PC Cable Resistance

[frank_gary]

<p>I have been trying to set up a low cost PC cord solution but it isn't working. I am running a PC/3.5mm cable through a 12ft length of headphone extension cable then back through another PC/3.5mm cable. I figure doing this I can change lengths and ends as needed for different flashes with little cost. Basically treating the extension cable like a pocket wizard. However, it is only successfully triggering the flash about 70% of the time. I thought the problem might be be resistance in the cables and just measured it at about an ohm each way. Is this normal? What do genuine cords of this length have in the way of resistance? Anything else that could be causing my poor triggering rate?<br>

Frank</p>

[dbcooper]

<p>Maybe there's a possibility the stereo (3-conductor tip/ring/sleeve) headphone extension socket is interfering with/isn't making proper contact with the 2-conductor 3.5mm-to-PC setup?</p>

<p>For something DIY that's cheap and reliable, have you considered PC-to-HH (household) adapters? A HH extension is mighty hard to plug into the house wiring by mistake (it has 2 female ends...just don't plug the male HH-to-PC adapters in to a wall outlet). Inexpensive zip cord (bulk 16 gauge 2-conductor lamp cord) works fine. Most places sell it cut to length for 12 to 30¢/ft., and the thicker conductor has less resistance than a small ones like the ones inside a headphone cable so you can do really long runs if you want to. http://www.flashzebra.com/short_pc_sync/0055.shtml</p>

[richard_cochran]

<p>With a typical multimeter, it's hard to precisely measure resistances of around an ohm or less. Your test leads probably have a resistance that's a substantial fraction of an ohm anyway.</p>

<p>It doesn't matter much, though. Anything under a few ohms should trigger with no trouble. I've made many sync cords out of the cheapest speaker cable I could find, up to 100 feet of the stuff, and I've never had one be unreliable, unless the cable was actually broken inside. 12 ft of headphone cable should be fine. My bet is that D.B. Cooper is right, and there's a problem going to stereo and back. Or a poor or dirty connection somewhere.</p>

[frank_skomial]

<p>The problem could be in the headphone socket/plug pair, and the jitter of the cable could possibly misfire or more likely not fire at all.</p>

<p>You need to measure sync voltage, and not necessarily resistance. This way you will actually measure sync voltage and not a static resistance of cables that tells you little or nothing. However, you will need a digital storage sampling scope to catch any intermittent voltage level sync signal disruptions.</p>

<p>Design your cable with something much better than the headphone socket/plug. </p>

<p> E.g. use houshold electricity extension cables with solid socket/plug set... it was good for Sunpak pro line of older flashes, and it could work for you. Just make sure you never plug it into household electricity socket on the wall.</p>

<p>Another idea, use a long microphone cables that have XL prong solidly build and locking plugs/socket set. The kind used on stage with musical or vocal performers, who usually jump up with the cables, and never disrupt the audio signal.</p>

[pete_s.]

<p>The cable resistance doesn't really matter.</p>

<p><em>The trigger signal on a modern flash is really an input that is pulled up with a resistor to the internal low voltage power supply (Vcc). The flash fires when you pull that input down to ground. At the time of triggering the current flows from the supply voltage through the pull up resistor and then through the pc cable, through the camera, back trough the pc cable and then to ground. The resistance of everything else in this circuit makes the cable resistance non-relevant.</em></p>

<p>The household connectors are not optimal because nowadays the pc sync circuit is low voltage and low current. The best connector would be something that is gold plated to avoid corrosion.</p>

<p>Anyway, the problem you have is that the cable is broken inside which means it works sometimes and sometimes not or that the connectors wiggle. It is not a problem going from mono to stereo and then back. The problem with 3.5mm connectors are that they are usually manufactured with only price in mind and the tolerances can be very sloppy.</p>

<p>If you want you could go from 3.5mm up to the bigger 1/4" connectors that many strobes uses (for instance profoto). Cables and connectors are more solid but bigger as well.</p>

<p> </p>

[frank_skomial]

<p>You could just stop your suffering, by getiing industry proven cables, that start at some $9, and of any length you desire.</p>

<p>See at B&H or Adorama for hundreds of ready made cables.</p>

<p><a href="http://www.bhphotovideo.com/c/buy/Sync-Cords/ci/1214/N/4294550949">http://www.bhphotovideo.com/c/buy/Sync-Cords/ci/1214/N/4294550949</a></p>

[frank_gary]

<p>After playing with the setup some more it seems that the problem is in my studio strobe rather than the sync cable. When hooked up to my SB-28 I got 100% fire rate. I'm thinking that the problem is if any piece of the audio equipment converts between stereo and mono as it shorts the tip and sleeve. Once I get hold of a 3.5 mono to 1/4 mono male cables I"ll be able to test more. I'll keep everyone up to date on it. </p>

[frank_gary]

<p>I just got a normal 3.5mm mono to 1/4 mono cable to use in the setup and the old studio strobe fire rate has certainly gone up it seems. When I tried hooking up a cable splitter so that both the strobe and SB28 would be triggered from a single PC connection to the camera I got an interesting result. the SB28 will fire while the strobe does not. Any idea why this would be happening? I'm thinking that it's possible that due to different sync voltages the SB28 triggering is preventing the strobe from working but I'm clueless. Any thoughts?</p>

[pete_s.]

<p>You never mentioned you are triggering two strobes in parallel with the same cord.<br>

Besides different voltages you can have different polarity.<br>

I would string together a bunch of nikon strobes and fire them in parallel with the same pc sync cord any day but I would never mix them with old strobes that could easily put out 200-300V on the sync cord. It's a bad idea.</p>

<p>Maybe the old studio strobe have an optical slave inside that you can use.</p>

<p> </p>