Detecting the outer foil of a non-polarized cap

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dbishopbliss

New member
I've been hanging out in the guitar forums lately and I came across a post where someone was asking how to detect the outer foil of a cap.  I always thought that non-polarized caps weren't really directional and never gave much thought to it.  However, this video has made me realize that there may be benefits.  Subsequently, another poster added this comment:
The proper way to connect the outside foil is to the low impedance side of the circuit, which, in the case of coupling caps, will normally be the plate of the previous stage. If it is a bypass cap to ground, connect the outside foil to the grounded side. If it is a bypass cap from a signal to B+, connect the outside foil to B+. The outside foil will act as a shield against electric field coupling into the capacitor, so you want it to have the lowest impedance return path to ground.
Click to expand...

Has anyone here considered this?  I've just never heard it mentioned before except when asking about the leads of auricaps in the Foreplay.
 
I have heard about this before but hadn't given it too much thought.  I remember reading that some of the polys are marked.  I just bought a couple of 100uf Solens for my Crack and I didn't see anything that would look like obvious markings to me.  My guess it that you would have to email the manufacturer to know for certain.

Thinking out loud made me go get the Solens and examine them again.  One lead is longer but there is no indication of what that means.  Just downloaded the PDF from the Solens site and that didn't help either.  I will send an email and see what they say.

 
For some reason I only hear this when someone is using the Auricaps.  Maybe because the manufacturer clearly states how to wire the caps.

Here is the link:

http://www.audience-av.com/capacitors/a_applications.php

I don't have a guitar amp to test with and the audio was such that I didn't understand his explanation anyway.  But I do have a scope and I really followed that one.

Edit:

So I don't have to wait for the video to load again the oscilloscope method is to clip the ground and signal leads of one channel to the cap leads.  Put an AC cord that is being used against the body of the cap.  Read the scope, then swap the scope probe leads.  Whichever orientation has the lower impressed voltage on it the ground lead is on the outer foil.
 
Even though the guitar amp method is a little tough to follow, you can easily hear that there is more noise when the cap is oriented one way versus other.  Next time I build an amp (actually I'm restoring a guitar amp now), I will be marking the outer foil end on my caps and installing them as described.  It would be interesting to compare the two in an audio circuit.
 
Usually, the shorter lead is the foil side, which you also want to try to get as close physically to the lowest impedance point.  Generallly the foil is also related to the direction of the writing on the cap, but it's early, and I'm somewhat confused on what the rule about this is, so I won't even guess as to which end the writing goes to.

HTH,

Jim
 
The writing is known to go in both directions, sometimes even on different models from the same company!

You can test it. Put some AC across the capacitor, and wrap a bit of foil or copper tape around the cap. Then measure voltage from each lead to the foil - no voltage = outer foil, some voltage = inner foil.
 
Some manufactures will mark a transformer's (or inductors) DOT with whether it should go to AC ground or AC hot.  I haven
 
Paul Joppa said:
  .  .  .  You can test it. Put some AC across the capacitor, and wrap a bit of foil or copper tape around the cap. Then measure voltage from each lead to the foil - no voltage = outer foil, some voltage = inner foil.
Click to expand...

I love it.  I'm a pragmatic person.  So the oscilloscope and ac across the cap with foil on the outside makes a lot of sense to me.  This is what I will be doing in the future.

Thanks to David and Paul for the good "how to."
 
I got an answer from Solen about how they mark their caps.  According to Chris, the longer lead is connected to the outer foil and should be connected to the negative side or ground.

Keep in mind that this may not be a universal standard across manufacturers. 
 
Paul Joppa said:
You can test it. Put some AC across the capacitor, and wrap a bit of foil or copper tape around the cap. Then measure voltage from each lead to the foil - no voltage = outer foil, some voltage = inner foil.
Click to expand...

Sorry to resurrect an old thread, but I'm wondering about the results I got:  I put 15VAC across the cap with foil wrapped around it, and tested the VAC from the foil to each lead.  On one side, I got ~0.7VAC, and on the other ~2.5VAC.  Is the side with the lower voltage the outer foil?

Thanks!
 
I seen that video posted on DIYAudio the other day and built one just like it.  It doesn't do anything you cant do manually with a scope, except for one scenario. Sometimes you will find a capacitor where the scope trace signal difference is very small between ends, to the point you cant tell the difference by the time you disconnect the leads and flip the capacitor around.  With this tool it switches ends every few seconds which makes it much easier to detect the difference.

Its actually so easy to use i went through my entire cap stash and marked every single one of them in no time.  The end of the capacitors with the outer foil was completely random for just about every cap, orange drops, red drops, misc polyfilm, known audio brands, and Russian surplus caps.  The only exception where caps that had the outer foil marked, vintage caps, and Auricaps, all of which were marked correctly as you would expect.

Parts cost is ~$3-4, enclosure was $6 i think from Mouser, its handy, and it works.
 
I've known guys with that much test gear and more. I've had five times that, mostly inherited, in the past. It took years to get rid of it all. It's a particularly odd obsession. Usually half of it is obsolete and doesn't work right and they really only use about 1/10 of it.
 
Paul,

Did you count the oscilloscopes?  I see 11. 

The one he uses is the same Tektronix that I used in school.  It is tubed.
 
Caucasian Blackplate said:
I don't think he has enough test gear to be trustworthy...
Click to expand...

I just happened to see his vid on his lab about an hour before I checked this post.  He has a few rooms going. 
http://youtu.be/7A_NqNyvaBM
 
I'm impressed that all the stuff works. I still suspect he never uses some of it.
 
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