Electronic Design

Bob's Mailbox

Hi Bob: I have been following your silicon-dioxide (SiO2) articles and just saw Steve Krueger's response ("\\[\\[Bob-s-Mailbox14550|Bob's Mailbox\\]\\],") naming it under the trade name Cabosil. We use Cabosil at our company as a thickening agent for adhesives and epoxies during manufacture. When I had my first exposure to it, I was told it was small glass spheres. (There are some epoxies with air-filled glass spheres, but that's different. Those are usually to yield lower dielectric constant and less capacitance. /rap) I was concerned about the size of the "spheres" (breathing them in) and did some research. I found out it is the thickening agent for ketchup—the reason we had to go to squeeze bottles versus glass bottles (not an excess of tomatoes)? (I have the impression that there is a lot of variance in the natural thickness of tomatoes or ketchup. My wife solves this by simmering until the texture is right, but Del Monte ain't gonna waste time like that! /rap) I was shocked about glass or sand added but still love my ketchup.
Kevin Wagner
Pease: Check! Best regards.

Hi Bob: We are hoping you can shed some light on the issue of capacitor soakage (or dielectric absorption, or "DA" /rap). We have read your publication on the subject entitled "Understanding Capacitor Soakage to Optimize Analog Systems". It was very helpful in understanding DA. We are currently struggling with an integrator that requires a reset to zero. After the reset, the integrator slowly goes in the direction of the previous output. This integrator is used in a closed-loop feedback circuit to create a very low-frequency high pass filter (0.01 Hz). We are using 1 µF and 3.6 MΩ as the current RC values. After studying your article and trying many different dielectrics (polypropylene, PPS, polyester) (First of all, get the "polyester" or mylars out of there. They are at least 10 times inferior. /rap) we have found with all the capacitors we have tested that the internal soakage elements have a substantially longer time constant than what you have shown in your article.
David Muir
Pease: All capacitors—the ones I measured and the ones you measured—have many more time constants. You are quite right. I never intended Figure 4 in my article to cover all of the time constants—just the ones relative to 5-second shorting periods. I shoulda indicated with dashed lines that there are more Rs and Cs out there. On and on...I do not know which time constants will change (nor how much) as you short out the cap for longer times. Bigger Cs? How much? I can't guess. You will have to measure the actual data on your favorite caps. I know that on my favorite polypropylenes, as the time period goes out into days and weeks, if you let the soakage settle, it seems to subside into months. The observed leakage rates (half of which are probably soakage) goes out into years! No kidding!

Hi Bob: I read with interest your "Mailbox" in the Jan. 18 issue, specifically the exchange concerning the metric system. You say "Even in England, many older people don't like using the metric system." I would say this is true even of younger generations here in the U.K. Although we are supposed to be metric, in keeping with the rest of Europe, the reality is that imperial measures persist in many areas of life. Typical examples of such measures would be miles for distances, miles per hour for speed, gallons for petrol, pints for beer, and pounds, ounces, and stones for weight. There have even been instances where market traders have faced prosecution for selling goods in imperial measures (to much public outcry, I might add). Imagine winding up in court for selling two pounds of bananas! (instead of 0.9 kg... /rap) The mind boggles. It seems to me, though, that the approach manufacturers have taken, particularly in relation to foodstuffs, is to sell things in imperial quantities but label them with the appropriate metric measure. So for instance, you can still buy a pint of milk but it will be labeled as 0.45 litres (or whatever the correct conversion is). (Otherwise, you'd have to throw out a lot of pint bottles... /rap) For my own part, I tend to use metric measures in my engineering life (simply because it's easier) and mainly imperial measures for everything else. I went to school at a time when the U.K. was just starting to move from one system to the other, and so I'm somewhat caught between the two stools, so to speak. If you asked me to imagine walking a mile or lifting 10 pounds, I could do so quite easily, but the same would not be true of walking a kilometre or lifting a kilogram. Also, if you were to ask someone over here how far it is to the next town, I think it is almost inconceivable that they would give you an answer in kilometres rather than miles. (If the guy who asked me had French license plates, I could do it... /rap) So, imperial measures are definitely not dead over here. We Brits value our independence. (Bravo! And we too, until Hell freezes over, which could happen any week now. /rap) And if you want further evidence of that fact, you should see how strenuously most of us are opposing the Euro!
Christopher Hill
Pease: Check! Best regards.

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

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