++++++++++++++++++ Reply-To: From: "Sverre Holm - LA3ZA" To: "'Bob Tellefsen'" , "'Timothy A. Raymer'" , Subject: RE: [Elecraft] Capacitors...simple yet complex question Date: Tue, 24 Dec 2002 17:25:09 +0100 One difference is due to the max temperature, something like 85 C or 105 C. Another one is the ESR - Equivalent Series Resistance - the smaller the better, but also often larger and more expensive. Sverre LA3ZA www.qsl.net/la3za > -----Original Message----- > From: elecraft-admin at mailman.qth.net > [mailto:elecraft-admin at mailman.qth.net] On Behalf Of Bob Tellefsen > Sent: 24. desember 2002 00:30 > To: Timothy A. Raymer; elecraft at mailman.qth.net > Subject: RE: [Elecraft] Capacitors...simple yet complex question > > > Tim > I believe the difference in size is due to voltage ratings. > Small caps usually have a low voltage rating. High voltage > caps are much larger. 73, Bob N6WG > > -----Original Message----- > From: elecraft-admin at mailman.qth.net > [mailto:elecraft-admin at mailman.qth.net]On > Behalf Of Timothy A. Raymer > Sent: Monday, December 23, 2002 12:45 PM > To: elecraft at mailman.qth.net > Subject: [Elecraft] Capacitors...simple yet complex question > > > Kind Listees, > > I have a question about capacitors. When working on two > power supplies > recently, I came across two types of capacitors. One type > are the regular > axial or radial Electrotic Caps that are, for example, 4700uF > Capacitors. Then, there are the Computer grade monster cans > that are in > Astron supplies. Same capacitance value, but always much larger in > physical size. I know there are differences between the two > in how they > are constructed, but my question is do the differences make > one or the > other better? > > Thanks. > > Tim Raymer > Timothy A. Raymer > Missouri Department of Health > and Senior Services ++++++++++++++++++++ Date: Tue, 24 Dec 2002 10:50:40 -0600 To: From: "Timothy A. Raymer" Subject: RE: [Elecraft] Capacitors...simple yet complex question What I was wondering about was differences in types of Electrolytic caps. I know within a type, Caps will vary with voltage rating. But I was wondering specifically about the differences in construction between the Capacitors such as the Panasonic TSW Caps that we all see in stuff, and the Mallory or Panasonic AA "Computer Grade" Capacitors. A 6800uF TSW is 25mm x 40mm, while the 6800uF AA it 35mm x 79mm. I guess the question I wanted answered by one of you technical types is: "Can the electrons tell the difference?" Or, "Do the bigger ones last longer?" Thanks. Tim Raymer 73 de KA0OUV At 15:29 12/23/2002 -0800, Bob Tellefsen wrote: >Tim >I believe the difference in size is due to voltage ratings. ++++++++++++++++++++ From: "Lyle Johnson" To: "Timothy A. Raymer" , Subject: RE: [Elecraft] Capacitors...simple yet complex question Date: Tue, 24 Dec 2002 09:46:44 -0800 Hello Tim! If you look up capacitors in the Handbook, or any basic reference, you'll see that the capacitance is related to the area of the "plates", the dielectric constant of the insulator between them, and the thickness of the dielectric. What we're generally seeing is improving technology providing higher dielectric constants and thinner insulation. This leads to smaller parts with essentially the same ratings as earlier, larger parts. But often the chemistry that makes this work has other issues. For example, early tantalum caps could fail spectacularly. And many times the material inside "electrolytic" caps had to be kept wet. As it dried out, the part would lose capacitance. (For example, in man of the electrolytic caps in AO-40, we sealed the rubber seal on the cap with a drop of high-vacuum-rated silicone to prevent drying out the electrolyte in a hard vacuum.) Temperature ratings play a big part here, and parts rated to work at higher temps often are better sealed, or the electrolyte has a higher "boiling point." Often, really small parts have limited lifetime ratings. Look through your digi-key catalog (or the on-line PDF version) and note the differences in sizes and price and temps and lifetime specs for the various capacitors. As pointed out in an earlier post, there are other factors to consider, such as "equivalent series resistance" (ESR) which plays an important role in switching supplies or high current supplies. Leakage current is another factor that may play a part in a particular circuit. It turns out for PLL loop filters you often have to use a polystyrene or polyester cap rather than a ceramic cap to keep noise down. Finally, like any real world part, a capacitor is also a set of resistors (each lead is a series resistor, and there is a parallel resistor - leakage - across the plates), a pair of inductors (each lead, even in a tiny surface mount part) as well as the capacitor itself. Sometimes you can use these parasitic elements to your advantage; usually they sneak up behind you and catch you when you aren't looking (think of Golem in the Two Towers). 73, Lyle KK7P +++++++++++++++++