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Old 9th Jul 2009, 9:23 pm   #1
Station X
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Default World's leakiest capacitor?

I turned on my Farnell E350 power supply this evening and noticed that the front panel neon wasn't lit. There was no HT (350V) output either. A quick check showed that the 2.5A anti surge mains input fuse on the rear panel had blown.

It didn't take me long to find out why. The 0.1uF 1000V Wima Durolit capacitor shown below sits right across the mains transformer secondary HT winding. It was replaced with a 1000V polypropylene type and all was well.

All I can say is thank goodness for fuses. With its multiple windings and taps it would have cost a fortune to get the mains transformer rewound and the power supply would have been BER.
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Old 9th Jul 2009, 9:46 pm   #2
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Default Re: World's leakiest capacitor?

Well it's a capacitor pretending to be a 0.1 Ohm resistor. I've had this on Electrolytics too.

Scary in a way.

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Steve P.
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Old 9th Jul 2009, 9:51 pm   #3
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Default Re: World's leakiest capacitor?

I suppose it must have failed at the instant of switch on, as the PSU was working fine about a week before. I'm pleased that I didn't replace the fuse and try again as it might have let some smoke out.

I guessed it was a capacitor at fault and the fact it was so bad made it possible to find it with just a DMM which must have less than 9V output on the resistance range. Unsoldering one end of the cap confirmed the diagnosis.
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Old 9th Jul 2009, 10:15 pm   #4
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Default Re: World's leakiest capacitor?

Quote:
Originally Posted by Station X View Post
I guessed it was a capacitor at fault and the fact it was so bad made it possible to find it with just a DMM which must have less than 9V output on the resistance range. Unsoldering one end of the cap confirmed the diagnosis.


All's well that ends well. Good result there, Graham.
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Old 9th Jul 2009, 11:32 pm   #5
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Default Re: World's leakiest capacitor?

Should fit an 'X' type really. Chances are it wouldn't have failed or if it had, it wouldn't have blown the fuse.



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Old 10th Jul 2009, 12:26 am   #6
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Default Re: World's leakiest capacitor?

Can you get Class X capacitors rated at 1000V AC?
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Old 10th Jul 2009, 7:47 am   #7
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Default Re: World's leakiest capacitor?

Quote:
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Can you get Class X capacitors rated at 1000V AC?
Class X caps don't carry a voltage rating. They are just specified as OK to use across nominal 220V/240V mains. They are high reliability but must not be used where failure would expose somebody to a shock hazard. That honour goes to class Y caps.

I don't know if there's an similar spec for using between phases of a 3 phase system.
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Old 10th Jul 2009, 8:44 am   #8
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Default Re: World's leakiest capacitor?

Quote:
Originally Posted by ppppenguin View Post
Class X caps don't carry a voltage rating.
That may be true - but X2 capacitors certainly do. I have in front of me right now, a 0.1µF 660V AC X2 capacitor, type Rifa PME264. And, I seem to remember that these are listed as standard catalogue parts from 275V AC to 760V AC (sorry, Station X, I haven't got 1000V AC else I'd stick one in the post to you!)
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Old 10th Jul 2009, 9:00 am   #9
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Default Re: World's leakiest capacitor?

You're right that they are marked with a voltage (275V on the ones I've just looked at) but I thought that it was incidental. The actual requirement is that it's safe to connect them between phase and neutral on a standard LV supply.

I didn't realise that there were other voltage ratings. Your 660V part looks good for phase to phase.

PS: Research in RS catalogue shows 630VDC parts but nothing over 300VAC
http://uk.rs-online.com/web/search/s...94955324&Nty=1
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Old 10th Jul 2009, 9:45 am   #10
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Default Re: World's leakiest capacitor?

The Wima cap I removed was probably a replacement. The original was specified as a Rifa PME26?? with TCC as an alternative about 30 years ago. I'm not sure what the cap actually does. It's switched across the used part of the secondary of the mains transformer before the bridge rectifier. Maybe it's there to prevent arcing of the switch when the voltage is changed.
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Old 10th Jul 2009, 10:46 am   #11
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Default Re: World's leakiest capacitor?

The capacitor is to reduce RFI caused by charge storage in the rectifier diodes.
A semiconductor diode remains conducting for an instant when reverse biased until the stored charge is swept from the junction, the diode then turns off abruptly causing a fast transient.


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Old 10th Jul 2009, 10:54 am   #12
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Default Re: World's leakiest capacitor?

Indeed, JLH discussed this (I forget where, either an article or one of his books)

In his opinion, it made no difference whether one cap was placed across the TX secondary, or, as is often seen, each diode is shunted by a cap.
He would opt for one cap across the TX secondary as it was easier!

I always fit one of these caps as a matter of course, in my projects.
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Old 10th Jul 2009, 11:34 am   #13
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Default Re: World's leakiest capacitor?

I have, or had, always thought those Wima caps to be some of the most reliable.
Of course, there is always the exception.
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Old 10th Jul 2009, 12:13 pm   #14
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Default Re: World's leakiest capacitor?

It would be interesting to know the construction. I've never known a plastic film cap fail short circuit like that, though it's relatively common with old waxies of course, as the paper layer burns though and the two foil layers touch.

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Old 10th Jul 2009, 1:21 pm   #15
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Default Re: World's leakiest capacitor?

I do believe that Class X capacitors, while designed as OK for across-the-line applications (with their attendant surges and spikes), don't have a defined failure mode - as distinct from Y types which must fail open-circuit.

Class X types are therefore used in applications where failure would blow a fuse. That being so, if you do connect an X capacitor (or any capacitor) across your HT secondary, make sure the fuse will blow and not the transformer, in case of failure! Using Class X will just ensure the failure is less likely.
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Old 13th Jul 2009, 12:09 pm   #16
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Default Re: World's leakiest capacitor?

The capacitor showed a circular crack around one of the leads with further cracks radiating out from it. This was at one end only. Of course this damage could have been caused when it was removed from the circuit.

I sawed off a slice and found that the foils and dielectric were so tightly wound that it was impossible to see them with the low power magnifiers I have. It just looked like a solid brown mass. I unwound the foils and the dielectric seemed to be paper rather than plastic. There was no sign of moisture. I think this cap must have had a physical contact between the plates somewhere rather than suffering insulation breakdown. I wasn't able to find out where the contact was though.
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