dry closure would be a relay contact, or a plain old switch. So, I need to use an SCR, and I guess I can use the batteries from the flash to power the pc circuit with a fdew volts to trip the scr. Thanks. Link to comment Share on other sites More sharing options...
tommyinca Posted June 16, 2008 Share Posted June 16, 2008 Being so old and has so much variations from mfg to mfg and models to model from same mfg, I don't think there is a electrical connection specification. However, there is a spec on the mechanical connection (ISO 519). It used to be a mechanical switch that trip by the shutter. Later on a bi-polar (semiconductor) latch (to ground) was used. Today, my guess is most people use a CMOS FET of (your pick) any sizes. No pulse, Dry Closure (a dead short). For how long? Link to comment Share on other sites More sharing options...
tommyinca Posted June 16, 2008 Share Posted June 16, 2008 Some older flashes has negative trigger voltage. For one flash (the flash you have) use of SCR is not a problem. Just make sure you get the + and - bias hook up right. Link to comment Share on other sites More sharing options...
mark u Posted June 16, 2008 Share Posted June 16, 2008 The PC connection is specified at 250V max. I suspect it (and the hotshoe centre connection) is controlled by an SCR which is toggled on and off by other circuitry: IIRC shutter curtain movement is now monitored optoelectronically (possibly via an IR photodiode). Link to comment Share on other sites More sharing options...
jason_hall4 Posted June 16, 2008 Share Posted June 16, 2008 >>>"rolls a bit above 250 with fresh batteries."<<< I can tell you now that it is not safe for the camera, no questioned asked. There is a way around it. You can get a Wein "safe sync" adaptor. It will take any voltage and drop it to the safe 6V. They come in various configurations. So that will need to go between the flash and camera, or radio trigger if you decide to use one. Here is a link to what B&H offers. http://www.bhphotovideo.com/bnh/controller/home?O=search&A=search&Q=&shs=wein+safe&ci=0 As far as connecting to the PC of the camera....the contact on the camera is polarity sensitive. I think the center is positive so you will need to determine the polarity of your wires. Once you get the voltage under control you can try it on the camera. If the polarity is wrong it simply will not fire the flash, switch the wires and you will be in business. You can get a PC cable and cut off the end you do not need, then wire in any type of connector you like. I like "House hold" myself, but you are in danger of some sap plugging it into a wall socket andﾅ.well you can guess the outcome. Link to comment Share on other sites More sharing options...
scherbi Posted June 16, 2008 Author Share Posted June 16, 2008 I believe I now have all the info I need to move forward. Thanks everyone! Link to comment Share on other sites More sharing options...
jason_hall4 Posted June 16, 2008 Share Posted June 16, 2008 >>>"A an example of a dry closure would be a relay contact, or a plain old switch."<<< >>>" Dry Closure (a dead short). "<<< Interesting, none of my books, includeing "The Illustrated Dictionary of Electronics-Four Edition", make no reference to the term. Oh well you learn somehthing new every day. Jason Link to comment Share on other sites More sharing options...
jimstrutz Posted June 16, 2008 Share Posted June 16, 2008 Some older Canon cameras were rated for no more than 6 volts from the flash, but the newer ones are rated for 250 volts. The 5D, 20D, Rebel XT/350D and newer are all good for 250V on both the hotshoe and PC terminal. Watch, momentarily there will be somebody reporting that the hotshoe is not included in the 250V spec., and they will quote some Canon rep who hasn't kept up with his tech journals. There always is. :) Link to comment Share on other sites More sharing options...
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