Why are these Grey RS caps high in value?

[Lucien Nunes]

Last night I was testing some of those grey plastic-cased caps with the RS logo moulded into the case that were popular service parts in the 60s & 70s. I can't recall if they are mixed dielectric and/or rebadged Dubilier Greycons - perhaps someone could confirm? A dozen caps, various values from 0.005 to 0.22µF, all tested OK for insulation at 500V, typically 500MΩ to 2GΩ. The 1000V rated ones gave the same reading at 1000V. But all tested high in value, at least 20% and up to 60% in one case, on a simple handheld digital LCR meter. I would have used a proper bridge but I am 120 miles away from it and the handheld was giving correct readings on other types of cap. It will read high if there is severe leakage, but that needs to be orders of magnitude greater than in the grey caps to influence the reading. For example, a new 0.22µF that read 0.21 was still reading 0.21 when shunted by 1MΩ.

My question is; assuming that they were all within 20% tolerance when new, by what mechanism is the capacitance increasing without affecting the insulation?

[threeseven]

Dimensional tolerance creep due to material aging?

[pmmunro]

Lucien,

How long after the insulation tests did you make the capacity measurements?

Dielectric absorption comes to mind. (I'm sure you'll be familiar with the phenomenon, but for others - http://www.wima.com/EN/absorption.htm).

PMM

[Heatercathodeshort]

These capacitors were Dubiliar mixed dielecric [MDC] and have served us well. I have a large stock of NOS ones but recently I have discovered that many are increasing in value often exactly 100%. I can only think that the dielectric is shrinking somehow reducing the gap between the foils. It's most odd but only this week I scrapped a NOS .22uf that read .44uf on the tester and gave severe lack of height in a vintage TV frame oscillator circuit. I guess it was around 40 years old. John.

[emeritus]

While it is probably nothing to do with your situation, a table of colour codes for components in general in the 6th Edition (1975) of the ITT Reference Data for Radio Engineers includes a tolerance called GMV (Guaranteed Minimum Value) having a tolerance range of 0 to +100%, with a code colour of yellow.

Another table of colour codes for ceramic dielectric capacitors refers to (but does not provide tabulated data for) "Class 3, low voltage ceramics specifically suited for transistorized or other electronic circuits for bypass, coupling, or frequency determination where dielectric losses, high insulation resistance, and capacitance stability are not of major importance." The tolerance ranges are stated to be either ±20% (Code M), or -20% to + 80% (Code Z). I cannot recall having come across any examples of capacitors with these codes.

[Heatercathodeshort]

The MDC types were almost spot on when new and were very reliable for around 30 years when they probably had a design life of 10. Everything goes back to nature eventually including ourselves. Cheerful John.

[cmjones01]

Here's a thought. We think these are mixed dielectric capacitors, paper and plastic. If the paper and plastic are layered, what happens if the paper absorbs moisture? The dielectric constant of water is about 80, which is higher than most things, so the capacitance is likely to increase. The insulation, however, might remain unaffected because it's mostly provided by the plastic layer.

Chris

[GMB]

My research into restoring old capacitors found that capacitance does indeed increase somewhat as dielectrics age. Although everyone assumes moisture is absorbed, I think that chemical decomposition of the dielectric is also a major factor, especially where they are well sealed.

It might be interesting to test the insulation to destruction and compare with a modern part.

[G8HQP Dave]

There was a debate about this in a thread a few years ago. Two main camps emerged:
1. leakage resistance is confusing capacitance meters so they read high
2. water is increasing the permittivity of the dielectric
I belong to the second camp.

[Lucien Nunes]

I remember that thread, although in the case of the original paper caps under investigation, the increased value was always accompanied by leakage. As Chris suggests, in this case the plastic film might be responsible for maintaining the insulation and the paper for the drift. I'll see what can be learned from dissecting one.

BTW, what was the reasoning behind using the mixed dielectric?

[Maarten]

@G8HQP Dave: I think actually both camps are right, but in this case it's definitely the water changing the permittivity, proven by the absence of leaking (due to the mixed dielectric).

@Lucien Nunes: What's the second dielectric? Polyester or polypropylene? The reasoning could be different depending on which one and the physical construction. If it's polyester, the paper was probably used to increase the allowed dV/dT. If it's polypropylene (which already has a better dV/dT rating than paper) the paper may have been used as a carrier for the conductive layer to be deposited on, since it used to be hard to get the metal to stick to the polypropylene.

[Lucien Nunes]

According to the RS catalogue, the plastic film dielectric is polyester. This is corroborated by a late 1960's Cables & Components catalogue entry for Bluecon 660M types. The Dubilier data sheet itself doesn't identify the film material but does state that the paper is impregnated with a solid synthetic resin and that extended foils are used for the leadouts.

A few evenings ago I took a scalpel to an 0.022µF 600V example. As well as the value and voltage rating, moulded into the polypropylene case are the patent number for Dubilier's patented encapsulation method (which applies to some of their other ranges too) and an identifying mark for the outside foil.

Having cut the case away, the element was unrolled revealing a foil-paper-film construction. Whilst it was easy to separate the film, the paper and foil were well stuck together, peeling apart only reluctantly. To confirm that the film was polyester, a burn test was carried out. Despite scrunching the film into a tight ball, its surface area was still large enough that it burnt vigorously in seconds with a yellow flame and characteristic smell of polystyrene. As it is unlikely to be polystyrene, it is probably polyester rather than polypropylene.

A drop of water and a drop of light oil were placed on the paper surface exposed after removing the film. The water formed a ball while the oil immediately spread out, indicating the hydrophobic nature of the impregnant. The same appearance remained after 48 hours. When the liquids were removed, there was no visible evidence of either the oil or water having penetrated or softened the material.

I don't know what we learn from this, other than that the capacitor is constructed as expected and that there is no visible degradation of the element. When I can test some capacitors on a bridge, we might discover that the dissipation factor has deteriorated or some other indication that the composition of the dielectric has changed.

[Maarten]

Seems typical behaviour for a paper cap, even if it has a hybrid dieelectric.