Electronic Design

What's All This Southern Crossing Stuff, Anyhow? (Or: The Law Of Unintended Consequences)

A couple of months ago, the San Francisco Chronicle printed a big headline on page 1: "Feinstein Endorses Southern Crossing." Many years before, arguments had erupted over whether or not the state should invest a few thousand million dollars on a new bridge across the San Francisco Bay.

This bridge would be situated several miles south of the SF-Oakland Bay Bridge, and north of the San Mateo Bridge, perhaps landing in San Francisco near Candlestick Park. Even though this route hasn't been fully engineered or located, it has been called "the Southern Crossing." The debate had died out and nobody had raised the issue for several years.

But then Senator Dianne Feinstein went over and poked a sharp stick at the sleeping lion, which stirred up much howling and caterwauling again. Some Developers nodded that this was a GREAT idea because the existing highways were filling up too fast (even during late evening hours), and new ways were necessary to get taxpayers, shoppers, and employees into and out of the city. Conservationists, on the other hand, bleated that any new roads would become over-crammed as soon as they were opened.

Nobody asked the engineers. But I wrote a letter to the editor of the Chronicle (which was ignored), pointing out that opening up a new road could not only fail to make traffic better—it could actually make traffic worse.

Meanwhile, the citizens down in San Diego were screaming that energy rates had risen horribly. The recent laws had deregulated the cost of electricity in such a way that they expected the electricity prices to go down, but that wasn't happening. How come? The Law of Unintended Consequences seems to apply here: "Be careful what you ask for: You might get it."

Let's build the "circuit" of Figure 1, using rubber bands or springs, and strings. I select just the right amount of weight and suspend it from this simple network of strings and springs. I volunteer to cut a string at L2. Do people think the weight will rise up, or fall down? Of course, everybody says that when I cut the fixed link, the weight will descend. I cut the string—and the weight does what it does.

So, when there are a number of roads, wide and narrow, leading from point W (Work) to point H (Home), as shown in Figure 2, and a new path is opened—Link Road L2—of course the typical commute speeds and times will be improved, right? Yeah, sure... Not a perfect analogy, but a good one is the electrical analogue shown in Figure 3, made of resistors and zeners. If I break the link L2 that connects up the 6-V zener, will the negative output voltage rise or fall? Of course, it must go down. Just run this in Spice. Or build the circuit, and snip the link going to the 6-V zener.

The mistake they made in San Diego was to assume that the laws of Supply and Demand would benefit the consumer in the most generous way. Every time a new technique was found to generate electricity cheaply, the net cost to the consumer declined. Great theory. As long as electricity usage was decreasing, the price of the cheapest source tended to decrease. But the law was written so that was just one possibility of what would happen on the curve of supply versus demand. Every time the demand increased enough to bring in a new high-priced supplier of electricity, the price for ALL suppliers—even those that could make electricity cheaply—went UP, UP, UP. The average wasn't held down. Thus, the law-makers who expected these laws to lower the average price actually achieved the opposite result: the price for all suppliers rises to that proposed by the highest seller. Although that's not what anybody wanted, except maybe the power-generating companies, that's what they got. It will take a while before they can talk their way out of it, too.

Yes, the weight in Figure 1 will go UP when you cut the string. (This assumes that you choose the "right" weight, and correctly pick the lengths of the strings versus the lengths of the stretched springs. This jump UP is most noticeable when there's a maximum tension in link L2 and only a minimum tension in the long strings.)

Also, the commute time in Figure 2 will probably get worse when the new road is opened. (This presumes that the average speed on a narrow road decreases when you try to shove a lot of traffic onto it, as we know is frequently the case.)

Finally, the size of the output voltage will decrease when the 6-V zener is disconnected. Refer to Figure 4. When the current applied is 14 ma, disconnecting Link L2 will cause the output voltage to shrink by several volts, as if there were a negative resistance. Yet, no active elements are present to cause a negative resistance. Analogues are analogous when Figures 3 and 4 are compared to Figures 1 and 2. Are you smart enough to avoid the Law of Unintended Consequences?

All for now. / Comments invited!
RAP / Robert A. Pease / Engineer
[email protected]—or:

Mail Stop D2597A
National Semiconductor
P.O. Box 58090
Santa Clara, CA 95052-8090

Bob's Mailbox
Dear Bob:
Do you know of any stash of MM5369/8 chips that may be around? I am looking for however many I can get for a couple of one-off projects. (These were really neat chips.) I have not used them in years and only recently foundout that they have been out of production since '97.
Jim Prince
via e-mail

I called up Halted Specialties, which is one of the best surplus stores for miles. They had none. I think you better use something else! Myself, I never used one or held one in my hand.—RAP

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