Every time I have asked someone about that "Black Line" you see on some caps I get a different answer. Some tell me to have the line more positive some tell me to have the blk line negative.
The other day I watched a rather unusual video where the guy was checking his caps with his scope and thought why didn't I ever do that. Watching his video I found out that the line doesn't mean anything. He checked several caps from the same vendor with different results. Could this possibly be why some of us have battled noise when we do a build?
The other day I watched a rather unusual video where the guy was checking his caps with his scope and thought why didn't I ever do that. Watching his video I found out that the line doesn't mean anything. He checked several caps from the same vendor with different results. Could this possibly be why some of us have battled noise when we do a build?
That line signifies the lead connected to the outside foil on a non-polar cap, that can be important for sensitive high gain circuits, so that lead should be connected to the lower impedance node in the circuit generally. On a polar cap, like an electrolytic, it indicates the negative lead.
Mike
Mike
Last edited:
Well its supposed to indicate outside foil, but often doesn't as the production line jumbles them up before the legend-printing machine!
Outside foil in a high impedance circuit should be the lower impedance side of the cap in order to shield the other from stray capacitive cross-talk / interference. In practice this mainly matters in valve circuitry, most solid-state is too low an impedance to care about this, which is why its fallen by the wayside.
Outside foil in a high impedance circuit should be the lower impedance side of the cap in order to shield the other from stray capacitive cross-talk / interference. In practice this mainly matters in valve circuitry, most solid-state is too low an impedance to care about this, which is why its fallen by the wayside.
I set up the scope and checked a 1/2 dozen of different caps. My findings were in most cases the black line was incorrect. This is why I posted the thread. The youtube video I watched was an eye opener since we always tend to believe what is marked on the cap. The scope however shows in most cases the marking is incorrect. So, I'm suggesting that in the diy mindset that we check the caps first and maybe the chances of lower that nasty hum will improve.
What is your actual scope setup for testing? Can you share a link to the video from post#1?
Regards, Gerrit
Regards, Gerrit
Thanks for sharing the link to the video. I wasn’t aware the industry was so careless in marking capacitors.
Regards, Gerrit
Regards, Gerrit
The line or mark means different things for different types of capacitors: for regular E-caps (liquid, aluminum), it indicates the negative side.
For solid electrolyte caps, aluminum or tantalum, it indicates the positive side (it is often a + sign, but a dot or a band is also possible).
For non-polarized types (paper, mylar, PS, etc), it indicates the outer foil.
If it was actually the case (more than 50% wrongly marked), it would indicate a reversed correlation: this would mean that you should always install those caps backwards.
I don't think it is the case: my own informal statistics show a proportion of mismarked caps slightly larger than 10% (which is already very large).
I have built (and shared) a small, low-cost automatic detector:
ƑƠɭįƠ is a simplistic, yet automatic outer foil tester for capacitors
I am not sure it is the best option: many boutique caps do not even manage to have basic parameters right, unlike industrial products.
Why would they be better on that particular aspect?
I put this up as a possible help for those experiencing hum and to try to possible alleviate some of the problem associated with diying a project. So that there is no misunderstanding I will try to explain a bit further. I'm going to once again mention some of my experience when asking electronics store owners the relevance of the "black line" on caps. I asked two owners of two different electronics stores and two repair technicians to explain the polarity of a cap with the magic line and received a 50/50 split on the answer. This is why I challenged the concept of polarity possibly being wrong or being mistaken with respect to the correct installation and looked for a positive proof on the subject and found a video made by an engineer on the subject.
My particular testing prior to me posting this thread and approximately 1 hour of testing caps in my parts bin revealed that in "my experience" I've found no problems with electrolytic caps only non-electrolytic. I'm talking the run of the mill type non boutique caps. This afternoon I tested 55 caps of the film and mylar composition. The orange caps of a brand I'm not going to mention tested 66% wrong polarity. It seemed that the newer caps in my bins were more susceptible to mis marking than the older variety. Now, your particular array of parts may yield different results. I will have to disagree with your particular logic of the installation of a cap in reverse polarity to what is indicated simply because of the results of a random sampling. My suggestion still stands that based on the video and my sampling that one should consider checking their caps prior to installation in a project and or in repair work they are engaged in.
With respect to the automatic tester....
The gentleman that made the video that I put a link to also designed and shared his concept, design, and schematic. I generally will sit down and used less than a 1/2 dozen caps at a time will probably find it simpler and easier to do it the old fashioned way and simply hook the cap to the scope observe the amplitude of the waveform and reverse leads to determine proper polarity and mark cap with a magic marker.
Last edited:
The black line tended to be more important in the days of valves with high voltages and higher impedances where things were more prone to interference.
Well I guess I'm mistaken as I thought one of the goals of DIY was to possibly teach people the correct way of doing things. I guess you can throw the caps in any which way and blame the noise on something else because apparently tube circuits are longer prone to noise and interference.
I learned only recently about the outer foil end marking on polystyrenes, but I'd always connect as described, outer foil to ground or earthier end.
It's pretty much a pointless exercise if you cant verify the marking is correct/a large proportion are incorrectly marked/you simply cant tell one end of the wound foil, from the other.
It's pretty much a pointless exercise if you cant verify the marking is correct/a large proportion are incorrectly marked/you simply cant tell one end of the wound foil, from the other.
My thoughts are its no longer pointless since the video explains simply how to test / check polarity if you have a scope. In the event you don't have a scope surely you probably know someone that does. Some of the older parts places the old Mom and Pop stores have a scope in the backroom and if you ask they will check them for you.
In other words if there is a will there is a way.
In other words if there is a will there is a way.
Redacting intended, this is customary, here on DIYA, no matter what. 🙂
Narrowing down to film caps, the band is purely cosmetics nowadays. I've recently tested a big handful of them for a tube preamp, polystyrene's and polyprop's, and about 1/3 have the band placed on the inner foil side.
So if you care about orientation (I do), you have to test them; no big deal. It can be even nicer if everybody joined in and cheer and encourage Elvee to do a PCB + GB for his magic polarity tester.
My thought summed up really.
I have loads of surplus Wima TFF/TFM film caps, and I can't honestly see which spiral end wire goes where.
I'd love a simple checker box, this is the type of thing I make for work, to solve a simple issue - it MAY be tested using a scope, but for me that involves:
Checking my son isnt asleep.
Going in to his room and getting my scope down from high level shelf
Turning into and waiting 30 secs for the screen to warm up
Test.
So I know you can correctly find the outside foil, on plenty of radial leaded polystyrene, and I will orientate correctly (when I can verify the line means anything), but some of the axials are less easily discerned by eye.
I have loads of surplus Wima TFF/TFM film caps, and I can't honestly see which spiral end wire goes where.
I'd love a simple checker box, this is the type of thing I make for work, to solve a simple issue - it MAY be tested using a scope, but for me that involves:
Checking my son isnt asleep.
Going in to his room and getting my scope down from high level shelf
Turning into and waiting 30 secs for the screen to warm up
Test.
So I know you can correctly find the outside foil, on plenty of radial leaded polystyrene, and I will orientate correctly (when I can verify the line means anything), but some of the axials are less easily discerned by eye.