On Mon, 23 Sep 2013 02:52:27 GMT, the renowned John K <spam@me.not> wrote:>Joerg <invalid@invalid.invalid> wrote: >> >> Doesn't always work. Another incident I had was a 6h endurance test of >> a new design for a customer. I sat in the office next door doing >> something else. Then ... sniff .. sniff ... what on earth ... ran into >> the lab and sure enough a transistor in a linear bench supply had >> shorted. Instead of a regulated 9V my prototype now got close to 40V >> at several amps. It was not a pretty sight and took out several other >> pieces of equipment. > >I just lost a brand new motherboard to a defective power supply. I'm >thinking of putting 50 amp SCR's across all the critical voltages for the >pc and lab supplies. Trigger the SCR with a TL431 set to 10% above nominal. >Real cheap - may only cost a few pennies. Better to blow a fuse than lose >an important piece of equipment. > >JKDecent power supplies claim to have OVP built in. Eg. http://www.seasonicusa.com/G-series-450-550-650.htm Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
X-capacitor failure modes?
Started by ●September 21, 2013
Reply by ●September 23, 20132013-09-23
Reply by ●September 23, 20132013-09-23
On Mon, 23 Sep 2013 13:03:18 +1000, the renowned "Phil Allison" <phil_a@tpg.com.au> wrote:> >"Spehro Pefhany" > >"Phil Allison" >> >>>"Mike Perkins" >>>> >>>> X rated capacitors are generally metallised film, such that where the >>>> film >>>> breaks down the metal is vaporised destroying and preventing any further >>>> current path. As a result the capacitance will generally degrade over >>>> time, so find the apparent 0.28uF a little hard to explain. >>> >>> >>>** Class X caps are specially wound so as to not suffer from internal >>>corona >>>discharge at 250VAC - the technique is called "two in series" which >>>creates a floating electrode inside the cap that sits at half the applied >>>voltage. >>> >>>Corona is caused by tiny trapped air pocket inside the cap during winding, >>>only avoided by winding in vacuo. Discharge initiates at about 150VAC ( >>>depending on the air pressure) - so having series pair solves the problem. >>> >>>A damaged X cap might lose one of the pair and so the capacitance goes up. >>> >> >> >> Interesting information, Phil! > > ** You mean you did not already know that ?Nope. Maybe it's common knowledge, but not for me. I've been involved in specifying such caps (griping about the size), and even in the test equipment to verify the long-term reliability at 85�C, but not the internal construction.>FYI: > >There are also " three in series " X caps for higher voltages like 440VAC. > >It is possible to vacuum impregnate some types of X caps ( eg film, foil, >paper ) and eliminate all trapped air. > >But few cap makers will attempt winding caps in vacuo. > >Staff hate wearing those clumsy space suits ...I've only been in one film cap plant, and I don't remember seeing much of the winding stuff. They were powder coated rather than the Euro potted-molded "box" type.> > >.... Phil > > >Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●September 23, 20132013-09-23
"Spehro Pefhany"> >>> Interesting information, Phil! >> >> ** You mean you did not already know that ? > > Nope. Maybe it's common knowledge, but not for me. I've been involved > in specifying such caps (griping about the size), and even in the test > equipment to verify the long-term reliability at 85�C, but not the > internal construction.** If you read Wireless World magazine - you might have come across a couple of articles by " Cyril Bateman" published in the late 1980s IIRC. Cyril worked as an applications engineer for Hunts Capacitors in the UK. In the article, the story of the development of class X caps is told. In the days of ( fully impregnated ) paper/foil caps, it had been common to use 600VDC examples as suppression caps at 240VAC with little problem. When metallised polyester film caps arrived and folk tried the same thing - BIG problem !!. Cyril's investigated, found it was internal corona damage and advised all customers to cease using metallised film caps in this way. The first solution was to make a component that was simply two 600V caps wired in series - then someone devised the cunning trick of winding a single cap with a floating metal layer inside. In the days before class X caps became readily available, some dealers here in Sydney advised the use of 1000V or 1600V metallised polycarbonate caps ( Philips 341 series) for the job - which rapidly caught fire when exposed to 240VAC. I like to think I had a little to do with changing that situation.... .... Phil
Reply by ●September 23, 20132013-09-23
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:> Decent power supplies claim to have OVP built in. Eg. > > http://www.seasonicusa.com/G-series-450-550-650.htmYes, and short-circuit protection. Unfortunately, I think the short- circuit protection relies on the 12V. If it is crowbarred the protection is lost. I'm starting to think of putting overvoltage protection at the input to the circuit. The IPB075N04L is p-channel, 40V, 50A(?) and 7.5 mOhm. It costs $0.2018 at Arrow: "http://octopart.com/partsearch#search/requestData&q=IPB075N04LGATMA1" A simple TL431 driving a 2N4403 could short the gate to the source and turn the mosfet off. This could work for input voltages from 5V to probably 30V or so. Now the problem is how long would it take to shut down given an instantaneous jump to a higher voltage, for example +5V shorted to +12V? Would it be fast enough to protect sensitive logic? JK> Best regards, > Spehro Pefhany
Reply by ●September 23, 20132013-09-23
On Monday, September 23, 2013 4:52:27 AM UTC+2, John K wrote:> Joerg <invalid@invalid.invalid> wrote: > > > > > > Doesn't always work. Another incident I had was a 6h endurance test of > > > a new design for a customer. I sat in the office next door doing > > > something else. Then ... sniff .. sniff ... what on earth ... ran into > > > the lab and sure enough a transistor in a linear bench supply had > > > shorted. Instead of a regulated 9V my prototype now got close to 40V > > > at several amps. It was not a pretty sight and took out several other > > > pieces of equipment. > > > > I just lost a brand new motherboard to a defective power supply. I'm > > thinking of putting 50 amp SCR's across all the critical voltages for the > > pc and lab supplies. Trigger the SCR with a TL431 set to 10% above nominal. > > Real cheap - may only cost a few pennies. Better to blow a fuse than lose > > an important piece of equipment. > >When I worked with a design for space flight, we did that. Added a giant SCR and a trigger circuit, to protect the DUT in case of a PSU malfunction (or a programming error since it was a programable PSU) Cheers Klaus
Reply by ●September 23, 20132013-09-23
On 22/09/13 12:14, Phil Allison wrote:> "Joerg" > >> Is it possible that an X-cap self-heals to the point where you can >> barely see a thing, yet release a serious plume of smoke? > > ** Yep. > > >> It is a Rifa GPF-series film cap 0.22uF/250VAC with all kinds of agency >> logos on there. > > ** I have been in the room twice when one of them blew itself up - RIFA > brand both times. > > The first one was years ago inside a fan heater and the second in a portable > TV late last year. > > Filled the room with yucky smelling smoke and set off the alarm. > > > > ... Phil > > >Hewh !! wish I could make my stuff self healing :-) Probly have to install more extractor fans though.
Reply by ●September 23, 20132013-09-23
Klaus Kragelund <klauskvik@hotmail.com> wrote:> When I worked with a design for space flight, we did that. Added a > giant SCR and a trigger circuit, to protect the DUT in case of a PSU > malfunction (or a programming error since it was a programable PSU)> Cheers> KlausI'd hate to think of triggering an SCR in a spacecraft. How do you turn it off? How do you protect the PSU against a short on the output? You can't simply blow a fuse. Why use a programmable power supply? Why have the power supply deliver enough voltage to damage a load? Power is hard to generate in space. Any excess would be wasted as heat, which is difficult to get rid of in space. Why not design the power supply so it cannot deliver enough overvoltage to damage a circuit. For example, spacecraft need batteries to provide power when the solar arrays are in the Earth's shadow. Batteries are capable of some known maximum voltage. They also have some minimum voltage when discharged. Have the power supply deliver some fixed ratio of the battery voltage, and design the load to operate between those limits. For example, a simple forward converter output is determined by the transformer turns ratio: http://en.wikipedia.org/wiki/Forward_converter Then the only problem you need to worry about is what happens if the power supply fails and doesn't deliver enough voltage. I'd think you would have backup power supplies and simply switch to a known good one. JK
Reply by ●September 23, 20132013-09-23
On Mon, 23 Sep 2013 10:26:22 GMT, the renowned John K <spam@me.not> wrote:>Klaus Kragelund <klauskvik@hotmail.com> wrote: > >> When I worked with a design for space flight, we did that. Added a >> giant SCR and a trigger circuit, to protect the DUT in case of a PSU >> malfunction (or a programming error since it was a programable PSU) > >> Cheers > >> Klaus > >I'd hate to think of triggering an SCR in a spacecraft. How do you turn >it off?Remotely cycle the power to the affected module. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●September 23, 20132013-09-23
Joerg <invalid@invalid.invalid> wrote:> Klaus Bahner wrote: > > On 22-09-2013 00:17, Joerg wrote: > >> Folks, > >> > >> > >> I can't find any source and upon powering it up the analyzer worked > >> fine. That was before I changed anything. As if it had repaired itself. > >> The only thing I could see is a crack in the plastic of the X-capacitor. > > > > I've once had a fluorescent lamp which produced popping sounds for some > > time and also smelled a bit of burned plastics. Was hard to localize > > where it came from hence it took some time to find the culprit. > > > > It turned out to be an X-capacitor, which also had a crack in the > > housing. Had to look carefully, because besides the rather unimpressive > > crack, it looked just fine. > > > > Ah, ok, so it could indeed be the X-cap then. I was impressed how much > smoke it let off. The whole room (size of a bedroom) was full of smoke.My local garage had a similar capacitor failure in their gas analyser abut a month ago, it filled the whole workshop with smoke and left a dirty streak up the wall behind the machine. They could hardly believe that the cause was so trivial. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
Reply by ●September 23, 20132013-09-23
On Monday, September 23, 2013 12:26:22 PM UTC+2, John K wrote:> Klaus Kragelund <klauskvik@hotmail.com> wrote: > > > > > When I worked with a design for space flight, we did that. Added a > > > giant SCR and a trigger circuit, to protect the DUT in case of a PSU > > > malfunction (or a programming error since it was a programable PSU) > > > > > Cheers > > > > > Klaus > > > > I'd hate to think of triggering an SCR in a spacecraft. How do you turn > > it off? > >Well, this PSU was on ground, testing the modules to be used for space flight. The SCR was to protect the space modules, not the PSU. Cheers Klaus
