Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!ut-sally!im4u!rutgers!ames!ucbcad!ucbvax!zion.berkeley.edu!max From: max@zion.berkeley.edu (Max Hauser) Newsgroups: sci.electronics Subject: Re: Appropriate use of capacitors Message-ID: <19984@ucbvax.BERKELEY.EDU> Date: Thu, 6-Aug-87 00:24:22 EDT Article-I.D.: ucbvax.19984 Posted: Thu Aug 6 00:24:22 1987 Date-Received: Sat, 8-Aug-87 08:38:33 EDT References: <1108@rtech.UUCP> Sender: usenet@ucbvax.BERKELEY.EDU Reply-To: max@eros.berkeley.edu (Max Hauser) Organization: U.C. Berkeley Lines: 125 Keywords: capacitor Summary: Signal-path caps and !@#$%& electrolytics In article <1108@rtech.UUCP> jchan@rtech.UUCP (Jeff Chan) writes: > >Here's a question for you old farts, audiophiles, and EEs: > >What are the appropriate uses and benefits/disadvantages of all the >various types of capacitors? For example, electrolytics are often >used in power supplies. I suspect that polycarbonates are thermally >quite stable... Back before I got involved with microelectronics, I had some design jobs involving critical capacitors (all the capacitors I deal with now are flat, very small, and "custom"!). Rather than give opinions about all the myriad dielectric types available in discrete capacitors, I'll mention those with salient good features that stood out. The trade mags like Electronic Design often show tables of this data in feature articles, every year or two. My comments are relevant to high-accuracy applications like signal paths and instrumentation (I will leave power-supply filtering to someone -- evidently everyone -- else). Electrical properties important to these applications include linearity, stability and low dielectric relaxation. The last of these is especially important in pulsed applications like sample/hold circuits, and also the hardest to get hard facts on; you pretty much have to measure. An example of the effect is for an abruptly discharged capacitor to exhibit some "memory" of its charging voltage after the discharge, due to residual polarization of the dielectric. In many dielectrics the magnitude can be 0.01% or more, which is bad news in some applications. I regret that I don't have any DR figures handy. 1. Polycarbonate (Lexan (tm)) caps have some near-ideal properties, especially their phenomenal temperature stability, which can be as good as 0.1% over the military temperature range. They are also very expensive (dollars apiece in quantity, instead of cents). While that fact alone would no doubt commend them to some audiophiles ;-), audio is not one of the applications that needs their principal attribute of temperature stability. Instruments like precision oscillators exploit that property. Also, I believe TFE (Teflon (tm)) caps share similar properties with polycarbonate, but are even more expensive, thus making them even more suitable for one-upmanship. Only kidding, of course. 2. Polystyrene caps have temperature stability approaching that of polycarbonate, but are manufactured to a coarser initial tolerance and are MUCH less expensive (order of few cents in quantity 100). They are useful for applications where stability may be important but initial value is not (a trimmed function generator, for example). They are also clean in other respects. A drawback is physical size; they are typically more bulbous than, say, Mylar (tm) caps of the same value and voltage, though the Mylar units are much less stable. Mylar caps are more common by far than polystyrene and denser, as I've said, but no cheaper, and their second-order dielectric properties as I recall are largely inferior to polystyrene. 3. Ceramic caps, especially the monolithic versions developed for digital-IC decoupling service, are relatively dense in the large values (0.1 uF and larger), and their high-frequency response is superior to those with plastic dielectrics (polywhatever), which typically contain a spooled sandwich structure with larger series inductance. The ceramic caps also have poorly controlled value, usually rated GMV (guaranteed-minimum) or -20, +80% and often, in my experience, show enough leakage conductance to be a problem in high-impedance instrumentation circuits, and their temperature stability is among the worst. They are therefore most useful in wideband and power-supply decoupling applications. The foregoing dielectric types are about all you need for low-value (1 uF and smaller) applications. A rub comes if you need something larger, where these types all get bulky, in signal-path applications. A large divide exists between electrolytic and non-electrolytic types, so much so that it is unfortunate they are both called capacitors; this causes lots of confusion among the inexperienced, who don't realize the fundamental difference. Electrolytics work of course by forming a (temporary) insulating layer between a metal surface and an electrolytic gel or liquid, in response to an applied DC voltage. The ONLY reason they are used is to gain the extremely high capacitance density possible from the very thin dimension of the electrochemically- formed insulating layer. They are inherently polar devices; must have a steady DC bias (and are, simply from that, nonlinear); and show, in most models, limited lifetime. Because of these three drawbacks they must be used with great care and awareness, and they are very successful in certain applications, like the low-frequency filter capacitors in DC power supplies. ************* Anti-electrolytic flame follows **************** Unfortunately, designers have been sticking electrolytics in everywhere they needed a large-value capacitor. The result? Audio distortion, for one thing (as already noted here and in rec.audio); but also, in-circuit failures where the electrolytics experience no or inadequate DC bias (they need a "forming" voltage equal to a fraction of the "working" rating) or simply from their limited lifetime. I have been repairing audio equipment since about 1970 and I cannot count the times I have seen "right channel is out, or nearly out, and (maybe) comes back with loud music." (If the right channel is completely out, it may well be due to an electrolytic interstage coupling capacitor that failed; if the volume is way down, quite possibly it's an emitter- or source- or opamp-resistor bypass capacitor, also electrolytic.) Often these capacitors fail because they are rated for 25 volts and are operated with a two-volt DC bias. Anyway, it's worth avoiding electrolytics of all kinds in analog signal paths and relegating them to power supplies where they are fat and happy facing DC and some 60-hertz ripple. It's just hard to do this if you need 100 or 1000 microfarads in your signal path. (One place you generally *don't* need capacitors, at least fundamentally, is in series with the speaker at an audio output stage. With proper design it's not hard for modern amp circuits to maintain stable zero-DC output voltages if the power supply is roughly symmetric.) If you can figure out a way to pack, say, 100 uF at 50 volts into a cubic centimeter or so, using cheap, dry, stable, voltage-independent materials, the world will beat a path to your door. Unfortunately, with readily manufactured insulators, physics is against you. Max Hauser, UC Berkeley EECS Department, IC Design Group UUCP: ...{!decvax}!ucbvax!eros!max Internet (old style): max%eros@berkeley Internet (domain style): max@eros.berkeley.edu (PS: for those who asked, I will post answers to the old-fart quiz soon.)