Electronic Design

Bob's Mailbox

Hi Bob: Your tip on how to phase a generator into the electric grid would work ("\\[\\[What-s-All-This-Others-Stay-Lighted-Stuff-Anyhow-1|What's All This ‘Others Stay Lighted' Stuff, Anyhow?\\]\\]"). Phasing sets of the not too distant past used a couple of incandescent bulbs and a guy poised with his finger on the button, just as you said. I wanted to explain, however, that in the electric power industry, we go to great pains to discourage people from connecting a generator into their house wiring because of the potential safety threat it represents. Our linemen go to work on circuits expecting (but not trusting—more on that in a minute) that the area with the outage has only one source of feed—from the electric grid. Once they detect any other source, all repair work comes to an immediate halt while they troubleshoot it back to that source. (Okay, we sure agree. A bus that connects to the power grid must not be corrupted by a second, uncontrolled power source. But in the rapidly growing number of cases where people want to sell power back into the grid (and make the power meter "spin backwards"), you have to plan to get things disconnected! Obviously, an "inverter" that's trying to put a kilowatt back into the grid at 220 V will not be happy if the grid loses power and turns into 0.01 ? to ground! /rap) That is what I meant by not trusting. Our work rules require testing it dead, sectionalizing, and then grounding prior to starting work. If they follow all those rules religiously, they should stay safe. (We agree. Safety first. Gotta have a high-reliability shorting bar! /rap) If they fail to follow all the rules, and they get hurt or killed by the unexpected source, then the homeowner has tragically caused the accident. For engineered systems where the homeowner is contracting to supply the grid via photovoltaics, wind, etc., we require installation of relays to detect a problem on the grid and automatically open an isolating breaker. (So, these relays and breakers open automatically and then require good planning before a decision to reconnect? /rap) It would be unusual for a generator to stay online during a widespread outage. But it is not impossible, and in almost 30 years, I have seen two or three cases where it did exactly that. The situation has to be just right (or just wrong! /rap), but it can happen. I hope your readers do not attempt to connect their generators to the grid. Maybe you can advise against it in a future column.
Gerry A. Akin
Pease: I guess that's a fair reminder. We need reliable relays, and plans, to disconnect any local power sources per strict rules that didn't exist 100 years ago. You are right. Professional installations have these relays and plans. Thanks for reminding us.

Hi Bob: Streetlights were once connected in series too, with an automatic shunting bar in case a lamp failed open. See the history section at http://en.wikipedia.org/wiki/Street_light. (Thanks for the historical info! It's kind of odd to think of an "anti-fuse" that goes short if overvoltaged! /rap) See the brochure for constant-current transformers (to feed such series strings) at www.specialtytransformers.com/ST%20Acrobat/CCR%20BROCHURE.pdf (That's quite weird! A transformer with moving parts! I guess fact is much stranger than fiction. /rap) and government spec (p. 11) at www.wbdg.org/ccb/DOD/UFGS/UFGS%2026%2056%2019.00%2040.pdf. For more on Christmas lights, see www.planetchristmas.com/Minis.htm, especially the update at the bottom.
Jim Harman
Pease: This does seem to be a complete story about "self-repairing" Christmas lights. It seems as if the "SEMI-conducting" path to the wire has to be pretty reliable or it would short out and steal all the current away from the actual incandescent filament! I guess I gotta go connect up several examples of dead bulbs to a curve tracer to see what kind of non-linear stuff is in there. I wonder what ohms we will find...

Hi Bob: In the Feb. 15 edition, you refer to PPS capacitors as polypropylene. While I share your appreciation for polypropylene as ideal for audio applications, PPS refers to polyphenylene sulfide, a cat of quite a different color. (I had not appreciated the PPS material. I've never worked with it. /rap) PPS capacitors are available in surface-mount packages and are alleged to survive typical reflow processes satisfactorily. PPS as a high-temperature dielectric has a lot of appeal but unfortunately is very expensive and does not self-heal very well. We have at my company decided not to pursue manufacture of capacitors using this dielectric. I have no idea as to "sonic characteristics" of PPS capacitors. In spite of the fact that I play lead guitar in a country rock band with homemade (sand state) gear, I don't think that there is much, if any, difference in sound with various film capacitor dielectrics. (I tend to agree. In any well-designed audio equipment, all films will work pretty well.) I try to avoid discussions on the sonic differences of different capacitor types. Those discussions have often resulted in "flame-o-grams" from so-called "gurus," and I no longer try to add value to related newsgroups. It's definitely a "reader beware" environment on the Internet as far as audio and electronic music equipment is concerned.
Terry Hosking
Pease: It is also a difficult place to get any honest comparisons with any two kinds of audio equipment. Bringing up the concept of A-B-X comparison is just too horrifying for some of these grossly opinionated people.

Comments invited! [email protected] —or: Mail Stop D2597A, National Semiconductor P.O. Box 58090, Santa Clara, CA 95052-8090

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