Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!rutgers!ames!ucbcad!ucbvax!cbosgd!cwruecmp!neoucom!wtm From: wtm@neoucom.UUCP (Bill Mayhew) Newsgroups: sci.electronics Subject: Re: Appropriate use of capacitors Message-ID: <645@neoucom.UUCP> Date: Sat, 8-Aug-87 13:03:23 EDT Article-I.D.: neoucom.645 Posted: Sat Aug 8 13:03:23 1987 Date-Received: Sun, 9-Aug-87 13:22:25 EDT References: <1108@rtech.UUCP> Organization: Northeastern Ohio Universities College of Medicine Lines: 99 Keywords: capacitor Summary: Three types of capacitors I like to use In the capacitor wars, I've got to agree that electolyic capacitors are terrible. Perhaps, it would be a good idea to start a Jihad against them. As mentioned, electrolytic units are appropriate for brute force bulk regulation of power supplies. In power supplies, the continuous application of DC keeps the liquid dielctric material formed and acting as an insulator. One point to keep in mind is that capacitors integate current. When a large brute force filter capacitor is present in a supply, it will draw a very large current while it attempts to nearly instantaneously reach operating voltage. This property can result in blown out fuses and/or rectifier diodes if percautions are not used. A number of methods are possible to slow down the initial charging. Often, a current limiting resistor is placed between the rectifier and the filter capacitor. A modest timing circiut can be used to close a relay contact across the resistor. One other thing is electrolyitic units is that there is finite leakage current. Sometimes, the leakage current is considerable. I once saw a guy burn up a plate supply transformer in a tube preamp by using a humungous filter cap bank. It turned out that the filter bank drew more leakage current than the preamp itself drew to operate. I try to avoid electrolytic capacitors in audio circuits. I'm amazed at how many times I've seen electrolytics used to directly couple the audio path in Compact Disk players. They'll almost certainly fail after a few years because there isn't a DC bias (usually) of sufficient magnitude to maintain the integrity of the dielectric film. Perhaps this is Japan getting even for WW II by building in planned failure into their products? Note that smaller "working voltage" values are useful if you insist in using an electrolytic cap in an audio circuit. A 5 volt rated capacitor is more likely to stay working than a 25 volt rated electrolyic capacitor in a circuit that has a 3 volt DC bias. It's difficult to believe that with all the fancy electronics, optics and mechanics in CD players that engineers can't concoct a stable direct coupled audio output circuit. For general non critical applications, I use stacked mylar (tm) capacitors. These are OK for most audio applications such as for bypassing the Miller-effect feedback resistor in the emitter of a transistor circuit. Values are commonly available in the nF to 1 to 2 uF range. Mylar has a quite modest dielectric absorption, and is value stable at consumer temperature ranges. A product example is Panasonic V-series. They are also pretty cost effective. On drawback is that tolerances are about +/- 5 to 20%. For critical needs such as sample & hold capacitors and highly stable audio oscillators, I use polystyrene capacitors. Polystyrene is very temperature stable and has very low dielectric absorption. Mallory offers two lines of polystyrene units. One line has a leakage R of 100-1000 megohms. The other series is up over 1000 megohms. The disadvantage of polystyrene is that it results in physically large capacitors. It is also modestly expensive. Values range from pF to nF typically. Tolerances are available as low as +/- 1%. Polystyrene degrades in dielectric performance at RF frequencies. For RF, mica capacitors are quite good. Silver/mica capacitors are quite expensive relative to ceramic types. Mica offers very good temperature stabilty and quite good dielectric performance. Mica caps are moderate in size. C vales are .5 to 1000 pF approximately. Tolerances are available to +/- 1% or better. A typical vendor for mica units is Cornell-Dublier Electronics (CDE). Polycarbonate capacitors are better than polystyrene in performance of the dielectric. Polycarbonate units are also available from several sources with very tight tolerence values. Unfortunately, polycarbs often cost several DOLLARS each (ouch!). They're mostly used for applications where high untrimmed accuracy and wide, wide temperature range stability is needed, for example space based radios. Most electronics industrical suppliers such as Newark or Pioneer should be able to burry you in a ton of capacitor data sheets (well, almost) if you phone-up and request some. Much of the discussion present in magazines such as "Audio Amateur" seems more opinion than fact-- especially in re capacitors. I'll admit some opinion that I'm a relatively demanding listener; I really can't acknowledge any real noticable sonic differences in mylar, polystyrene and similar type capacitors when used appropriately in audio coupling circuits. Electrolyic units do certainly add noticable coloration to sound, but then electrolytic capacitors are readily measurably inferior in performace on very inexpensive test gear (such as a $14.95 pocket multimeter from Radio Shark). I get a laugh when I see electrolytic capacitors used in coupling applications in CD players. In an apparent move of appeasement, they often have a red plastic sleeve (as opposed to the more common blue or orange) that is emblazoned "AUDIO GRADE". I wonder just who the manufacturers are trying to impress, since the same CD players bear labels on the outside of the cabinet saying something to the effect of "dont open-- danger of hideous government punishment.." or some similar rot. Seemingly, "AUDIO GRADE" == "inferior to industrial grade". --Bill