Electronic Design

Bob's Mailbox

DEAR EDITOR:
At first I thought I picked up an April Fool issue, but no, you have Al Gore in the Tech Year in Review section honoring him for imploring engineers to turn green (Dec. 1, 2007, p. 45; ED Online 17958). What crap. Please cancel my subscription.
–"RAY"

Hello, Ray: There are several reasons to “turn green,” and Al Gore’s bleating about warming theories is only one of those reasons. Energy is expensive. Or haven’t you noticed?

Wasting energy is expensive. If you have an inefficient oven or refrigerator or hi-fi in your house, you may have to run your air conditioning harder and longer to get the waste heat out. That is expensive. And that applies to server installations and computer farms. And 100-W bulbs, too. So saving energy is going to continue to be important. (I don’t even own an air conditioner. I do have a lot of 100-W incandescent bulbs, as warming up my house in San Francisco is almost always a good idea. Especially in the summer when it’s cold outside…)

My boss is going to ask me to work on low-power and higher-efficiency designs. I better not plan to ignore him. My customers are going to ask me for lower-power circuits and systems. I can’t tell them to bug off. They are my real bosses, as they pay my salary. My competitors are gonna beat me up if they can brag about lower-power products. I certainly don’t want to get beat up by them.

I have always been in favor of making low-power products. Yes, I will sell you my LM137K regulator that will regulate 1.5 A, but I’d sooner sell you an LM337MP that puts out just 0.5 A, if that will make your system happy. I also designed an LM337LZ on an even smaller chip, for 100 mA, and an LP317LZ that ran on 2 mA quiescent, rather than 10 or 5 mA.

So you can say that, but at your peril. Go ahead. Tell your boss you don’t want to work on “green” products and systems. I’m not gonna argue that you have to like Al Gore, but he is telling some of the truth. It ain’t just politics.

And what is the opposite of “green”? Do you like to pay George Bush’s friends $3.59 per gallon? I don’t know anybody who wants to do that while filling up their 38-gallon tank. But hey, you vote with your pocketbook, I’ll vote with mine. /rap

HI BOB,
Years ago, you published a “mystery” circuit that consisted of just a transistor and a resistor, I believe. You applied a positive voltage, and it produced a small negative one. If I remember right, you explained (in the next issue) that one junction was acting as an LED and the other as a photodiode. I’ve lost the issue. Could you give me the details again?
–TERRY PERDUE

Hi, Terry: Take any NPN silicon transistor—metal can (2N2222) or plastic (2N3904) or even monolithic (LM114). Ground the base. Connect the emitter through 1k to +12 V so it will zener. (This may damage or degrade the transistor, so you should throw it away when you are done.) What is the V at the collector? A high-Z DVM will read –0.3 V. /rap

BOB,
I’ve tried using LEDs as light sources for microscopy, but the output was always too low. Now with the newer devices, I can get all the light I need from either the color or white sources with virtually no heat as compared to older incandescent sources. (Yeah, you can get a lot of light now. But if you used the right kind of mirrors or optics, even old incandescent bulbs would reflect the heat away... and put the light where you want it. /rap)

When using a low-voltage dc source, I just use a currentlimiting resistor. But if I need to power directly from the 120-V ac line, I have been using a line-voltage-rated capacitor whose impedance limits the current. I don’t seem to have any inrush current problems, and I usually put a small rectifier diode in reverse polarity parallel with the LED to avoid reverse breakdown. (If you put the LED in a bridge of four diodes, it can run on the current both coming and going through the cap. /rap)

Sometimes I just use another LED in reverse parallel. I have been doing the same for LED indicator lights in ac circuits as the power consumption is very low (mostly VARs), and there is little heat generated. I have not seen this concept used and wonder if I’m missing something or they are missing something.
–OWEN MULKEY

Hello, Owen:
If everybody did this, the power company would be very annoyed! But if just a few of us do it, no harm. It’s a good idea to put ~1k in series with the cap to limit in-rush transients… and I usually put two line-rated caps in series, so if one fails shorted, you just get a little more brightness—and don’t fry the 1k. /rap
BOB PEASE

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

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