Determining a likely value of a small low voltage polystyrene cap

N Cook: Unless you know the "working voltage" rating of the cap, value estimates based on physical are just about meaningless. As you undoubtably know, with the same value cap, the higher the voltage, the bigger the physical size and visa-vera. Hopefully, somewhere else on the board you can find a similar cap with intact markings to get some kind of idea of what you are dealing with. electricitym

Implicitly allowed for in the value of d

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

On Wed, 12 Oct 2005 11:36:54 +0100, "N Cook" put finger to keyboard and composed:

The units are wrong. I would expect something like ...

C = k * (L * W / d)

But then that doesn't fit nicely with your empirical data.

The formula for a parallel plate capacitor is ...

C = k eo A / d

k = dielectric constant eo = permittivity of free space = 8.85 x 10 -12 F/m A = plate area d = distance between plates

Plugging your data into the above formula gives:

See this URL:

"On Capacitor Dielectric Materials - A Chemist's View":

According to the author, polystyrene has a k of ~2.55 which is in disagreement with your results, although not by much. However, something looks very wrong with the 680pF and 2220pF data. I like the way you've approached this problem, though.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Is this just a supply decoupling cap, or something mre critical like a timing circuit or tuned circuit? If it's a supply decoupling cap, it's not hard to make an educated guess on a suitable replacement. If it's the latter, a capacitance box or substition of likely values until it works satisfactorily might be good enough.

Try 1nf or 10nf and work up/down from there!

Dave

cap

being

Going by memory I thought the relation was inverse square law with "d".

The other factor is they are spiral wrapped with the same dielectric material on the other 2 surfaces, not nessarily same thickness and when wrapped these become double thickness - 3 dielectrics with 2 "plates" all spirally wrapped - Ive no idea what the design formula for such construction is

I like his approach, too.

If you consider that the wrapped construction uses both sides of the foil strips, this doubles the effective area, which effectively halves the calculated k values above. This starts to look much more reasonable now, except for the 37 and 52, for which there must be some other explanation.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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On Wed, 12 Oct 2005 22:57:47 +0100, "N Cook" put finger to keyboard and composed:

I would think the capacitance would roughly double.

If the diagram on the left represents a cross section of the spiral, where T & B are the top & bottom plates, and D is the dielectric material, then I would think that the diagram on the right would be its electrical equivalent.

T B T B T B B T T B T D B D T D B T D B B D T T D B T D B D T D B T D B---B D T---T D B T D B D T D B T D B B D T T D B T B T B T B B T T B

V+ V- V+ V- V+ V- V- V+ V+ V-

Let's say C is the capacitance of a single wrap of the spiral. It follows then that the capacitance of 2 adjacent wraps is 3C. Therefore the capacitance of n wraps would be C x (2n-1), ie roughly double, given a sufficiently large number of wraps.

So the figure of k in two of the calculations above now approaches 2.8 which is in the ball park for polystyrene.

BTW, the value of 2220pF is strange. Shouldn't that be 220pF? If so then k = 5.2 /2 = 2.6 which is once again looking good.

That only leaves 680pF. Was the cap marked 680? If so, how can you be certain that this means 680 rather than 68 x 10^0? If the latter, then k=1.9 which is again pretty close.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

polystyrene

and

become

yes bouncing key , should read 220pF i will try repeating another unwrapped 680pF measurements today and check on ac cap meter also

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

polystyrene

and

become

Must have mangled the wrong "680pF" cap The construction is different being 2 foils 48mm * 4.5mm with 0.015mm dielectric film to one side only. Plugging those figures in gives the 5.5 value Spiral structure factor 2 seems to not come into play unless the plate area A is defined as the combined area of both plates.

So general formula is C in pF = 5.5 * 8.85 * L * W / (1000 * d )

C = 0.0487 * L * W / d, C in pF and dimensions mm