Electronic Design

Bob's Mailbox

Bob: In “What’s All This Capacitor Leakage Stuff, Anyhow?” (March 29, 2007, ED Online 15116) you have a diagram of a test circuit using the LMC662. This circuit is quite a bit different from the “capacitor soakage” test circuit you referenced on your Web site. (After you get the capacitor charged up, and after you get the soakage elements charged up (which takes hours and days), the rate of change of VOUT is caused by the leakage, which you cannot really see until you have waited some days. Any rate of change before that is mostly caused by soakage. So in this experiment, I was trying to separate the soakage from the leakage. After a few days, this did work right. For a soakage test, try charging up high first and then pulling it to ground. The leakage is not involved. /rap) For the leakage circuit, I was curious what the “ball hook” and the coax were for. What’s that all about? (The ball hook lets me connect to several different capacitors in sequence. It is insulated with cheap nylon. That isn’t a very good low-leakage insulator, but if I keep my body biased up to 8.8 V, it is not too bad. /rap) Why is the coax 12 in. long? (I’m guessing it could be shorter but not a lot longer.) I figured the ball hook is the same as what I call a “chicken stick.” We hams use them when working on high-voltage circuits to make sure all the caps are for sure discharged by grounding the chicken stick with a clamp to chassis, then running the “ball-hook end” over all of the high-voltage components. (Yeah, but when you use the chicken stick, do you put it in series with 1 O? Or 0.1 O? How often does the 1-O resistor have to be replaced because you blew it out? I am not working at low-ohms levels, but at 10 to 100 MMO. Different range, eh? By 13+ orders of magnitude. I hope that you have learned that even your chicken stick can not discharge a big highvoltage oil capacitor properly because it can recharge itself up to hundreds of volts due to soakage if you wait a while. A big highvoltage cap needs a resistor to discharge it for a long time to make it stay at low voltage, not just for a second. Over near Livermore, some movers were moving some big high-voltage capacitors. One crate got busted, the shorting bar got knocked off the capacitor, and a moving guy bumped into it and got killed—electrocuted— even though the capacitor must have been shorted out for hours! Dangerous things. /rap) Here, you’re probably using it to prevent distorting the experiment through human-body discharge. (Yeah, it is an adequate insulator. It seems to have less than 2 pF of capacitance and 1 lousy MMO. If I had a Teflon ball hook, that would be nice. But the ordinary nylon one is okay. I don’t need perfection. I can get good measurements if it is just good enough for a short time. /rap) Just curious about the specifics with the coax. (Even though I could not see it, I remember that the coax was to prevent leakage into the + input. It had to be at least 12 in. long to reach all the capacitors. If I had a bigger array of capacitors, I could have used 2 or 3 ft of coax to reach them. No big deal. /rap) This looks like a fun little project. Have you done any more work on it since?

Jason A. Dugas
Pease: No, but I will next week.

Bob: A year ago you had an article on the Sallen-Key filter (Sept. 28, 2006, ED Online 13480). In that article, you mentioned that using an op amp with higher input impedance will allow the use of larger values of R1 and R2 and smaller, cheaper, and higher-quality capacitors. What is the tradeoff of staying with smaller R1 and R2 values to reduce Johnson noise and having to use X7R caps instead of NPO caps? To use all NPO caps in my four-pole, two-opamp design, I must cross over the 1-MO level for some R values.

Bruce Allen
Pease: What BW are you trying to handle? Low pass or high pass? If you used 0.01-µF NPOs and 1 MO, you would be talking 16 Hz. A 1-MO resistor doesn’t have much noise in a 16-Hz bandwidth. It would have (125 nV x4) rms or 3 µV p-p. Most op amps have more noise than that. These days, you can buy NPOs bigger than 0.02 µF. Look in Digikey. You ought to build it and try it both ways. Avoid X7Rs, which have long soakage tails.

Hi Bob: Nikola Tesla was an amazing man. He is now credited as the inventor of the induction motor, wireless transmission, Tesla coil, logic gates, Tesla turbines, lighting ioning gases, etc. (No argument. His induction motors and several other items were quite brilliant. /rap) I’m most interested in his claims of wireless transmission of power. If anyone could have pulled this off, he was capable of doing it. He invested a lot of his time and money in this endeavor, only to fall through by being under-budgeted. (I think he was over-ambitious with schemes that could not possibly work. Some of his schemes were loony, and even Tesla couldn’t make them work. /rap) I think we are just coming to grips with what he knew. He knew the Earth’s capacitance and telluric currents, resonance, distance of the ionosphere, the difference between transverse and longitudinal waves, etc. What is your opinion on this? In general, do you think it’s possible? Do you have any technical insights?

• Tony A. Wittic
Pease: I am not 1/20 as smart as he was when he had his head screwed on right, because I am not an electric field expert. I’m just dumb. That’s okay. When he had nutball ideas, which he often did later in life, he may have been dumber than me.

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