Ken <tylereng@pacbell.net> wrote: > Yesterday in the components group I saw a request >for a 0.1 pf capcitor. Now I have been a hobbiest for quite >some time now but that value seems almost incomprehensible >to me. I've read many times that lead length alone on capacitors >can greatly impact the true value of a capacitor used in a circuit. >Especialy at lower values. > >Now for the questions: > >1.) Is there really a practicle use for a capacitor of that value ? Yup. At very high frequencies and/or voltages, such a capacitor would carry a good deal of current. >2.) Would it be possible to make your own by simply providing >two conductive plates/pins/wires with a little space between them >and use the air as a dialectric ? Yes, and this is generally how it's done. You'll often see a small capacitor made by simply placing two copper traces on a printed circuit board close together. These are sometimes adjusted by scraping away some of the copper trace. >3.) Does the dialectric value of air change with enviromental variations ? Not very much. Air is pretty close to vacuum as far as dielectric constant goes. Note that in the printed-circuit-board capacitor, the dielectric isn't air, but both air and plastic, or whatever it is that the circuit board is made out of. >4.) Does wire itself have a certain amount of capacitance in a circuit ? It sure does. And you can't get rid of it, either--at least not very easily. The capacitance values are low--like your 0.1pf--but these can be very significant at high frequencies. And if you think you're not going to be dealing with high frequencies because your circuit is meant, say, for audio frequencies, you may be in for a surprise: the capacitance of the wires may, under the proper circumstances, provide nice feedback pathways for the signal. When the output of an amplifier is thus connected back to the input, you get a high-frequency oscillator whether you wanted one or not. It's enough to drive you nuts and can be very difficult to get rid of. One method that's used is to apply other capacitors that will re-route any high-frequency signals. I suspect that this is the intended purpose of the capacitor you saw requested. The method is called "neutralization." >5.) Instead of the above use of air as a dialectric could you not use >a piece of wire extended above the circuit, looped once, give a close >approximation of a 0.1 pf capacitor ? Yup. Back before there were printed circuits, this was, as others have mentioned, called a "gimmick." You'll find them in a lot of places. If you're interested in the art of tuning a high-frequency circuit, have a look inside the tuner of an FM radio or a UHF television set. You'll see all kinds of unlikely components: one-half-turn coils (with center taps, yet,) strange ground plane designs which cut down (but don't eliminate) the capacitance between components, and other arcane tricks known only to rf guys. >6.) Does equipment exisit to accuratly measure a 0.1 pf capacitor ? Yup. You could do it yourself if you had a high-frequency oscillator and an impedance bridge circuit. We also worry a lot about small capacitances in high voltage work. The frequencies are often low, but even a very small capacitance that's charged to a million volts and then to minus one million volts every 1/120 second is pretty significant. In fact, the capacitance of electric power lines to ground and each other is always taken into account in electric power work. That +/- 1MV figure isn't an exaggeration. That's about the peak voltage you'll see on the 765kV lines that are powering your computer right now. M Kinsler still trying to figure out what the hell ThinkQuest was supposed to have been... -- ............................................................................ Interpretation and instruction of physical science and technology Athens, Ohio, USA. "http://www.frognet.net/~kinsler"
Date: 28 Sep 1998 04:48:22 GMT
Original Subject: Re: A Question of Capcitance