Capacitor outer foil test device

[martin manning]

Here's a simple device for quickly determining which lead is connected to the outer foil in rolled film/foil caps. The isn't much to it: A Hammond enclosure, some jacks, a switch, and a couple of solder-less banana plugs with paper clip wire for quick contacts. In operation an oscilloscope is connected to the BNC, and the cap under test is held between finger and thumb and rested on the contacts. The switch is cycled, and when the lowest signal amplitude is observed the switch bat position indicates the outer foil lead. The banana jacks next to the BNC can be connected to an audio amp in place of the scope. As a bonus, the device can also be used to sort or check component values by connecting it to a multimeter.

[FourT6and2]

Cool little box.

I've done the trick where you attach the cap's leads to an instrument cable plugged into an amp so you can hear the noise. But to be honest... I never really noticed a difference either way when I've tried it.

[sonicmojo]

Cool little box.

I've done the trick where you attach the cap's leads to an instrument cable plugged into an amp so you can hear the noise. But to be honest... I never really noticed a difference either way when I've tried it.

You should hear the difference for sure if you are holding the cap in your fingers.

[M Fowler]

I like your little box very helpful tool.

Mine is the BlueBear version connected to a small amplifier on the work bench.

outer-foil-tester9-300x262.gif

2884272356_fffbfb6546_m.jpg

Foil cap tester.jpg

[FourT6and2]

Cool little box.

I've done the trick where you attach the cap's leads to an instrument cable plugged into an amp so you can hear the noise. But to be honest... I never really noticed a difference either way when I've tried it.

You should hear the difference for sure if you are holding the cap in your fingers.

Yup. That's how I've done it. Can't tell a difference. My building is right next to a giant power line/transformer though. Maybe there's just so much interference that it's noisy either way.

[pompeiisneaks]

That's weird, normally the more noise there is the easier it is to see, because they pick up so much. I've always been able to see a difference, except with the higher capacitance caps, as I recall. 0.1 ones seem to have very nominal difference, but say .0022 has a very clear one. (I hope I've got it right, I haven't done this in a while, maybe I've got it backwards and the smaller the cap, the better the noise rejection lol!)

~Phil

[pompeiisneaks]

Oh and another trick I remember someone saying was to put either a fluorescent or led light right next to them, they're noisy as hell. the led's have poorly filtered dc rectified signal usually, and the fluorescent, we all know

~Phil

[romberg]

This guy has a series of electronics related videos I find interesting. He has one on this subject:

https://www.youtube.com/watch?v=BnR_DLd1PDI

His switch is automatic and run by a small IC circuit. Maybe a bit of overkill. But the geek factor makes it pretty cool!

The thing I found interesting is that you can't trust a lot of modern caps which have a marking. So, if your new caps are marked you want to test them anyway since the marking only shows which direction they entered the labeling machine not which side has the outside foil .

Mike

[martin manning]

Maybe a bit of overkill...

Ya think? And, I would argue, more difficult to use too.

[martin manning]

Here's another simple way to find the outer foil on film caps: Just insert the banana plugs with bent paper clips shown at the top of this thread into a DMM. The cap is placed across the terminals and held tightly between thumb and forefinger. The resulting noise voltage is read in mV. I find it takes about 10 seconds for the reading to stabilize, but there is a clear difference in the reading between orientations, as seen in the photos below. The cap shown is a 20n SBE6PS, and I also tried a 50n 6PS and a 100n CDE tubular. All showed a 2x or greater difference in voltage. Larger caps will have lower absolute levels due to their lower impedance at the dominant 60Hz test frequency. As mentioned above, these plugs are also very nice for quickly checking resistor values too.

[2tone]

So the lead on the common multimeter (black) input would be the outer foil when it reads lowest mv?

[martin manning]

So the lead on the common multimeter (black) input would be the outer foil when it reads lowest mv?

Correct, same as a scope trace shows lower amplitude when the outer foil is connected to ground.

[stelligan]

Here's another simple way to find the outer foil on film caps: Just insert the banana plugs with bent paper clips shown at the top of this thread into a DMM.

This is awesome!! - says scopeless me......

I can shit can my guitar cable with alligator clips.

[JD0x0]

Cool little box.

I've done the trick where you attach the cap's leads to an instrument cable plugged into an amp so you can hear the noise. But to be honest... I never really noticed a difference either way when I've tried it.

You should hear the difference for sure if you are holding the cap in your fingers.

Yup. That's how I've done it. Can't tell a difference. My building is right next to a giant power line/transformer though. Maybe there's just so much interference that it's noisy either way.

For me, depends on the cap, even the value. Some caps are very subtle, some I've wasn't able to perceive a difference, some were night and day.

[Pmacaula]

Sorry for resurrecting a zombie thread, but this looked like the best spot to ask a few questions.

Thought some might be interested in what testing looks like. Used the scope on an Analog Discovery 2 & zip-tied the cap to a plugged-in 120VAC line. You can see material differences in induced voltage based on which end ground is connected to for a range of axial foil capacitor (2.2nF, 4.7nF, 100nF) values and some non-axial Sprague Orange drops (47nF).

Below are a few photos of the setup and results. I am doing this as part of a build documented on TDPRI (https://www.tdpri.com/threads/5e3-micro ... 418/page-1)

The AD2 is the green/clear box in the lower left and the cap under measurement is zip-tied (loosely enough for me to swap caps) to the power cord of a battery charger.

The first photo shows lower amplitude than the 2nd, so the black alligator clip end (ground) is connected to the outside foil in the first photo.



Here are all the axial foil caps after measurement (- on the outside foil end, + on the other). Note the small printed + on the 4th cap from the
left. While it was correct on that particular cap, it was not on the cap to its right. In short - the manufacturer's marking is not a reliable indicator.


There was also a meaningful difference for the two Sprague Orange Drop caps.
Below are shots from the 47nF cap. The induced signal amplitude in photo 1 is about 2x that in photo 2.



Would be curious to know if others have measured Orange Drop or other non-axial caps for noise levels based on which way they are connected and if it makes a material difference to the actual noise heard in the amp's output.