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#2
January 22nd 04, 06:37 PM
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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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#3
January 22nd 04, 09:32 PM
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In article ,
says... Hi, Frank, 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. Wow! I didn't know this... Thanks for the neat bit of history. I will add that most of the tantalums I'm finding in my gear are localized filters for the power-input traces in 7000-series O-scope plug-ins. Everything still works great, though. ;-) -- Dr. Anton Squeegee, Director, Dutch Surrealist Plumbing Institute (Known to some as Bruce Lane, KC7GR) kyrrin a/t bluefeathertech d-o=t c&o&m Motorola Radio Programming & Service Available - http://www.bluefeathertech.com/rf.html "Quando Omni Flunkus Moritati" (Red Green) |
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#4
January 22nd 04, 09:49 PM
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Frank Miles wrote:
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) I was there at the time, too. Tantalums were essentially verboten unless the source impedance supplying the tantalum cap was at least 3 ohms/volt. That's because it was found that the short circuit failure mode was aggravated by high inrush current, so the source current had to be limited. One of the chief reasons we had been using tantalums in the first place is that they have very good bypass characteristics up to quite high frequencies -- so a single capacitor could handle a very wide range. When the source impedance was high, the capacitor didn't need to be so good in the first place, and of course adding a physical resistor in series with a supply bypass pretty much defeats the whole purpose. Consequently, the 3 ohms/volt rule pretty much eliminated tantalums as a viable choice for most applications. Fortunately, it was at just about the same time that very big improvements were made in aluminum capacitor technology. As the aluminums shrunk in size, they became much better at bypassing higher frequencies. So they took over from tantalums pretty rapidly. There was a glitch for a while, though -- boards were being cleaned with Freon at the time, and it was discovered that Freon could migrate past the seals on some or most aluminum capacitors and corrode the aluminum, leading to poor reliability. The solution adoped by some manufacturers was to add a rubber seal at the lead end of the capacitor. That increased the length of the leads between the outside of the capacitor and the inner body, increasing the lead inductance and decreasing the capacitor's high frequency bypass capability. . . but that's just another example of the day-to-day problems an engineer faces and has to overcome. Incidentally, I got a Tek 1502 TDR on eBay not long ago. It had a shorted tanalum power supply bypass capacitor. A couple of other anecdotes -- A time base plugin I designed had gotten through the entire extensive pre-production test phases, accelerated life tests, etc., and was in pilot production. I walked past the production line technician's bench every day, and began noticing several tantalum capacitors of the same type in the replaced-component box. They had come from a sweep circuit I had essentially copied from an instrument which had been in production for some time. Puzzled, I analyzed the circuit carefully, and discovered that at an extreme setting of one control, the tantalum cap could have a very small reverse voltage applied. I modified the circuit to eliminate the possibility of any reverse voltage of any level, and the capacitors quit failing. Servicing data from the instrument I had copied the circuit from showed noticeably reduced reliability of the capacitor, also. The lesson learned is that tantalums won't tolerate _any_ reverse voltage. If they don't fail immediately, a disproportionate number will fail eventually. The other anecdote involves a QRP rig. As a crude reverse-voltage protection, I had reverse-connected a 3-watt diode (actually, a 36 volt zener I had lots of) across the power supply terminals. My battery supply normally had an-line fuse which would blow. Just before Field Day one year, the fuse holder broke and I didn't have a spare in the junk box. I'd never blown a fuse in 20 years of Field Days, so went without. The battery was a 12 volt, 5 Ah sealed lead acid unit, capable of a few hundred amps if shorted. As I'm sure you've guessed, that was to be The Year of the Reverse Connected Supply. The wires to the battery immediately melted out of their insulation, burning some holes in the tent floor. I managed to disconnect the battery without getting burned and before a real fire started, and checked the damage. The rig's (recently installed) power switch was fortunately off, so the innards didn't get any reverse voltage. The diode had gotten so hot that the plastic case had fractured and probably burned -- it was gone. The diode's solder joints had melted, and the two separated diode leads were dangling. But there was still a dead short across the terminals -- a small 6.8 uF dipped tantalum capacitor was also across the terminals, and it had become such a good short that it hadn't gotten hot enough to explode. (The power supply wires were something like #24 or #26, so they'd limited the current.) My guess is that it went short just as soon as the diode opened, and made a better quality short than the diode had. The fuse is now back in place (along with new diode and capacitor), so of course I haven't reverse connected the supply since. Roy Lewallen, W7EL |
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#5
January 22nd 04, 10:22 PM
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"Roy Lewallen" wrote in message
... analyzed the circuit carefully, and discovered that at an extreme setting of one control, the tantalum cap could have a very small reverse voltage applied. I modified the circuit to eliminate the possibility of any reverse voltage of any level, and the capacitors quit failing. How much voltage? We talking tens, hundreds, or thousands of mV? |
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#6
January 23rd 04, 12:08 AM
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Walter Harley wrote:
"Roy Lewallen" wrote in message ... analyzed the circuit carefully, and discovered that at an extreme setting of one control, the tantalum cap could have a very small reverse voltage applied. I modified the circuit to eliminate the possibility of any reverse voltage of any level, and the capacitors quit failing. How much voltage? We talking tens, hundreds, or thousands of mV? As I recall, it was a couple of tenths of a volt. The capacitor was probably a 12 or 25 volt unit. A quick scan of the web shows that some manufacturers claim their tantalum capacitors will withstand something like 10% of rated voltage (not to exceed 1 volt) at 25 degrees C, and 3 - 5% of rated voltage (not to exceed 0.5 volt) at 85 degrees C, with a time limit on application of reverse voltage. Because of my experience, though, I'd consider it to be bad design practice to allow any reverse voltage at all until I saw some reliability figures for capacitors used under those conditions. The problem is that it doesn't cause immediate failure, or even assured failure -- it just increases the probability of failure. Some applications can tolerate the increased failure rate, and some can't. At Tek, a great deal of importance was placed on reliability. Roy Lewallen, W7EL |
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#7
March 20th 04, 02:47 AM
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Frank Miles wrote:
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. Ah ha! I have a 465 (w/DM44) that I purchased in 1978 for personal use (no commercial abuse). Shortly after the warranty expired, it would not power up. I traced the problem to a shorted tantalum filter cap on the +15 volt line. But of course, it wasn't in the power supply, but rather on one of the boards. Pain to get to, IIRC. BTW, I did not get option 5 (TV sync separator). I wonder if it is feasable to install it myself? Documentation is listed as 465 option 5 supplement 070-2191-00. Anyone have this info? -- Jerry wa2rkn no email @ present |
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#8
March 21st 04, 10:41 PM
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Jerry Koniecki wrote:
Frank Miles wrote: 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. Ah ha! I have a 465 (w/DM44) that I purchased in 1978 for personal use (no commercial abuse). Shortly after the warranty expired, it would not power up. I traced the problem to a shorted tantalum filter cap on the +15 volt line. But of course, it wasn't in the power supply, but rather on one of the boards. Pain to get to, IIRC. I can imagine. Tantalum caps are a real pain in the ass. If you reverse the polarity, they burn right through the PCB after a few months! We got a board in for repair last week with that problem. Luckily, everything on the board (yes, even the stickers with the serial numbers) is self extinguising. -- Reply to nico@nctdevpuntnl (punt=.) Bedrijven en winkels vindt U op www.adresboekje.nl |
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#9
March 22nd 04, 08:30 AM
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Jerry Koniecki wrote in message ...
Frank Miles wrote: ..snippety... BTW, I did not get option 5 (TV sync separator). I wonder if it is feasable to install it myself? Documentation is listed as 465 option 5 supplement 070-2191-00. Anyone have this info? Yes, you can install it youself. The OPt.5 is a small extra board, basically a video clamp circuit. There is a small ek special IC on the board, but when you analyze the circuit you will recognize that a quad op-amp of gain-BW 5 MHz will work well. At least at mine it does so. Scopes with option 5 have a differnt front panel layout. So, you might get in trouble to realize this with your non-opt5 scope. But basically: Yes, it can be field installed. hth, Andreas |
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#10
March 21st 04, 10:41 PM
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Jerry Koniecki wrote:
Frank Miles wrote: 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. Ah ha! I have a 465 (w/DM44) that I purchased in 1978 for personal use (no commercial abuse). Shortly after the warranty expired, it would not power up. I traced the problem to a shorted tantalum filter cap on the +15 volt line. But of course, it wasn't in the power supply, but rather on one of the boards. Pain to get to, IIRC. I can imagine. Tantalum caps are a real pain in the ass. If you reverse the polarity, they burn right through the PCB after a few months! We got a board in for repair last week with that problem. Luckily, everything on the board (yes, even the stickers with the serial numbers) is self extinguising. -- Reply to nico@nctdevpuntnl (punt=.) Bedrijven en winkels vindt U op www.adresboekje.nl |
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