NON-Electrolytic Caps. Polarity??? - Page 1
Forum section: electronics / beginners

author: GlennSprigg
author_rank: Super Contributor
timestamp: March 17, 2020, 12:29:06 pm
content: I recently re-found a 2015 Youtube post from 'Mr. Carlson's Lab'. He goes into great detail explaining about noise reduction in circuits, attributable to the requirement of orientation of virtually ALL capacitors! (That have one side 'grounded' at least?).He talks about & shows a lot of old/new brands, that even have a black line on one end, which one may be excused for thinking that is the 'grounded' end, but he shows there is no rhyme or reason to these markings, even on many of the exact same Capacitors!He talks about the need to understand the 'FOIL-END', that is, the wire that is connected to the OUTER most turn of the foil.He shows this, by connecting the capacitors to an oscilloscope and alternating the connection direction. He uses his thumb & pointer finger to grasp the capacitors, resulting in his body acting like a 50hz/60hz aerial which shows up as varying amplitudes on the Scope. When connected the wrong way, the amplitude is much higher!! I've never heard of people discussing this aspect before!!Paul Carlson seems to be a very clever, experienced & very clearly spoken teacher too...

author: coppice
author_rank: Super Contributor
timestamp: March 17, 2020, 12:37:13 pm
content: This is a well known issue, but it really doesn't matter in most applications. Its only the large value rolled up capacitors which have significant asymmetry in their sensitive to noise pickup, and they have become a lot less common. Large value capacitors mostly go into low impedance circuits where pickup is suppressed by the low impedance. If you are working in a niche where large rolled up capacitors are used in high impedance applications you should be very well aware of this effect.

author: TimFox
author_rank: Super Contributor
timestamp: March 17, 2020, 12:54:22 pm
content: Back in Boy Scouts, I was taught that the black band ("outside foil") was closer to the surface of the paper capacitor, which was just covered in paper and wax. Therefore, as a safety matter, that end should be connected to the lower voltage point (often ground) to avoid breakdown if something contacted the capacitor body. Modern film capacitors have the same construction, but the outer case is more robust.During the vacuum tube era, with voltages above 100 V or so, the most common locations for these capacitors was the screen-grid bypass capacitor, where the outside foil was grounded, and plate-to-grid coupling capacitor, where the grid voltage was much closer to ground than the plate voltage.

author: GlennSprigg
author_rank: Super Contributor
timestamp: March 17, 2020, 01:35:56 pm
content: Thanks 'coppice' and 'timfox'. Thanks for your replies, and I understand.And I understand that the importance would greatly depend on the type of circuit.What I wondered is why the manufacturers would bother with their black bands at all, if they were usually 'wrong'?

author: coppice
author_rank: Super Contributor
timestamp: March 17, 2020, 01:43:18 pm
content: Quote from: GlennSprigg on March 17, 2020, 01:35:56 pmMost caps don't have the band. The ones that do are targeted at applications where it matters. You'll find lots of the caps made for RC power supplies, which have an AC rather than DC voltage rating in their specs, have a band. This means you can ensure the lower voltage side is the outer layer. If the capacitor gets overstressed. and pops, this reduces the scale of the resulting disaster.Unfortunately the marking is not universal. Sometimes the band means the outer layer side. Sometimes it means the inner layer side. You need to check the spec for the individual part.

author: ucanel
author_rank: Regular Contributor
timestamp: March 17, 2020, 04:38:14 pm
content: Quote from: GlennSprigg on March 17, 2020, 01:35:56 pmThat is just a claim,where is the statistical meaningful proofs.Is there a research that sayssearched 10k datasheets from reputable brandscompired the real caps and found X percent of them labelled wrong.

author: floobydust
author_rank: Super Contributor
timestamp: March 17, 2020, 11:26:16 pm
content: I found Mr. Carlson's technique to ID the outside foil didn't work with large value capacitors such as 1,000pF to 10nF. A scope has about 15pF input capacitance and with larger caps, AC hum on either side is pretty much the same.

author: Urshurak776
author_rank: Contributor
timestamp: March 19, 2020, 01:25:43 pm
content: I dabble in the guitar world now and then. On some tube guitar amps, the guys that make those always hook up the caps to a scope to see which way to orient it for the least noise possible. I'm currently in process of building a Fender 5E3 clone, so I'll take a look at it to see what the hub bub is about.

author: GlennSprigg
author_rank: Super Contributor
timestamp: March 23, 2020, 12:30:49 pm
content: Quote from: ucanel on March 17, 2020, 04:38:14 pmHi thereucanel. I'm not sure if you watched the actual Youtube link at the start. What 'Mr. Carlson' was saying & showing, that even when he tested many caps from the exact same brand/batch/type/value, he found them inconsistent. He was not putting the manufacturers down, as maybe the 'black' band has some other meaning to them, and not indicating the 'end-of-foil' termination. He's purely showing a relatively simple way to 'test' them, for when such noise is important in a particular circuit.

author: John B
author_rank: Frequent Contributor
timestamp: March 24, 2020, 08:29:16 pm
content: Quote from: floobydust on March 17, 2020, 11:26:16 pmI was able to repeat the technique using a 4.7u film cap. I used a straight coax connection, not a 10x probe. I also had to touch a macbook plugged into an adapter to get enough induced voltage in my body through the suppression caps to make it obvious on the scope. But yes, one side had more induced noise. I didn't get a measurement, but maybe a 5:1 difference in noise. In the video, he's getting a really clean signal though.

author: floobydust
author_rank: Super Contributor
timestamp: March 25, 2020, 01:03:27 am
content: It all relies on capacitive coupling, maybe 100pF from the human body fingers coupling say 10VAC of 60Hz AC hum to the test capacitor.So you would get about 10mVRMS which is close to what Mr. Carlson sees using 2mV/div on his scope with 0.1uF caps.4.7uF cap's reactance is about 564 ohms at 60Hz - quite low. My math says it drops to 0.213mV which is unusable. How did you get anything noticeable on your scope? Even picking up 100VAC from the Macbook gives 2mV which I would say is too low.I only worry about the outside foil on polystyrene capacitors and ended up using a microscope to ID the outside foil.P.S. His workbench top shelf has one tiny 10-32 machine screw for the rebar support arm?

author: John B
author_rank: Frequent Contributor
timestamp: March 25, 2020, 10:06:15 am
content: This time I placed the cap on the macbook with weight to remove the human variable, ie how much hum is being picked up and how much pressure is being applied to the cap. The film cap has a flat surface area around 384mm^2.I mean, personally I'm not too fussed as I put audio projects in die cast boxes, and there's nothing inside nearby like suppression caps and HV switching. But still the proof of concept works.

author: Zero999
author_rank: Super Contributor
timestamp: March 25, 2020, 01:45:34 pm
content: Could this be the reason why a quasi-polarised capacitor symbol is often used in some schematics?

author: floobydust
author_rank: Super Contributor
timestamp: March 26, 2020, 02:42:19 am
content: I went back in time to the 1930's as I thought the curve was symbolic for an arc horn for RF transmitter air capacitors, or something Niklola Tesla would worry about, but I did not find anything.Radiotron Designers Handbook 1933used straight-plate symbol for capacitors.Electrical drawings and Ladder Logic use two parallel plates for a N.O. contactor/relay contact symbol. It's easily confused for being a capacitor. I've taken flack for using it and default to the curved-plate symbol for a capacitor but some Europeans and noobs complain that it's an 'old' symbol.From Radiotron Designers Handbook 1954:"Note: The curved plate of the symbol used for a condenser indicates the earthed (outer) plate of an electrolytic or circular paper capacitor ; when this is not applicable the curved plate is regarded as the one more nearly at earth potential."ANSI Y32.2, IEEE Std 315, Z99electrical/electronic symbol conventions.

author: wraper
author_rank: Unknown
timestamp: March 26, 2020, 03:06:39 am
content: Quote from: Zero999 on March 25, 2020, 01:45:34 pmIt's usual polarized cap symbol, not something "quasi".

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2020, 11:58:13 am
content: Quote from: wraper on March 26, 2020, 03:06:39 amNo, it's a non-polarised capacitor symbol, because there's no + sign. Someone else asked the question in another thread.https://www.eevblog.com/forum/beginners/component-ratings-in-an-moc3020m-ac-mains-240v-light-dimmer-circuit/msg2973170/#msg2973170I theorise that the reason for this symbol might be because of the polarity of some non-electrolytic capacitors, although it's not important in the schematic discussed in the other thread, so I could be wrong or it's being used inappropriately.

author: wraper
author_rank: Unknown
timestamp: March 26, 2020, 12:30:21 pm
content: Quote from: Zero999 on March 26, 2020, 11:58:13 amBy quick googling I cannot find if "+" sign should or should not be there according to any standard. But such "polarization" in that optotriac circuit makes no sense just as well as using polarized capacitor.

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2020, 01:22:03 pm
content: Quote from: wraper on March 26, 2020, 12:30:21 pmYou need to practice your Googling technique. I found this in less time than it took me to type this message. The note below indicates the curved plate is ground, which would backup my theory that it would be the outer plate of a film capacitor.https://uk.farnell.com/capacitor-types-and-performance

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: March 27, 2020, 02:41:57 am
content: Quote from: Zero999 on March 26, 2020, 01:22:03 pmWould it? I see no connection, implied or otherwise, between the curved plate symbol and the outer layer of a film cap.I believe that the foil-paper caps made during the tube era were much more reliably marked and marked to mean the outer layer of foil than what you can get today. Much of today's caps with a black stripe come from China. Those guys will make anything that sells and if copying antique caps with a black stripe will sell then they will do it. And will do it without regard to what that stripe signifies as long as it whets the appetite of amateur guitar amp builders who yearn for that genuine vintage NOS component or something like it.I have watched nearly everything that Paul Carlson has produced and he is one of the most expert and methodical technicians/engineers I know. I'm a P.Eng with 40 years experience and I have learned much from him.

author: floobydust
author_rank: Super Contributor
timestamp: March 27, 2020, 06:37:19 am
content: I think the curved-plate symbol came about from the variable capacitor where the curved rotor is at chassis ground potential and the stator is free. Like the symbol in this schematic.I found when American electronics symbols changed and got somewhat standardized, it wasbeginning of 1945at the post-war conference of American Standards Association (ANSI) and Armed Services. Books and magazines followed, adopting the new symbols. They kept the straight-plate capacitor/contactor symbol, deleted the round plate variable capacitor symbol, and a few otehr symbol changes.Articlehttps://www.americanradiohistory.com/Archive-Radio-Craft/1940s/Radio-Craft-1945-01.pdfpg. 211 (pdf pg. 27/70)edit: actually a terrible example schematic circuit (mains powered, no transformer)

author: TimFox
author_rank: Super Contributor
timestamp: March 27, 2020, 01:40:38 pm
content: In that 1945 article, the curved-plate symbol replaces the two-straight-plate symbol, to avoid confusion with the contactor symbol. Furthermore, the curved plate represents the outside foil of a non-polarized paper-foil capacitor.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 27, 2020, 02:07:41 pm
content: Quote from: Urshurak776 on March 19, 2020, 01:25:43 pmI used to repair audio equipment. And indeed, this is the outer foil which is most sensitive to noise pickup.I was also advised of the same rotation trick many moons ago.That and to twist the heater leads for the vacuum tubes.