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In Reply to: RE: Film Capacitor Orientation Question posted by Barry on July 12, 2021 at 17:15:59
It makes little difference in most circuits which end is attached where. The capacitor exhibits a low impedance between its two leads, usually much lower than the common mode impedance of either point to which it is connected. Because of this, noise picked up on the outer foil (if there is an outer foil) will appear at virtually the same amplitude on both ends of the cap.
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Follow Ups:
If it doesn't matter, help me understand then why the videos showing film capacitors connected to an oscilloscope show an induced signal larger in one direction than another when the test leads are switched. I actually can see this when my caps are connected to a scope. Is this test not representative of actual circuit conditions?Since I could install the caps either way, can I test resistance on either side of the cap to ground with a DVM to determine the lower impedance point?
I really appreciate your help here. Thanks.
Edits: 07/13/21
"Is this test not representative of actual circuit conditions?"
That is correct. The scope test is only useful for identifying which lead is connected to the foil. It doesn't represent actual circuit conditions. In actual use, the impedance of the capacitor itself reduces the differential induced across it. In addition, the outer foil is not a continuous, all-encompassing shield. Currents induced on the outside of the shield are free to travel to the inside surface, where they can, in turn, induce additional current on the adjacent layer. Even the scope test makes it evident that the outer foil is grossly imperfect as a shield. Connecting the probe to the inner connection does not entirely eliminate the external coupling, even when the foil is grounded. Consider how much less effective the shielding is likely to be when the foil is elevated above AC ground by the impedance of the stage driving the capacitor.
This is not to say that both ends of the coupling capacitor will be identically affected by voltages resulting from external fields. However, the real result is much less significant in practice than many people realize. In addition to these caps, unwanted pickup also takes place through the many unshielded component leads, vacuum tubes, etc. For all these reasons, "correct" orientation usually offers no real benefit in real world use.
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Buy Chinese. Bury freedom.
"can I test resistance on either side of the cap to ground with a DVM to determine the lower impedance point? "
No. The impedance on the source side will be the output impedance of the stage. You can't just measure that with an ohm meter. On the destination side it will be the grid resistor so that can me measured with a meter.
In almost all cases the output impedance of the source side will be lower than the grid resistor value of the destination.
Tre'
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