15v circuit, 35v capacitor, but only have 25v - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: February 10, 2020, 02:21:13 am
content: I just wanted to ask what you guys think about this. It decides whether i finish a repair tonight or place a mouser order for just a couple parts lol.I'm replacing a failed capacitor on a power supply (rated 4700uf, tested about 800-900uf) and doing the other two while i'm at it. It's for a 5v line and is rated 16v... no problem.... i have a 16v 4700uf here.The other two are 35v 1000uf. One is for -15v and one is for +15v. I have 25v 1000uf on hand. I don't know if 25v is okay because i think 15v is 30v peak-to-peak, but each capacitor is only having +15 or -15 running through it, so isn't 25v enough room to be safe?I don't want to push it too close so i'll buy 35v's and wait if you all think it's best to wait and use 35v.

author: Whales
author_rank: Super Contributor
timestamp: February 10, 2020, 03:19:26 am
content: Quote from: Mp3 on February 10, 2020, 02:21:13 amWhat? Are these capacitors for DC power rails or AC signal filtering?EDIT: possibly you mean that -15 to +15 is a 30V difference? If the capacitors are only wired from 0 to +15 and 0 to -15 then the capacitors only see 15V, not 30V. You would have to wire a cap from -15 to +15 to get the 30V (but that's probably not what this circuit requires).

author: Mp3
author_rank: Frequent Contributor
timestamp: February 10, 2020, 03:39:03 am
content: Quote from: Whales on February 10, 2020, 03:19:26 amYeah, i don't think they are actually seeing peak to peak voltage. These are DC power rail capacitors coming after some voltage regulators, so i was quite sure these capacitors only see 15v. But, i kept second guessing myself if they were 35v for a reason. But then i thought 35v is too close a margin if they actually saw 30v all the time. Thanks!

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 10, 2020, 04:07:20 am
content: Some designers like to double or even triple the voltage rating for electrolytic caps. This probably isn't necessary for name brand caps but I guess the designer will sleep better at night if they use cheap caps and derate the maximum voltage by a large factor.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: February 10, 2020, 04:28:46 am
content: Sometimes the capacitor ratings are chosen based on availability. May be during the company had surplus stock of 35V capacitors and hence they decided to use that. It may also depend on the cost. It may so happen that if the distributor had 35V in large quantity, they may actually sell the 35V cheaper than or equal to the 25V one.So in my opinion, it is perfectly safe to use a 25V cap for a 15V DC line.

author: Mp3
author_rank: Frequent Contributor
timestamp: February 10, 2020, 05:09:51 am
content: Quote from: EEEnthusiast on February 10, 2020, 04:28:46 amThis is exactly what i was imagining was the case. Especially since 16v is needlessly high for a 5v line but 16v is so common. I have the new capacitors in place and everything's working nicely. I am ordering new voltage regulators anyway since they were very old and crusty. I dont particularly trust 80's IC's for power delivery. The 5v was very "fuzzy" looking on the scope.

author: rdl
author_rank: Super Contributor
timestamp: February 10, 2020, 05:16:51 am
content: It's actually not a good idea to use an aluminum electrolytic cap at voltages far below its WVDC rating.

author: tggzzz
author_rank: Super Contributor
timestamp: February 10, 2020, 10:04:00 am
content: Quote from: Mp3 on February 10, 2020, 02:21:13 amRipple current can be a consideration, since it causes internal heating. That's why capacitors include that in their specification.Apart from that, you need to understand the DC and ripple voltages on the capacitor. In other words, disambiguate your statement "i think 15v is 30v peak-to-peak"

author: fcb
author_rank: Super Contributor
timestamp: February 10, 2020, 10:33:52 am
content: Quote from: rdl on February 10, 2020, 05:16:51 amCan you explain why? Or provide a citation please?Like many others I overrate capacitors (16WV on a 5V rail, 25WV on a 12V rail, 50WV on a 30V rail etc)- it's something I've always done and it hasn't bitten me in nearly 30 years.

author: tggzzz
author_rank: Super Contributor
timestamp: February 10, 2020, 10:48:25 am
content: Quote from: fcb on February 10, 2020, 10:33:52 amHand waving explanation... The dielectric is formed by the passage of a DC current, sometimes called a leakage current. Too little voltage can mean too little current and imperfect dielectric formation.

author: Mp3
author_rank: Frequent Contributor
timestamp: February 10, 2020, 10:59:39 am
content: The caps that were in it were no name caps from the 80's, neither brand name on either cap i recognized or had any meaningful google presence. They were also shockingly light when removed from the power supply, but that isn't surprising since the 4700uf was putting out just under 1000uf capacitance. I used some high temperature caps to replace the old ones which had some of the lowest listed ripple current on Mouser when i bought them.I think most of the 16v 4700uf caps showed something like 2-2.5A ripple, and the ones I'm using have 1.2A ripple. The service manual made no reference to ripple or anything like that, just listed what model # of capacitor was used, but then again i wouldn't be posting in the Beginners section if i presumed to really know what I was talking aboutAnyway, the device has been happily working for the last few hours straight since I put these new capacitors in and touched up a lot of old crusty looking solder.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 10, 2020, 12:04:46 pm
content: If you have no idea of the actual ripple current present, one way is to install the candidate capacitors and monitor their case temperature. As some sort of rule-of-thumb, it shouldn't rise more than roughly about 5 deg C over the ambient right next to the capacitor.

author: fcb
author_rank: Super Contributor
timestamp: February 10, 2020, 12:17:18 pm
content: Quote from: tggzzz on February 10, 2020, 10:48:25 amOK. So what rule-of-thumb would you use then?

author: tggzzz
author_rank: Super Contributor
timestamp: February 10, 2020, 01:13:26 pm
content: Quote from: fcb on February 10, 2020, 12:17:18 pmI would read the data sheet to glean whatever info I could. I would google for (informed) answers.

author: rdl
author_rank: Super Contributor
timestamp: February 10, 2020, 01:16:24 pm
content: Dielectric deterioration of aluminum electrolytic capacitors used at voltages far below the WVDC rating is something I read about probably 40 years ago and has stuck in my mind. I don't know how much of a problem it is because it's rarely mentioned anymore. Maybe modern dielectrics are better and less affected or maybe it's becoming lost knowledge. I've always tried to stay at around 20-50% just to be safe.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 10, 2020, 01:21:08 pm
content: Quote from: rdl on February 10, 2020, 01:16:24 pmAs far as I understand the mechanism correctly, if it is a problem at all, then it's only a problem if, at some point after long usage at low voltage, the actual voltage is suddenly increased by a lot. This wouldn't happen with fixed rail voltages.

author: pcmad
author_rank: Regular Contributor
timestamp: February 10, 2020, 05:27:49 pm
content: if the psu has no load in it it can rasie the voltage keep to the rating set or higher

author: Zero999
author_rank: Super Contributor
timestamp: February 10, 2020, 08:49:56 pm
content: Quote from: tggzzz on February 10, 2020, 10:04:00 amI think he might be confused by negative voltages. According to his first post, it's a bipolar power supply, +/-15V and seems to be confused about the fact that the total voltage is 30V.Quote from: Mp3 on February 10, 2020, 02:21:13 amYes, that's fine. You're right, each capacitor will only have 15V across it, which will be safe.Another thing which can catch people out is the physical dimensions: it's easy to order a capacitor which is electrically suitable, but won't fit the PCB. Measure the original capacitors and the distance between the legs, i.e. pitch, before ordering replacements.