Non-polar electrolytics.  I'm confused. - Page 1
Forum section: electronics / beginners

author: Tech Gorilla
author_rank: Newbie
timestamp: March 13, 2020, 03:56:52 pm
content: I purchased a used DBX286A that needed love. I ordered all of the broken pots and all the electrolytics for giggle. As I was changing out all of the caps (to Nichicon), I pulled a cap out and was confused that the board wasn't marked for polarity. I kind of panicked and left the 4 or 6 non-polar caps alone. The caps I purchased from Mouser were 1uF and 22uF 25V.I am assuming that a polar cap cannot go where a non-polar cap goes. can anyone smarter than me help me understand this?First post. I hope I don't break a rule.

author: Benta
author_rank: Super Contributor
timestamp: March 13, 2020, 06:59:41 pm
content: Quote from: blueskull on March 13, 2020, 04:05:50 pmHuh? That's how non-polarized electrolytics are made. You'll need to use two caps of double value, though.

author: Tech Gorilla
author_rank: Newbie
timestamp: March 13, 2020, 07:04:59 pm
content: Quote from: Benta on March 13, 2020, 06:59:41 pmThis is what I understood from reading too. I will order the proper non-polar caps today.

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 13, 2020, 09:18:01 pm
content: Non-polar electrolytes, in general, are better then polar ones (THD).If you can't find needed non-polar ones you can change them to series connected polar ones, with twice larger value. (Minus to minus of cause).But it seems to me that it could be better to leave that caps alone - they can be fine. You may test them for capacity value and leakage current.

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2020, 10:05:19 pm
content: Quote from: Vovk_Z on March 13, 2020, 09:18:01 pmThe capacitors are in series, so it doesn't matter whether they're connected plus to plus or minus to minus, either will work.QuoteI agree. Simply replacing all of the capacitors is pointless.

author: floobydust
author_rank: Super Contributor
timestamp: March 13, 2020, 10:54:31 pm
content: Quote from: Benta on March 13, 2020, 06:59:41 pmIt doesn't work in practice because the two capacitors are not identical- in capacitance or leakage current. You end up with a net charge on one cap that is usually reverse polarity, with my luck. In theory two caps work, but not really. Just buy a bipolar cap, Nichicon makes them.

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2020, 11:04:49 pm
content: Quote from: floobydust on March 13, 2020, 10:54:31 pmDo you have an links to studies on this or have you done any experiments yourself?Electrolytic capacitors act like poor diodes, so if one gets reverse biased, it will simply pass the current straight through. One of the capacitors might develop a DC charge, but I don't see how it could be a problem.Edit: The electrolytic capacitor was originally developed from the electrolytic rectifier, which was a very poor diode.https://en.m.wikipedia.org/wiki/Rectifier#Electrolytic

author: floobydust
author_rank: Super Contributor
timestamp: March 13, 2020, 11:53:13 pm
content: This is from experience - I've had failures using two of the same polarized cap to make a bipolar one. EMG amp, POTS telephone line bridge and loudspeaker crossover networks.It fails with one cap getting "stuck" with high leakage current. It's some kind of unequal charge distribution or rectification across the dielectric. I ended up putting blocking diodes across each cap and/or high value resistors and then they work OK. This is with electrolytics or tantalums.You can prove it on a bench, with a signal generator and DC offset, the midpoint of the two caps should, in a perfect world be...volts, neglecting upset from the voltmeter loading.

author: Gyro
author_rank: Super Contributor
timestamp: March 14, 2020, 10:17:45 am
content: If youmustuse back to back electrolytics, then the simplest answer is to put a high value bias resistor from the junction to a suitable supply rail. Eg. connect the two caps +ve to +ve and connect a 1 - 10M resistor from the junction to the positive DC rail. That will keep both caps happily biased.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 14, 2020, 10:37:40 am
content: All these workarounds are interesting in that it teaches you about the physics of it, but it seems to me a lot simpler and space saving on the pcb to just buy some non-polarised electrolytics...

author: madires
author_rank: Super Contributor
timestamp: March 14, 2020, 11:05:04 am
content: Another source is Jamicon's NK non-polarity series. Small value caps can also replaced with film caps, but the size might be a constraint.

author: Zero999
author_rank: Super Contributor
timestamp: March 14, 2020, 11:15:34 pm
content: Quote from: Gyro on March 14, 2020, 10:17:45 amOr the negative rail, if the junction between the two capacitors is the negative terminals.Quote from: blueskull on March 14, 2020, 10:21:17 amI don't think it would be too bad, once the bias point has been established. It would be interesting to test, but I lack suitable test equipment for precise THD measurement.

author: Mp3
author_rank: Frequent Contributor
timestamp: March 15, 2020, 12:20:46 am
content: OP, have you found the service manual for the dbx unit you're working on?This thread has a lot of speculation but i know off hand when I repaired a DBX 160 there were at least one bipolar capacitor installed at the factory.So i think it's quite likely you will end up buying bipolar caps. I keep some bipolar muse caps around in common sizes just to be safe, though i almost never need them.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 15, 2020, 01:36:26 am
content: Be very wary if replacing bipolar caps in some circuits.An Electrohome picture monitor had a.bipolar associated with the horizontal output circuit, which had died.I looked up "bipolar" caps in the RS Components catalogue & ordered what looked like they would do the job, with a low ESR, etc.On their arrival, I was astounded by how small they were compared to the original, but looking back at the catalogue, saw that was the correct size.Marvelling at the wonderful advances in technology, I fitted one, the monitor turned on, & ran.-----For about 20 seconds, then smoke & steam started emitting from the "you beaut" new bipolar.It seems that the Electrohome cap was a very special one, which is why it was big.The chances of getting a new one from Canada anytime soon were remote, & the production people wanted the thing "yesterday", so I built a "Chrisrmas tree" of Polyester "Greencaps" to get the correct value.It wasn't pretty, but it did the job!