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Hi all,
tantalum caps seem to be too expensive for consumer-grade equipment. They contain pricey material - silver and, of course, tantalum, so making them cheaper is impossible. AFAIK they are widely used in military-grade equipment, where the price is not an issue. Their main advantages are a longer life (they do not dry out nor leak) and a bigger temperature range (frost resistance). About using cheap parts in consumer electronics: At least 80 percent of failures of certain types of TV sets were caused by dried-out aluminum caps. The good practice when repairing these sets was: first check all electrolyte caps by adding a good one in parralel. It was successful very often. BR from Ivan |
John Larkin wrote:
I think multilayer ceramics are pushing 100 uF these days. Yes, but alas, only with zero volts across them. Capacitance drops precipitously with DC bias. For a cap with Y5V dielectric, at half the rated DC voltage, there's only 10% of the initial capacitance left. Most manufacturers don't tell you. |
"Jeroen" wrote in message ... John Larkin wrote: I think multilayer ceramics are pushing 100 uF these days. Yes, but alas, only with zero volts across them. Capacitance drops precipitously with DC bias. For a cap with Y5V dielectric, at half the rated DC voltage, there's only 10% of the initial capacitance left. Most manufacturers don't tell you. I didn't think it was quite as bad as that. Also very temperature dependent. These type of ceramics are also pyroelectric as well as being piezoelectric! |
On Thu, 22 Jan 2004 05:05:22 -0800, the renowned Bill Turner
wrote: On 21 Jan 2004 19:09:37 -0800, (Lewin A.R.W. Edwards) wrote: Consumer electronics are costed down to the lowest possible level. If they can use something cheaper, they WILL use something cheaper. A TV set or VCR has had people go over the design hundreds of times with BOMs and catalogs, checking to see if they can shave a penny here or a penny there. _________________________________________________ ________ Well, maybe. Any manufacturer who has been nailed with thousands of dollars in warranty costs caused by saving a penny might disagree with your statement. I've seen it happen. Caused by bad engineering (or purchasing), I would say. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
On Thu, 22 Jan 2004 08:54:58 +0100, Jeroen
wrote: John Larkin wrote: I think multilayer ceramics are pushing 100 uF these days. Yes, but alas, only with zero volts across them. Capacitance drops precipitously with DC bias. For a cap with Y5V dielectric, at half the rated DC voltage, there's only 10% of the initial capacitance left. Most manufacturers don't tell you. Which opens up the possibility of using them as parametric amplifiers or modulators. I have a paper somewhere that uses the nonlinearity of ceramic caps to make a nonlinear transmission line - a shock line - that sharpens the rising edge speed of kilovolt pulses. John |
On Wed, 21 Jan 2004 22:21:31 -0800, "Tim Wescott"
wrote: I wouldn't assume that just because your test equipment comes to you broken is a result of tantalum caps -- perhaps your sample is skewed by buying at hamfests instead of burgling active technology companies? Maybe if you only acquired your home entertainment equipment from dumpsters you'd conclude that aluminum electrolytics are bad? I recently escaped from a company that does aero (but not space) systems. They get mounted on aircraft and are expected to survive being shipped in an unpressurized cargo hold at 50000 feet. At that altitude a wet aluminum electrolytic will dry out, but a tantalum will be fine. There are even wet-slug tantalums for high-altitude applications that will not dry out at these altitudes. Wet-slug tants are expensive (do they still have silver cases?) but don't blow up like the dry ones. The dry slugs coat the sintered tantalum (fuel) with MnO2 (oxidizer). The problems with tantalum are their fragility (we've had exploding caps on our boards, with one manufacturer's part being fine and another being horrid), cost, and the relative scarcity of tantalum. This is really erratic. One spool of tants will be bombs, another can't be made to fail by deliberate abuse. John |
"John Larkin" wrote in message ... On Wed, 21 Jan 2004 22:21:31 -0800, "Tim Wescott" wrote: snip Wet-slug tants are expensive (do they still have silver cases?) but don't blow up like the dry ones. The dry slugs coat the sintered tantalum (fuel) with MnO2 (oxidizer). Boy are they ever. I have some surplus ones, but they have cases that are more of a silver-gray than that nice yellowish-white look you get from silver-plated connectors. These are special parts, but if you want a generally high-performance cap in a (relatively) small package they're hard to beat. I worked for a while on a project to make a power-wire networking device. During testing I accidentally dragged a scope ground across a circuit that was referenced to the 115V power line, thereby exceeding the tantalum cap's voltage rating -- er -- "slightly". Little pieces of flaming capacitors bounced around the lab. After that all of my digital logic (3.3V and lower) coworkers _never_ messed with my bench. The problems with tantalum are their fragility (we've had exploding caps on our boards, with one manufacturer's part being fine and another being horrid), cost, and the relative scarcity of tantalum. This is really erratic. One spool of tants will be bombs, another can't be made to fail by deliberate abuse. Interesting. I'll have to remember that. Thanks. In any case when designing with _any_ electrolytic capacitor it's best to use specify a cap for 20-50% higher voltage than what you think it's ever going to see, particularly because many voltage regulators overshoot on power up and the output cap sees more voltage than you think. John |
"John Larkin" wrote in message ... On Wed, 21 Jan 2004 22:21:31 -0800, "Tim Wescott" wrote: I wouldn't assume that just because your test equipment comes to you broken is a result of tantalum caps -- perhaps your sample is skewed by buying at hamfests instead of burgling active technology companies? Maybe if you only acquired your home entertainment equipment from dumpsters you'd conclude that aluminum electrolytics are bad? I recently escaped from a company that does aero (but not space) systems. They get mounted on aircraft and are expected to survive being shipped in an unpressurized cargo hold at 50000 feet. At that altitude a wet aluminum electrolytic will dry out, but a tantalum will be fine. There are even wet-slug tantalums for high-altitude applications that will not dry out at these altitudes. Wet-slug tants are expensive (do they still have silver cases?) but don't blow up like the dry ones. The dry slugs coat the sintered tantalum (fuel) with MnO2 (oxidizer). snip Silver cased wet slug tantalums DO explode, most contracts that allow the use of wet slugs require the use of tantalum cased parts. |
In article ,
Dr. Anton.T. Squeegee wrote: In article , says... snippety from my personal stuff purchased new. One example is a MGA Mitsubishi rear projection TV that operated flawlessly for nearly 20 years of daily use. Most of my test equipment comes from hamfests and is surplus after becoming obsolete and non-operative in less than 20 years. That leads me to wonder what the real story is behind tantalum capacitors. What do the experts have to say? The ONLY problems I've ever had with tantalums are whe (1) The part was defective from the manufacturer. (2) The voltage rating was consistently exceeded. (3) The thing was installed backwards (reverse polarity). I have no less than five Tektronix O-scopes here, all vintage late-70's to mid-80's. This means not one of them is less than 20 years old. They all use lots of tantalums, and they all work great, but then again Tek was (in those days) proud of what they put out, and was most definitely engineer-driven (which means at least a 20% 'fudge factor' built into everything they made). Tantalum caps are very stable and durable, but they are much more costly than aluminum types. In consumer electronics, the manufacturers will try to shave every penny they can off the cost of the design, often contrary to good common (engineering) sense. Such considerations are (usually) not so critical when it comes to non-consumer stuff. Tektronix was, during that time, strongly discouraging all new designs from using tantalums. IIRC they had been taken to court over a case in which a 465 'scope (the original, not the plastic follow-ons) had spontaneously ignited and had resulted in an expensive fire. Forensics revealed that a tantalum power-supply bypass cap had started the conflagration. The drive to reduce tantalum usage was driven primarily by this liability issue, more than component cost. If you wanted to use a tantalum, you had to justify its usage to the component/design review committees -- which wasn't difficult if you had good reasons and your design was solid. -frank (ex-Tekie) -- |
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