directional placement film capacitors?

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seavote

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May 31, 2006
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while checking out a mic pre project from Elektor Electronics 2/2003 i came across this info. "coupling capacitors c4 and c5 are specially marked in the schematic diagram. the marking indicates the lead connected to the outer foil of the capacitor,which should be connected to the non-critical side of the circuit.Many types of film capacitors are correspondingly marked.The result is that the capacitor screens itself,thereby reducing the suseptability of the circuit to interference." i have never heard of this. i checked some film caps i have and could not find any indication of which lead was which. i have not (but will)checked the specs. is this technique based in sound theory? how much difference will it make in a circuit? it seems like it may be a good practice to adopt. but how can you tell which lead is from the outer end of the foil? the article is pdf otherwise i would post
 
[quote author="seavote"]while checking out a mic pre project from Elektor Electronics 2/2003 i came across this info. "coupling capacitors c4 and c5 are specially marked in the schematic diagram. the marking indicates the lead connected to the outer foil of the capacitor,which should be connected to the non-critical side of the circuit.Many types of film capacitors are correspondingly marked.The result is that the capacitor screens itself,thereby reducing the suseptability of the circuit to interference." i have never heard of this. i checked some film caps i have and could not find any indication of which lead was which. i have not (but will)checked the specs. is this technique based in sound theory? how much difference will it make in a circuit? it seems like it may be a good practice to adopt. but how can you tell which lead is from the outer end of the foil? the article is pdf otherwise i would post[/quote]

This is indeed a useful practice in sensitive circuits. In some film capacitors that are round or cylindrical one lead connects to the middle of the wrap, the other lead connects to the outside of the cylinder. In most circuits where one of the capacitor's two leads are connected to a low impedance circuit point (like opamp output or ground), observing this orientation to connect the outer wrap to lower impedance will provide some self shielding.

Back in my old hi-fi kits I used a lot of polystyrene caps and these marked the outer wrap end with a single black line. I've seen this on a few other round film caps. Of course with the clear polystyrenes you could just look at them and see which end was the outer wrap.

This is a fairly subtle phenomenon but just think about the example of such a cap connected from an opamp output to it's minus input. Properly inserted the minus input is shielded from noise by the cap itself. Connected the other way, the minus input is hanging out in the breeze with maximum surface area exposed to noise pickup.

JR
 
Hello Seavote,

Marking inner and outer foil on caps has been around since the 1950's and probably before.
I have a 65 Fender amp on the bench and the original caps have "outer foil" written on them.
On many it is indicated by a line.
Those that dont have a line I assume that the end of the writing is the outer.
I suspect that many new cap manufacturers no longer do this.
The technical reason given by the Elektor article is correct.
However the sound difference between which side you connect to the signal, has been discussed many times over the years in Hi-Fi circles.

Sorr
 
You can check for the outside foil on capacitor with a scope. With the scope set to say 20 to 200 mv, connect probe and ground of the scope across the cap and observe the amount of noise or hum on the scope. Then flip the probe leads and check it again. If there is a change, the cap indeed has an outside foil, and the capacitor lead that was connected to ground of the scope in the quieter setup is the outside foil.

You have avoid moving the cap when making these tests, because, like a non-humbucker guitar pickup, the amount and the character of the noise will often change if the cap is moved, even slightly. if you have trouble seeing the noise on your scope, place the cap near a soldering station, power transformer, or a power cord that has current flowing through it. These things are hum generators that will tend to couple noise into the cap and make the test results more obvious.

If there is an outside foil, it should be connected to the lower impedance point in the circuit. Often this will just be ground. It won't make much difference with coupling caps, but it will sometimes matter with bypass and EQ caps. It will be more important with smaller caps than large ones.

Some modern capacitors should be marked, but aren't. I guess the manufacturers just don't realize that it matters in some applications.
 
Welcome to the message board, DK.
It's actually a weird coincidence, you turning up just now. Only minutes ago, I was looking at your site (after landing there via a Google search) and was thinking, "someone should invite this guy over to our forum..."
 
Dave K rocks.

plain-n-simple, He's a gem.

I've already fawned over at the PSW forums, and I've thought before now that he'd be a great addition...

Welcome Dave. -Hope you find it pleasant here.

[Basil Fawlty]just don't mention the war![/Basil Fawlty]

:green:

Keef
 
Keef and everybody, hello from Burbank, and thanks for the warm welcome. I wasn't really aware of this forum till recently and it took a while to get registered, but I see a lot of interesting topics here, with opportunities to learn and contribute.

Happy Holidays all, and talk to you soon.

David
 

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