Electronic Design

Bob's Mailbox

Dear Bob: As I'm sure you know, field service work often requires extreme engineering. But sometimes, the quick fix doesn't do the trick. Some years ago, we were in a situation where we needed about 18 V ac from a 220-V source. A junior engineer dug through our bag of parts and came up with two

110-V transformers, a 6.3-V unit, and a 12.6-V unit. "Eureka!" he said. "We can just put the primaries in series and the secondaries in series and everything will work just fine!"

An older head said, "Whoa, what about the current rule? Here we have two different N's, which means two different current ratios, but we're trying to put the same current in both transformers. If you tried to put the 220 V across the two primaries, the 6-V primary might go to 140 V, which is excessive. The 12-V primary would go toward 70 V—not enough. (The old guy is surely right! The 6.3-V output can easily put out 1/3 more, but the 12.6-V transformer will likely put out 1/3 less, and the output voltage will be down a couple of volts. It might seem to function, but the 6-V transformer might overheat due to magnetic distortion—not a great idea. /rap) You may enjoy spending a few minutes figuring out just what does happen in a simple circuit like that."

Robert Dehoney (via e-mail)
Pease: For sure, this is a fun problem. If you put a couple of watts of suitable resistor across the primary of the 6.3-V transformer, and if the load is fairly constant, this might work quite well. My first guess is to put a 10-W, 10-kΩ resistor across the primary. The voltage might even out at 110 V in the primaries, and then you'd get about 18 V on the secondary at I would guess 1 or 2 A, or 18 W. Is this a bad guess? I now guess that a 10-kΩ resistor, dissipating about 1.5 W, would not be enough. You might need 3 kΩ, putting 5 W into a 10-W resistor, to help get the primaries to be equal in voltage for an 18-W (1-A) load.

I just picked up a piece of nylon fish line, about 300 yards of 32-lb test, that was all tangled. I found it in a tree. I eventually got it all untangled, into two pieces, on neat little bobbins. THAT was a good and fun, challenging problem.

Dear Bob: Given your views regarding Spice, what can you say about the selected few computer models that are driving the global-warming discussion?

Jim Paquet (via e-mail)
Pease: I think that most computer models are poor. The data I have seen indicate the world is NOT warming up nearly as much as the computers say—satellite data. So what's wrong? I'm not an expert. I'm just a suspicious old curmudgeon. Just two weeks ago, I read that diesel smoke provides about as much global warming as 1/4 of the carbon-dioxide theory. That means if we cut the carbon-dioxide emissions in half, there would still be an overheating problem due to the diesel smoke. There are no simple answers. Yet nobody knew this until two weeks ago. What do I believe? We should be skeptical. Who has a model of what kind of hell will break loose if the Gulf Stream stops flowing? I'm nervous about that.

Hey RAP: I have been asked to help with a small in-line amplifier project and I was thinking of using the LM384. But we would like to model it in Spice. Is there a way to get the characteristics of the internal devices so that we could make a subcircuit model for our simulation?

Tim Crouse (via e-mail)
Pease: The LM384 is quite old, and nobody knows anybody who can get very interested in making a good model, or any model at all. I mean, the LM384 was in our 1980 Audio-Radio Handbook. How many engineers who designed things in 1980 are still ALIVE? Why don't you just make a tiny model: gain (dc) = 50, bandwidth = 450 kHz, ZOUT = 1/2 Ω? Just run this and let it ride. How can you go wrong? And after you run the Spice work, you have to run a breadboard.

Comments invited!
[email protected] —or:

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

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