Posted on Saturday, 24th April 2010 by Auw Jimmy
I just got my Oscilloscope, so I can play more fun with my DIY stuff. Ok, let’s start with some basic “game”, to find the inner and outer foil of a capacitor.
Basically, most of the capacitor, has what we call the inner foil and outer foil. Because, most of capacitor’s construction is based on the winding of paper or other conductor (silver, copper, gold, etc), so we will have a start position (the inner foil) and the finish position (the outer foil).
Although for the film type capacitor, it’s just fine to connect positive or negative to the inner or outer foil. But due to some reason, it’s preferable to connect the outer foil to negative side or to the “nearest” negative side (input side on coupling application).
Why? Because the outer foil will catch the outside interferences. So better if you can put this outer foil to the place nearest to ground or negative pole. So it could completely transfer those unwanted noise faster to the place where it should belong – the ground.
Some capacitor like Audio Note, Jensen, Auricap, Hovland, VCap, etc usually marks their capacitor with different color lead or print some black line to mark the negative side or input in the coupling application. Some other popular capacitor, like Mundorf, doesn’t seem to care about this, means no marks at all. So you got to check it by your own. Other capacitor like Duelund, which uses the Stacked Foil design, I believe doesn’t have any polarity (it’s not winded, but stacked).
How to do the test with Oscilloscope? Simple by testing both leads, and give some “interference” outside the capacitor (touch by hand or put some electric field interference e.g. high voltage cable, etc). The side with higher noise, means the outer foil.
Below are some picture from my own measurement on some capacitors.
Audio Note Oil Filled Mylar Capacitor. The black line marking on the capacitor’s body means the negative or input side. We can see the noise is quite big if we put the positive probe on the side which has black line marking.