Obsolete Capacitor?

It could be a motor run capacitor, or used in a DC power supply.

Jon

This is an older paper-oil capacitor. They are only good for DC and audio-frequency applications. Caps like these are typically used in fluorescent lighting, mercury-vapor, or sodium-vapor ballasts, in ferroresonant voltage stabilizers/regulators, in induction motor "run" capacitors, and in DC filtering applications. They may also be used in audio crossover networks, thus making them susceptible to overpricing for the audiophool market.

The dielectric fluid in older Visconol units may contain PCB's, while newer units use mineral oil. Most newer paper-oil caps will have label stating that they contain No PCB's - if no such label, it likely contains PCB's.

BTW, these particular caps appear to be vastly overpriced... :)

Bert

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Stoneridge Engineering, LLC 
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Cursitor Doom wrote: 
> Greetings gentlemen, 
> 
> What kind of end product would a capacitor such as this be found in? 
> 
> 
> http://www.ebay.co.uk/itm/Plessey-visconol- 
> capacitor-8mf-600VDC-70c-/321396389935? 
> pt=LH_DefaultDomain_3&hash=item4ad4b7b82f 
> 
> Thanks. 
>

You might find something like this in a glassFET (firebottle, "vacuum tube" or "valve") amplifier, I suspect, as these tend to use rather high DC voltages. Possibly an audio amplifier, possibly RF (either radio-transmitter, industrial RF heating, medical diathermy, etc.).

nearly all of those are ac apps, yet you said dc & audio...

yes, be wary. I've seen canned oiled paper caps like that used in 1930s radios.

NT

Yeah - I should have said DC filter, AC-power, or audio frequency applications.

The paper-oil dielectric system is too lossy for direct use in higher-frequency power applications. In fact, they tend to overheat, bulging the case, and even blowing up if abused by an RF power application.

I'm surprised to read that. I have a box of these things and because I had nothing better to do I measured their ESRs and found them to be really tiny in relation to their smallish capacitance values.

** The type of cap in the link is rated for high voltage DC apps only.

They were used in the PSUs of valve equipment in the 50s and 60s as a high reliability replacement for electros. No doubt, they could be use in a speaker x-over too.

Similar looking caps, but with *AC voltage* ratings were for use in motor start and fluoro light PF correction.

I have see so many faulty ones (internally arcing or shorting) from Quad 11 amplifiers that they must be regarded as useless nowadays.

... Phil

** Any film/foil cap will show low ESR values compared to a similar value and voltage electro.

I tried this experiment one day:

I connected a 5mH air core inductor in series with a 47nF, 275VAC X2 suppression cap. Then, using a 50 watt amplifier with my bench sine generator set at 10kHz drove the pair until the voltage on the cap was 250VAC.

In a few seconds it poured smoke.

The same test with a ceramic cap showed no such issue.

... Phil

Must've been MKT type (polyester)?

PET has a nasty tendancy to run away; as the stuff melts, dielectric loss shoots way up (analogy: compare the microwave loss of ice to water) and it burns right through, oozing out goo and unpleasant vapors.

PP is good stuff, limited by the thickness of metallization.

Some very small parts can be made in that value; the smallest I've seen were Illinois Capacitor MKP (gray, 13 x 5 x 11 (L x W x H) mm, 10mm LS). They'll handle maybe 0.5A without getting too hot. They didn't quite melt at 2A, but they got unsafely hot:

(gray bank on left; some also used on the right)

Their PPB "snubber" type capacitors are quite good; I've ran them at about twice ratings with a modest temp rise.

Most manufacturers have a similar range of products.

Extremely robust film caps are the standard for induction heating, where solid copper end caps (for heatsinking), or water pipes, are soldered on to dissipate the heat. Rather than huge single rolls, construction is made from many small rolls of film and foil (or metallization), which distributes the heat -- and the current -- better (skin effect doesn't go away just because it's a capacitor!).

I wouldn't expect it to; for a C0G type, the losses are quite low, so it will be quite happy operating well above rated voltage. Even at RF. I've put >100VA through a 100pF 50V disc ceramic... didn't even flinch!

On the other hand, if it was a type II dielectric (X7R, Z5U, Y5P..), it probably just wheezed under that condition. Such dielectrics are lossy enough to raise the temperature to just under the Curie point, where the value changes rapidly, limiting power like a polyfuse.

And that's if you can even get that voltage on it, in the first place; even a 250V disc will drop value substantially during the swing of 250VAC, so that your average frequency at resonance will be much higher than

10kHz. Such a system is hysteretic with respect to frequency... so bizarre.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com

** The ones that blew up were RIFA metallised paper - so similar to the OP's but without the oil fill.

I just tried the test again and ruined another.

** Yep, MKP types ( polypropylene dielectric ) heat very little in the same test.

** 3kV rated Z5U ceramics heat slightly in the test and the resonant frequency shifts upwards a tad.

I just tried a 3.3nF example at 38kHz and 350VAC.

... Phil

"Removed or not available" Another reference please.

** FFS go back to the *original post* for the link.

... Phil

Hmm. It seems the item sold despite the outrageous priced demanded. Here's another example: