On Friday, May 25, 2012 01:02:05 PM Jon Elson did opine: > gene heskett wrote: > > For this it gets sticky, because you have 2 choices of how to attach > > the scope probes, BOTH of which leave the case of the scope lethally > > hot, so at the very least it has to be running on an isolation > > transformer, and sitting on an insulated table, no way around that. > > You'll obviously need to be standing on an insulated surface, and > > following the one hand in the pocket rule. > > The right way to do this is to get two identical 240 v to (something) > transformers. > 240 - 12 V would be fine. Then, the scope can be safe to touch. You > hook the > 240 V primary wires to the circuit to be measured, the scope can be > connected > to the secondary terminals. You don't even need a scope probe, you can > wire directly > to the input BNC. > > Jon
I thought of that too, Jon. But I tried that once, many years ago, and found that the transformers blocked the higher frequency stuff I was interested in. For this however I'd think it would be adequate as I don't think we are looking for megahertz plus response. But with my history, its certainly something I would be looking for in the scope traces however they are obtained. However, if the problem is related to the spikes on the line caused by slow recovery characteristics of the diodes in this kits high voltage DC supply, those spikes will be attenuated by the poor high frequency response of the transformers. They won't look as if they are big enough to be a problem so most would ignore them. At KXNE-TV, where I was the transmitter super for several years, we were plagued by 6 lines that rolled through the picture since the powerline rate was 60 hz, but the NTSC vertical sync was 59.94, we did try some large capacitors across the line, but they had no effect. They were caused by the full wave 3 phase bridge rectifiers for the 20 kilovolt beam supply, good for about 12 amperes if both klystrons were fresh and running at full song. The nominally 12 microsecond recovery time of a few thousand $ worth of common DO-5 rectifiers was causing the phases to be shorted together as one phase was rising above the diodes conduction point, and the phase that had been supplying the power was falling, so at every crossover point, the slow recovery of the bridge leg going off effectively shorted the phases together, and created a spike of ringing when they finally did turn off that was still 2 to 3 kilovolts by the time it got back through the trio of 75kw rated substation transformers that were hooked up backwards to make the 15 kilovolt rms that fed the bridge. Superimposed on the rest of the buildings 127 volt utility wiring, it raised hell with small equipment fuses, light bulbs, florescent ballasts etc. Because TPTB thought the transmitter would be better protected if it had a regulated AC supply when the place was originally built, there was a 225kva rated ferro-resonant 3 phase transformer on a pad outside the building that Wayne Co. Public Powers substation 3 feet away fed. That thing was a huge choke, trapping all that from getting back to the powerline where sheer miles of wire would have absorbed it. I tried to justify replacing every diode in that bridge with faster recovery devices, but this was the early 70's and 1 microsecond devices were then about $40 a copy, I'd need more than a hundred of them, so that got turned down. Finally, a lightning strike blew that overgrown Sola transformer all to hell with the resultant fire warping the enclosures steel roof. And of course we were off the air. Sola wanted about $120k for another core and around 90 days to deliver, not a stock item. So we bent the 4 pieces of 750mcm a phase around that puppy just to get back on the air. All in huge kearney bolts right there in the enclosure. With the substation off-line, and my boss Larry from the head shed in Lincoln, we used a 3/4" conduit hickey to get 2 of each phase in and out into the biggest Kearney Bolts they could find in NE, with Larry and I on the pipe cheaters of a pair of 48" mag pipe wrenches to pull them up tight, we had 6 of then installed in about 5 hours, then covered with cambric tape and a couple rolls of scotch 88 per bolt to keep the weather out. Called Ron at WCPP to come and make up the fuses to the substation again. Problem 99% solved, now that surge had someplace to go. Lines in pix almost subliminal, light bulbs lasted 3 times longer. Obviously we never did order a new core for the regulator. ;) The reason I relate this story is that there may be some similarities as to the tripouts John is seeing. Depends on how sensitive to these diode recovery spikes his Seimans kit might be. If that is the case, then he has an easy fix because he won't have to buy 100+ of a diode that is now 10% of the cost fast diodes were back then, and likely even faster. Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> Buck-passing usually turns out to be a boomerang. ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
