X-capacitor failure modes?

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.

--
Mike Perkins 
Video Solutions Ltd 
www.videosolutions.ltd.uk

X class caps have improved a lot with fusible links built into thin metal foil pattern so that less heat damage to vaporizing toxic plastic with only fractional loss of capacitance with many fuse links in the entire rolled metalized layer.

Check out Panasonic's design of X class PU and PE caps.

I don't know the MSDS of your part, but smoke from molten PVC wire insulation caused more lethal risk than the radiation at Three Mile Island.

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.

--
Regards, Joerg 

http://www.analogconsultants.com/

That 0.28uF is the main reason I posted my question. I expected half or more of the capacitance to be gone up in smoke, literally.

--
Regards, Joerg 

http://www.analogconsultants.com/

--
It could be that the pressure exerted on the film before the case 
cracked was enough to squeeze/compress it to the point where the 
thinner dielectric caused the capacitance increase.

"Mike Perkins"

** 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.

... Phil

Interesting information, 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

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.

JK

"Spehro Pefhany" >"Phil Allison"

** You mean you did not already know that ?

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 ...

.... Phil

Decent power supplies claim to have OVP built in. Eg.

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

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.

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.

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

"Spehro Pefhany"

** 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

Yes, 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:

"

"

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

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

Hewh !! wish I could make my stuff self healing :-) Probly have to install more extractor fans though.

I'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:

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

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

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

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