Why bipolar electrolytic caps? - Page 1
Forum section: electronics / repair

author: Vicus
author_rank: Regular Contributor
timestamp: February 06, 2021, 03:29:12 pm
content: I have in the lab a couple of JBL PRX712 (powered speaker). One of them have some intermittent noise comming out of the transducer (either the woofer and compresione driver). The noise is more like an hiss than a hum and sometime it's correlated to the speed of the fan. The amplifier it's composed by a psu + dual power amplifier (a Pascal S-PRO2 I think is the model) and a preamplifier board with the rest of the staff that nedded. I try to swap the preamp board so I know that the problem is it. No schematic available unfortuntely but the pinout of the ribbon connector can be found. On the board there are some 47uF 16V smd electrolitic capacitor but they are not linked to the low voltage power pin that the psu board supply. Those in the picture do. They looks to me bipolar so why this type of choose?As you can see there are also + mark on the silkscreen.Some opinion?

author: bob91343
author_rank: Super Contributor
timestamp: February 06, 2021, 07:15:41 pm
content: Why do you think they are bipolar?

author: Vicus
author_rank: Regular Contributor
timestamp: February 06, 2021, 08:23:21 pm
content: Nice question. Well because they don't have the black mark that indicate the negative terminal.

author: edavid
author_rank: Super Contributor
timestamp: February 06, 2021, 09:04:00 pm
content: If they are really bipolar, it's because they are used as audio coupling capacitors.P.S. "electrolytic"

author: Vicus
author_rank: Regular Contributor
timestamp: February 06, 2021, 09:44:00 pm
content: Quote from: edavid on February 06, 2021, 09:04:00 pmYeah, one of many mistake.Audio coupling capacitor means that they feed the audio signal into supply rails? I for shure measure one of the capacitor is ground and the other to the supply rail. So I don't think so.

author: Bud
author_rank: Super Contributor
timestamp: February 06, 2021, 09:55:38 pm
content: Quote from: Vicus on February 06, 2021, 08:23:21 pmThey have a small dot on the positive terminal side. The + mark is printed on the silkscreen of the board.

author: bob91343
author_rank: Super Contributor
timestamp: February 06, 2021, 09:56:42 pm
content: If they are bipolar they are used where the polarity gets reversed, most often in speaker systems for crossover when a paper or plastic capacitor is too large or expensive. They are also used for motor phase shifting, as in antenna rotator boxes.If no polarity is marked, and the size is such that it could be a paper or polymer unit, then it's not electrolytic.

author: Vicus
author_rank: Regular Contributor
timestamp: February 06, 2021, 10:17:20 pm
content: I still think that they are bipolar, I'll try to save one remouving carfoully than apply some inverse polarity voltage. As you see in this picture they also use "standard" marked electrlytic cap.

author: TheMG
author_rank: Frequent Contributor
timestamp: February 06, 2021, 10:21:12 pm
content: Quote from: Bud on February 06, 2021, 09:55:38 pmI agree, these are just standard electrolytic capacitors, just with different markings than the ones commonly seen on such capacitors.Note also the angled corners on one side of the capacitor, as well as the matching silkscreen outline.

author: Vicus
author_rank: Regular Contributor
timestamp: February 06, 2021, 10:28:01 pm
content: Smd bipolar eletrolytic caps have also one side angled. Crearly who did design the schematic and the pcb use standard eletrolytic cap but for some reason during assembly they put this one. Now I'm very curious to see if they will explode.

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2021, 12:09:14 am
content: I think they are bipolar for some reason (no idea why) and I think they are Panasonic EEE1VA100NP. The dot indicates ROHS compliance.

author: Gyro
author_rank: Super Contributor
timestamp: February 08, 2021, 10:23:05 am
content: Electrolytic caps are easy to blame when something goes faulty, but from the symptom you describe (intermittent hiss sometimes related to the speed of the fan) I don't think they are a natural suspect. You have said that the caps are not associated with the power rails, so they almost certainly are bipolar signal coupling caps which are in a low stress situation.I would be looking more for poor connector contacts, dry joints etc.

author: Vicus
author_rank: Regular Contributor
timestamp: February 09, 2021, 09:42:58 pm
content: The bipolar one is from psu rails and ground. The others one I don't know what they are doing.

author: tooki
author_rank: Super Contributor
timestamp: February 10, 2021, 11:43:17 pm
content: Quote from: Vicus on February 06, 2021, 10:28:01 pmWhy would you think they'd explode?!?Polarity in a capacitor is a fundamentally undesirable trait, one that we tolerate in electrolytics and tantalums in exchange for their high capacity in a small space. There's no situation where polarity in a capacitor has any advantage, it's purely a limitation to work around. All else being the same, you can always use a non-polarized capacitor in place of a polarized one, but not the other way around.Internally, a bipolar electrolytic is simply two double-capacity electrolytics in series, in opposite polarity. (So either +--+ or -++- internally, makes no difference in the end.) So for a given value and voltage, it'll tend to be a bit bigger, and won't be optimized for low ESR the way polarized power supply filtering caps are. And they're a bit more expensive, so there's normally no reason to use bipolar electrolytics except for signal paths, which is likely precisely what they're doing here.Quote from: bdunham7 on February 08, 2021, 12:09:14 amI agree. Normal electrolytics necessarily always have a polarity indicator; ones without it are bipolars.Quote from: TheMG on February 06, 2021, 10:21:12 pmNope. I am currently using Panasonic bipolar electrolytics in a project, and they lookexactlylike the ones in the pictures. They do have the angled corners — presumably it's cheaper to just use the same bases. As for the silkscreen, the component footprints for SMD electrolytics are always for polarized ones, so unless you wanna go to the effort of drawing your own footprints, you just use the regular polarized footprint and ignore the polarity marking.

author: perieanuo
author_rank: Frequent Contributor
timestamp: February 11, 2021, 08:46:32 am
content: hithe person who did the silkmask I assume is not stupid, + means polarised.between VCC and GND rail you put polarised and that's all folks.and cotrare to other oppinions on this thread, the caps can also be cause for humming.extract them and test with rlc tester or replace them or just test with non-smd ones after extracting, then you find your 'bonheur'.the non-polarised ones are in audio signal path only, we aren't suppose to reinvent the wheelregards