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Outer foil direction on circuit?

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tonescout

tonescout

Member
Joined 2020
#1
Hi, I have read and sort of understand the logic of outer foil is the lowest impedance path to ground. I have an old amplifier (1983) where I am replacing the old coupling caps and I wanted to make sure I have got the direction correct. I have labelled the outer foil end in red here below.

Any comments / advice welcome 🙂

Screenshot 2023-05-04 at 15.36.28.png
 
#2
The rule of thumb is to connect the outer foil to a low AC impedance point or ground, so that the capacitor will have better shielding against external electrostatic fields. At first sight, I think the indications on the schematics are correct.
 
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#6
But, ya know what? Everybody says this, but I'm not sure I really believe it. It's a capacitor; at frequencies within its working range there's not much difference between either end of it. If it's not a capacitor, what difference does it make which end is connected to lower impedance? Just never heard a convincing argument.

All good fortune,
Chris
 
#7
What I sid is as follows: take a piece of copper foil, wrap it around the cap, and test the capacitance to either of the leads. My capacitance tester works in the pF region, and shows the difference. The lowest capacitance is the lead connected to the outer foil. Connect that to the source (output/input) or to earth when it is decoupling.
- note the direction of print on the cap. The other caps will have the same “direction”.
Ah, and Chris, this ‘shielding’ will make just that difference of holding a finger near the cap and hearing a hum or not . . .
 
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#10
triode_al said:
- note the direction of print on the cap. The other caps will have the same “direction”.
Click to expand...
I read an article in Nuts and Volts I believe where the author talked to a number of cap manufacturers such as CDE and they said the printing cannot be relied on to determine the outer foil as the caps are printed in alternating directions. So 50% will be one way and 50% the other.
I'll look for the source.
 
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#13
Chris Hornbeck said:
But, ya know what? Everybody says this, but I'm not sure I really believe it. It's a capacitor; at frequencies within its working range there's not much difference between either end of it. If it's not a capacitor, what difference does it make which end is connected to lower impedance? Just never heard a convincing argument.

All good fortune,
Chris
Click to expand...
That's why it's unusual for large value capacitors to mark their outer foil. But for (say) a 22pF polystyrene, it makes all the difference in the world to connect the coloured end (outer foil) to a low impedance to ground because it reduces hum pick-up.
 
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#14
triode_al said:
What I sid is as follows: take a piece of copper foil, wrap it around the cap, and test the capacitance to either of the leads. My capacitance tester works in the pF region, and shows the difference. The lowest capacitance is the lead connected to the outer foil. Connect that to the source (output/input) or to earth when it is decoupling.
- note the direction of print on the cap. The other caps will have the same “direction”.
Ah, and Chris, this ‘shielding’ will make just that difference of holding a finger near the cap and hearing a hum or not . . .
Click to expand...
Would not the wire connected to the outer foil have the highest capacitance?
 
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